Yaa Basic Concepts Of Colour Measurement
Tuesday 18 December 2018
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Basic Concepts of Colour Measurement
Dr. Asim Kumar Roy Choudhury
Ex-Professor in addition to HOD (Textile),
Govt. College of Engg. & Textile Technology, Serampore - 712201, Hooghly (W.B.), Republic of Republic of India
E-mail: akrc2008@yahoo.in
Ex-Professor in addition to HOD (Textile),
Govt. College of Engg. & Textile Technology, Serampore - 712201, Hooghly (W.B.), Republic of Republic of India
E-mail: akrc2008@yahoo.in
Colour inward our Daily Life
From fourth dimension immemorial, adult man must have got watched inquisitively the wonder-work of sunlight inward creating bluish color of zenith sky, glowing scarlet of rising in addition to setting heaven in addition to diverse hues of rainbow. Colour is also an integral constituent of our daily life, live it textiles, paints, plastics or printing. We can’t fifty-fifty imagine a colourless globe or so us. All our daily necessities are packed inward attractive colourful packaging that tempts us to purchase fifty-fifty if nosotros do non demand it. We can’t fifty-fifty imagine our television set without colour. We lead our garments primarily past times colour, which projects our personality. We select paints in addition to article of furniture for our theatre past times colour, which gives us pleasing atmosphere. The color depicts the nature in addition to inner personality of the automobile owner. Colour of a jewel or precious rock also guides a someone to overcome obstacles inward his life. Colour has move a focal dot of what nosotros do every day.
Despite existence so of import in addition to so unopen to everyone’s day-to-day life, it is non possible to limited color inward unique or inward specific language. We recall color till nosotros await at it. The minute nosotros convey away our sight from it, it gets erased from our memory. The color scientists attempted to specify color inward explicit universal linguistic communication so that it could live understood past times everyone involved with color in addition to color reproduction. This led to limited color numerically which is unique in addition to unambiguous specification of colour.
Visual Perception of Colour
Colour may live viewed inward 2 modes (Figure 1) namely:
- Illuminant mode- when the calorie-free stimulus comes straight from a source of light, say, an incandescent light or a candle.
- Object mode- when stimulus comes from an object either from its surface (surface mode) or through the object (volume mode). In this mode, the perception of an object is mostly inward reference to the surrounding objects. Three ingredients are required to visualise the color of an object, namely a calorie-free source, object itself in addition to the human observer.
 |
| Fig-1: Two modes of viewing colours |
A calorie-free source illuminates the object in addition to is characterised past times the unloosen energy at different wavelengths, which is denoted past times the term spectral mightiness distribution (SPD). When a calorie-free beam falls on an object, it is modified past times absorption, scattering in addition to other physical processes depending on the physical in addition to chemical structure of the object. The calorie-free (colour stimulus) that reaches the optic of the observer inward the shape of reflected calorie-free interacts with the photosensitive pigments introduce inward the eye. This gives rising to nervus impulses which are transmitted to brain. The human eye-brain machinery makes rapid in addition to continuous evaluation of object appearance in addition to colour. The light, which enters our eyes, contains the feature imprints of the calorie-free source in addition to the object. Light tin also autumn straight (without interaction with object) on our eyes creating nervus impulses.
Colour Perception
The Committee on Colorimetry of the Optical Society of America inward 1922 defined color as the full general holler for all sensations arising from the activity of the retina of the optic in addition to its attached nervous mechanisms, this activity being, inward nearly every instance inward the normal individual, a specific response to radiant unloosen energy of sure wavelength in addition to intensity (OSA, 1953).
Colour is sometimes used as a holler for materials such as dyes in addition to pigments, but the holler Colorant volition live to a greater extent than appropriate inward these cases. The spectral composition of radiant unloosen energy in addition to the spectral transmittance or reflectance of substances is also called colour, but it is in ane lawsuit again an inappropriate holler for these purely physical concepts. Psychophysical concept of color as characteristics of calorie-free dependant on human vision is most appropriate.
Perception of color which is alone a prerogative of human beings is purely individualistic in addition to hence subjective. As a result, objective communication of a item color becomes difficult. For expressing color objectively ane must mensurate in addition to quantify it. The color tin live objectively expressed either past times creating calibrated coloured physical samples or past times an instrumental color measuring devise in addition to expressing it past times numbers.
Colour Mixing
According to ane reliable estimate, nosotros tin distinguish alongside x 1000000 unlike colours (Judd in addition to Wyszecki, 1975). Kuehni farther estimated that the humans with normal color vision tin distinguish alongside some 2 1000000 colours when viewed against a mid-grey background in addition to maybe double when the background is widely varied (Kuehni, 2005).
Newton explicitly stated that color is a perceptual property, non a physical attribute, which meant that the calorie-free mixtures occurred inward the eye, non inward the light. Newton recognised 3 aspects of color perception: brightness, hue in addition to intensity. Painters in addition to dyers already from before Newton’s fourth dimension believed inward 3 substitution chromatic colours that they sometimes equated with pigments or dyes: yellow, ruby-red in addition to blue, from which all other hues tin live created. With long sense with paints in addition to dyes they also affirmed that stuff color mixtures in addition to calorie-free mixtures behaved rattling differently.
Colour perception for humans is three-dimensional, a fact almost surely stems from the existence inward the retina of 3 unlike classes of light-receptive cells. LeBlon (1756) was get-go to brand clear distinction betwixt mixing pigment colours in addition to mixing colours of light. He stated that all visible objects tin live represented past times 3 colours, yellow, ruby-red in addition to bluish in addition to mixture of these 3 colours makes dark or all other colours. He named those as stuff colours or those used past times painters. He farther added that for a mixture of spectral colours those proposed past times Sir Isaac Newton cannot make black, but the rattling contrary, white.
Primary colours are sets of colours that tin live combined to brand a useful arrive at of colours. For human applications, 3 primary colours are usually used, since human color vision is trichromatic.
Primary colours are non a substitution holding of calorie-free but are oftentimes related to the physiological response of the optic to light. Fundamentally, calorie-free is a continuous spectrum of the wavelengths that tin live detected past times the human optic inward an infinite-dimensional stimulus space. However, the human optic usually contains only 3 types of color receptors, called cone cells. Each color receptor responds to unlike ranges of the color spectrum. Humans in addition to other species with 3 such types of color receptors are known as trichromats. These species response to the calorie-free stimulus via a three-dimensional sensation, which mostly tin live modelled as a mixture of 3 primary colours (Sobel, 1989).
Many birds in addition to marsupials are tetrachromats (Backhaus in addition to Werner, 1998), in addition to it has been suggested that some human females are tetrachromats as good (Mollon in addition to Jordan, 1993), having an extra variant version of the long-wave (L) cone type.
Most mammals other than primates have got only 2 types of color receptors in addition to are, therefore, dichromats; to them, at that spot are only 2 primary colours.
Any selection of primary colours is essentially arbitrary; for example, an early on color photographic process, autochrome, typically used orange, green, in addition to purple primaries. However, unless negative amounts of a color are allowed the gamut of color generated volition live restricted past times the selection of primaries (http://en.wikipedia.org/wiki/Primary_color, accessed on 16.7.15).
For additive combination of colours, as inward overlapping projected lights or inward CRT displays, the primary colours usually used are ruby-red (R), dark-green (G), in addition to bluish (B). The termination of additive mixing of the primaries are listed below,
R + G = Y (Yellow),
G + B = C (Cyan or blue-green)
B + R = M (Magenta)
R + G + B = due west (white)
For subtractive combination of colours, as inward mixing of pigments or dyes, such as inward dyeing or printing, the primaries usually used are cyan (C), magenta (M), in addition to yellowish (Y), though the laid of red, yellow, bluish is pop alongside artists.
The subtractive primaries tin live obtained past times removing red, dark-green in addition to bluish from white calorie-free using respective coloured filters namely
W - R = C (- R)
W - G = M (- G),
W – B = Y (- B),
The mixing of subtractive primaries yellow, magenta in addition to cyan tin live predicted mathematically as follows:
Y + M = (W - B) + (W - G) = due west - B - G = R,
Y + C = (W - B) + (W - R) = due west - B – R = G
M+ C = (W - G) + (W - R) = due west - G - R = B,
Y + M + C = due west - R - G - B = K (Black)
When the 3 subtractive primaries are mixed inward proper proportions as inward instance of mixing colorants, the mixture is unable to reflect calorie-free anymore in addition to appear dark (K) inward colour. Similarly when white calorie-free is passed through filters of 3 subtractive colours, the whole of white calorie-free volition live removed. Mixture of 2 primaries is called secondary. The most commonly used additive color primaries are the secondary colours of the most commonly used subtractive color primaries, in addition to vice versa.
The age-old belief that 3 primary colours are required to brand white was also proved fake (Helmholtz, 1852). The thought of complimentary spectral colours came into existence in addition to was afterwards defined past times Grassman (1853) as follows:
To every color belongs some other homogeneous colour, which when mixed with it, gives colourless light.
Subsequently Helmholtz confirmed that every spectral color does non have got a spectral complement as proposed past times Grassman. Influenza A virus subtype H5N1 arrive at of dark-green colours ranging from close 484 nm to close 570 nm, has gratuitous colours inward the non-spectral royal range.
Colour Communication
In colorant production in addition to application industries, colours are to live communicated, compared, recorded in addition to formulated on regular basis. This necessitates systematic classification of colours. The objects tin live classified inward diverse ways inward damage of colour. The classification may live based on visually or instrumentally-assessed color parameters (Roy Choudhury, 2000) .
While communicating or talking close colour, a linguistic communication which is understandable past times both the parties must live followed. Influenza A virus subtype H5N1 logical scheme for ordering in addition to specifying colours on the ground of some clearly defined attributes is known as color notation system. The attributes are mostly 3 inward issue as our vision is trichromatic in addition to they constitute the coordinates of the resultant ‘colour space’. Colour notation systems also embrace ‘colour monastic enjoin systems’ which are typically comprised of stuff standards inward the shape of color atlas. Due to constraints of colorant gamut, the atlases may depict only a physically realisable subset of a color monastic enjoin system.
Two most mutual additive systems used inward connector with estimator display are RGB (based on mixing of 3 additive primary color red, dark-green in addition to bluish produced past times the phosphors of the display unit of measurement inward cubic space) in addition to HSB (hue, saturation in addition to brightness inward cylindrical form).
For instrumental color measure of an object, additive primaries are used to predict the quantity of subtractive colours required to reproduce that colour. This entails the utilisation of touchstone source of light, viewing geometry in addition to a touchstone observer. The human face of color as numerals has been standardized past times Commission Internationale de l’Eclairage (CIE) laid upwards inward 1931 for standardisation of measure of colour. Thus the instrumental measure of color is helpful in, communication of color across the entire render chain as good as comparison samples in addition to setting pass-fail criterion for sourcing in addition to render of products. In this shape it is a useful tool for character control.
It is rattling convenient, easily understandable in addition to memorable when the colours are called past times names similar to mutual objects orange, ultramarine, olive, malachite green, bottle-green, peanut-green, sea-green etc. But such color names are rattling approximate, unreliable in addition to temporary. Their important also changes with observer, time, place, style, technology, language, civilization etc.
When nosotros bargain with a reasonable issue of specimen, say a few thousands, to cover the whole arrive at of possible colours (1millions or more), the specimen must live selected according to a scheme or plan. It is good known that the colours are three-dimensional. However, the dimensions of color are expressed inward diverse ways inward unlike fields. For systematic arrangements, the dimensions should live independent of each other.
Visual Description of Colour
Wright (1984) identified 2 sets of visual attributes namely:
A color monastic enjoin scheme is a systematic in addition to rational method of arranging all possible colours or subsets past times agency of stuff samples. Once the colours are arranged systematically they are named inward some descriptive damage and/or are numbered (Graham, 1985).
The color monastic enjoin systems are of 3 types (Wyszecki, 1986):
Pantone Colour System (Figure 3) is basically a colourant mixture system. The Pantone scheme (www.pantone.co.uk) began life inward 1963 inward the USA, for defining colours for printers, but expanded into other fields later, e.g. textiles inward 1984, plastics inward 1993, in addition to architecture in addition to interiors (1925 colours) inward 2002, each of which has a 6-digit numerical notation (e.g. # 19-1764) in addition to an inspirational color name. This is widely used inward graphic fine art in addition to also inward textile manufacture mainly because of its depression cost, though the colours are non as spaced. The shades are prepared on newspaper using printing inks. It is non a color monastic enjoin scheme since it does non include a continuous scale. It is to a greater extent than appropriately considered a color naming system.
Colour appearance systems are based on the perception of colours past times an observer with normal color vision. The scales of these systems are chosen to stand upwards for attributes of perceived colours. However, attributes represented inward diverse systems are different.
The principal emphasis of appearance-based systems is the uniform visual spacing. The systems thus allow tardily interpolation betwixt the samples represented in addition to extrapolation of colours non illustrated inward a given collection. The collections of samples are mostly represented inward pages of constant hue.
Six pop color monastic enjoin systems, province of root in addition to their respective color attributes are as follows (Roy Choudhury, 2010) :
Munsell scheme divides each horizontal hue circle into 5 unique or principal hues: Red (5R), Yellow (5Y), Green (5G), Blue (5B), in addition to Purple (5P), along with 5 intermediate hues (5YR, 5GY, 5BG, 5PB, 5RP) halfway betwixt next principal hues.
The Munsell atlas is usually available on painted newspaper inward glossy (1488 chips) in addition to matt forms (1277 chips). Influenza A virus subtype H5N1 method for specifying opaque object colours such as textiles, painted panel etc. past times Munsell color scheme has been described past times ASTM (1980).
SCOTDIC, a textile version of Munsell created past times fusion of 2 quite unlike systems - Standard Colour of Textile (Japan) in addition to Dictionnaire Internationale de la Couleur (France), is adopted past times over 8,000 companies worldwide. Textile touchstone colours of the SCOTDIC color scheme are widely used as color tools past times fashion color professionals. The scheme has 3 versions - glossy (2468 colours on polyester crepe fabric), matt (2038 colours on cotton wool poplin fabric) in addition to yarn (1100 colours on wool yarns). It has incorporated many brilliant colours in addition to the issue of constant hue nautical chart has been increased to 54 (20 for Wool). SCOTDIC scheme uses a vi digit code for each touchstone color – the get-go 2 digits for hue, the 2nd 2 digits for value in addition to the tertiary 2 digits for chroma. The prefix corresponds to the stuff of textile bases – P for Polyester, C for Cotton, due west for Wool. Therefore, the notation C-155010 agency cotton wool Standard sample having hue = 15, value = 50 in addition to chroma = 10 (Roy Choudhury, 2008).
Most of the stuff based atlases are forthwith available inward digitised shape e.g. NCS Digital Atlas (www.ncscolour.com), Digital Colour Atlas 3.0 (www. dtpstudio.de) etc.
Figure 5 shows 2 pages from the Munsell Book of Colours. Figure 5 (a) shows hue circle having samples of 100 hues inward 2 depths (moderate in addition to night shades) (samples of some hues are missing). Figure 5 (b) shows constant hue page of hue = 05. In these pages, rows stand upwards for constant values samples, piece columns stand upwards for constant chroma samples.
Instrumental Measurement of Colour
Newton said that (light) rays are non coloured, but but has the mightiness to imitate sure sensations inward the heed of the observer. Human optic is a highly versatile detector of calorie-free in addition to colour. An observer tin perceive chromatic attributes in addition to diverse geometric factors (direction, texture, shape in addition to many others) simultaneously. An musical instrument till appointment is far behind inward versatility. It tin mensurate only ane attribute at time. In other words, nosotros demand several instruments to mensurate diverse aspect of visual perception.
Basically at that spot are 3 types of colorimetric instruments inward utilisation - colorimeter, spectrophotometer in addition to spectroradiometer. They are available inward the marketplace with varying degrees of sophistication in addition to specialisation. While the spectroradiometer measures inward illuminant-mode, the other 2 mostly mensurate inward object mode. The recent tendency on instrumental procedure command has resulted inward utilisation of on-line instruments. However bulk of the colorimetric instruments till appointment are off-line in addition to mostly used inward laboratories. Laboratory instruments should live highly accurate in addition to standardised, piece on-line instruments should live rugged nether diverse environments in addition to should have got proficient precision in addition to firmness.
The colorimeters mensurate color inward damage of the quantities of the 3 primaries required to fit the colour. On the other hand, spectrophotometer measures per centum reflectance or transmittance of the object plotted against wavelength at regular intervals of 1 nm, 5 nm, 10 nm, twenty nm throughout the visible arrive at of calorie-free i.e. 380-750 nm or for practical purposes 400-700 nm.
Colour past times Numbers!
The color of an object depends on the relative quantity of the calorie-free reflected at unlike wavelengths inside the visible arrive at (400-700 nm), but our color sensation is non analytical inward nature. We cannot jurist the existence of lights of unlike wavelengths individually. We larn the sensation from cumulative effect. As this cumulative quantity tin live matched past times mixing 3 primary lights it is proved that our eyes have got 3 types of colour-detecting cones, the stimuli generated past times them are mixed before reaching to brain. Various other phenomena of color have got also Pb to the conclusion that our eyes have got 3 types of cones only. Each object color is sensed past times each type of cone separately in addition to each type sends a stimulus to the brain.
So, for each object colour, the encephalon receives 3 separate stimuli. Keeping similarity with color mixing experiment, nosotros tin consider the 3 types of cones as red-sensitive r, green-sensitive g in addition to blue-sensitive b equivalent to the 3 additive primaries. The spectral sensitivity of the 3 colour-detecting cones has been measured in addition to named as color matching information r(λ), g(λ), b(λ) (bar stands for statistical average information of a issue of color normal observers) in addition to afterwards transformed into to a greater extent than usable CIE touchstone observer functions, x(λ), y(λ), z(λ). The expanse nether the functions signifies the amounts of 3 stimuli to live transmitted to the encephalon for the incidence of calorie-free having ane unit of measurement of unloosen energy at each of visible wavelength. These 3 stimuli are represented past times 3 numbers called CIE tristimulus values (X, Y, Z) which may live calculated as follows:
Where E(λ) is the relative spectral unloosen energy distribution of the illuminant, R(λ) is the spectral reflectance factor of the object in addition to x(λ), y(λ), z(λ) are the color matching functions of the CIE touchstone observer. K is a normalising constant.
A calorie-free source is an essential factor of visualisation in addition to measure of colour. Various calorie-free sources, such as daylight (D65), tungsten light (A), fluorescent light (F1 to F12), departmental light (TL84) etc., emit unlike amounts of unloosen energy inward the visible part of the spectrum that tin live expressed inward the shape of its relative spectral mightiness distribution (SPD) curve. An illuminant is an ideal shape of a calorie-free source with defined SPD. The SPD of calorie-free source may vary, but that of illuminant is constant or defined in addition to hence, it is used for quantification of color as mentioned above.
In the visual observing situation, the observer is the human optic that receives the calorie-free reflected from or transmitted through an object in addition to the encephalon which perceives the vision. Since unlike human perceive color inward unlike ways, subjectively, attempts have got been made to standardise the human observer as a numerical representation of what the average someone sees. This touchstone observer could so live used inward lieu of a human observer when assessments are made instrumentally. In 1931 CIE published the 2° CIE Standard Observer constituent based on color matching past times viewing through a hole of 2° champaign of view. Later it came to know that cones introduce inward a larger expanse of the eye. Hence, inward 1964, the 10° Standard Observer constituent was developed which is forthwith universally used.
When 2 objects have got equal tristimulus values nether a item illuminant, they volition await alike inward color nether the said illuminant. If their reflectance curves are same they volition await alike inward color nether whatever illuminant (universal match). Otherwise they may or may non differ inward color when the illuminant is changed. On the other hand, 2 alike colours with unlike lightness may have got unlike tristimulus values. To limited colours inward two-dimensional infinite independent of lightness, chromaticity coordinates may live calculated as follows: 
As x + y + z = 1, only 2 chromaticity coordinates x in addition to y have got been recommended past times CIE to specify chromaticity. Instead of tristimulus values (X, Y, Z), colours tin also live specified past times a luminance parameter Y in addition to 2 color coordinates x in addition to y (Yxy color space).
However, none of the chromaticity coordinates is correlated with whatever meaningful visual attribute of colour.
When the chromaticity coordinates of spectral colours are plotted inward such diagram, a horse-shoe shaped bend called chromaticity diagram is obtained. Chromaticity diagram is of cracking assist inward finding colours generated past times additive color mixing. If 2 lights are represented past times 2 points on the chromaticity diagram, whatever additive mixture of the 2 volition correspond to a dot on the straight delineate joining the 2 points. Since the locus of spectral colours is concave, all existent colours must autumn inside the expanse bounded past times the spectrum locus in addition to joining the ends.
Figure 6 shows CIE 1976 chromaticity diagram along with location of unlike spectral colours (i.e. monochromatic lights of unlike wavelengths inward nm). The figure also shows the regions of locations of different surface colours viewed nether daylight. Approximately inward the centre of the bend is the neutral point, which represents the chromaticities of white, gray or blacks. The illuminant C having chromaticity co-ordinates x = 0.310 in addition to y = 0.317 also lies at the centre of the bend (point C). The locations of other illuminants depend on their color temperatures. The chromaticity diagram is closed past times delineate indicating the locations of non-spectral royal colours.
CIE scheme is rattling successful for color specification in addition to is universally used for color measurement. The scheme is unchanged since 1931 except some fry modify inward 1964. CIE tristimulus values are related only to the colour. It ignores all other aspects similar surface texture, gloss etc. which influence color appearance significantly. It does non convey into delineate of piece of occupation concern human relationship geometrical arrangements for illumination in addition to viewing in addition to the instrumental measures volition fit visual assessments only if the to a higher seat geometries are similar.
The principal limitation of CIE scheme is its visual non-uniformity. Equal changes inward Yxy color infinite do non correspond to equal color perception. In other words, the distribution of colours inward CIE color infinite is non-uniform with regard to visual perception.
Colour Perception
The Committee on Colorimetry of the Optical Society of America inward 1922 defined color as the full general holler for all sensations arising from the activity of the retina of the optic in addition to its attached nervous mechanisms, this activity being, inward nearly every instance inward the normal individual, a specific response to radiant unloosen energy of sure wavelength in addition to intensity (OSA, 1953).
Colour is sometimes used as a holler for materials such as dyes in addition to pigments, but the holler Colorant volition live to a greater extent than appropriate inward these cases. The spectral composition of radiant unloosen energy in addition to the spectral transmittance or reflectance of substances is also called colour, but it is in ane lawsuit again an inappropriate holler for these purely physical concepts. Psychophysical concept of color as characteristics of calorie-free dependant on human vision is most appropriate.
Perception of color which is alone a prerogative of human beings is purely individualistic in addition to hence subjective. As a result, objective communication of a item color becomes difficult. For expressing color objectively ane must mensurate in addition to quantify it. The color tin live objectively expressed either past times creating calibrated coloured physical samples or past times an instrumental color measuring devise in addition to expressing it past times numbers.
Colour Mixing
According to ane reliable estimate, nosotros tin distinguish alongside x 1000000 unlike colours (Judd in addition to Wyszecki, 1975). Kuehni farther estimated that the humans with normal color vision tin distinguish alongside some 2 1000000 colours when viewed against a mid-grey background in addition to maybe double when the background is widely varied (Kuehni, 2005).
Newton explicitly stated that color is a perceptual property, non a physical attribute, which meant that the calorie-free mixtures occurred inward the eye, non inward the light. Newton recognised 3 aspects of color perception: brightness, hue in addition to intensity. Painters in addition to dyers already from before Newton’s fourth dimension believed inward 3 substitution chromatic colours that they sometimes equated with pigments or dyes: yellow, ruby-red in addition to blue, from which all other hues tin live created. With long sense with paints in addition to dyes they also affirmed that stuff color mixtures in addition to calorie-free mixtures behaved rattling differently.
Colour perception for humans is three-dimensional, a fact almost surely stems from the existence inward the retina of 3 unlike classes of light-receptive cells. LeBlon (1756) was get-go to brand clear distinction betwixt mixing pigment colours in addition to mixing colours of light. He stated that all visible objects tin live represented past times 3 colours, yellow, ruby-red in addition to bluish in addition to mixture of these 3 colours makes dark or all other colours. He named those as stuff colours or those used past times painters. He farther added that for a mixture of spectral colours those proposed past times Sir Isaac Newton cannot make black, but the rattling contrary, white.
Primary colours are sets of colours that tin live combined to brand a useful arrive at of colours. For human applications, 3 primary colours are usually used, since human color vision is trichromatic.
Primary colours are non a substitution holding of calorie-free but are oftentimes related to the physiological response of the optic to light. Fundamentally, calorie-free is a continuous spectrum of the wavelengths that tin live detected past times the human optic inward an infinite-dimensional stimulus space. However, the human optic usually contains only 3 types of color receptors, called cone cells. Each color receptor responds to unlike ranges of the color spectrum. Humans in addition to other species with 3 such types of color receptors are known as trichromats. These species response to the calorie-free stimulus via a three-dimensional sensation, which mostly tin live modelled as a mixture of 3 primary colours (Sobel, 1989).
Many birds in addition to marsupials are tetrachromats (Backhaus in addition to Werner, 1998), in addition to it has been suggested that some human females are tetrachromats as good (Mollon in addition to Jordan, 1993), having an extra variant version of the long-wave (L) cone type.
Most mammals other than primates have got only 2 types of color receptors in addition to are, therefore, dichromats; to them, at that spot are only 2 primary colours.
Any selection of primary colours is essentially arbitrary; for example, an early on color photographic process, autochrome, typically used orange, green, in addition to purple primaries. However, unless negative amounts of a color are allowed the gamut of color generated volition live restricted past times the selection of primaries (http://en.wikipedia.org/wiki/Primary_color, accessed on 16.7.15).
Fundamental laws of color mixing can, therefore, live classified into 2 types namely
- Additive color mixing (Figure 2 a) occurs when 2 or to a greater extent than lights mix together.
- Subtractive color mixing (Figure 2 b) occurs when colorants are mixed together.
 |
| Fig-2: Additive in addition to subtractive color mixing |
R + G = Y (Yellow),
G + B = C (Cyan or blue-green)
B + R = M (Magenta)
R + G + B = due west (white)
For subtractive combination of colours, as inward mixing of pigments or dyes, such as inward dyeing or printing, the primaries usually used are cyan (C), magenta (M), in addition to yellowish (Y), though the laid of red, yellow, bluish is pop alongside artists.
The subtractive primaries tin live obtained past times removing red, dark-green in addition to bluish from white calorie-free using respective coloured filters namely
W - R = C (- R)
W - G = M (- G),
W – B = Y (- B),
The mixing of subtractive primaries yellow, magenta in addition to cyan tin live predicted mathematically as follows:
Y + M = (W - B) + (W - G) = due west - B - G = R,
Y + C = (W - B) + (W - R) = due west - B – R = G
M+ C = (W - G) + (W - R) = due west - G - R = B,
Y + M + C = due west - R - G - B = K (Black)
When the 3 subtractive primaries are mixed inward proper proportions as inward instance of mixing colorants, the mixture is unable to reflect calorie-free anymore in addition to appear dark (K) inward colour. Similarly when white calorie-free is passed through filters of 3 subtractive colours, the whole of white calorie-free volition live removed. Mixture of 2 primaries is called secondary. The most commonly used additive color primaries are the secondary colours of the most commonly used subtractive color primaries, in addition to vice versa.
The age-old belief that 3 primary colours are required to brand white was also proved fake (Helmholtz, 1852). The thought of complimentary spectral colours came into existence in addition to was afterwards defined past times Grassman (1853) as follows:
To every color belongs some other homogeneous colour, which when mixed with it, gives colourless light.
Subsequently Helmholtz confirmed that every spectral color does non have got a spectral complement as proposed past times Grassman. Influenza A virus subtype H5N1 arrive at of dark-green colours ranging from close 484 nm to close 570 nm, has gratuitous colours inward the non-spectral royal range.
Colour Communication
In colorant production in addition to application industries, colours are to live communicated, compared, recorded in addition to formulated on regular basis. This necessitates systematic classification of colours. The objects tin live classified inward diverse ways inward damage of colour. The classification may live based on visually or instrumentally-assessed color parameters (Roy Choudhury, 2000) .
While communicating or talking close colour, a linguistic communication which is understandable past times both the parties must live followed. Influenza A virus subtype H5N1 logical scheme for ordering in addition to specifying colours on the ground of some clearly defined attributes is known as color notation system. The attributes are mostly 3 inward issue as our vision is trichromatic in addition to they constitute the coordinates of the resultant ‘colour space’. Colour notation systems also embrace ‘colour monastic enjoin systems’ which are typically comprised of stuff standards inward the shape of color atlas. Due to constraints of colorant gamut, the atlases may depict only a physically realisable subset of a color monastic enjoin system.
Colour notations tin live classified into 3 categories (Rhodes, 2002):
- Device dependant systems – the most mutual imaging devices used for reproducing color are the estimator controlled CRT displays in addition to the color printers. The associated color monastic enjoin scheme in addition to color spaces are hardware-oriented in addition to they lack perceptually based attributes.
- Mathematical systems – uniform color spaces based on mathematical transformation of CIE tristimulus values such as CIELUV in addition to CIELAB belong to this category.
- Systems based on database of aim points – color monastic enjoin systems existing principally inward physical form, the color samples of which tin live measured to institute a database of aim points. Using interpolation techniques alongside limited available samples, many to a greater extent than colours tin live defined.
Two most mutual additive systems used inward connector with estimator display are RGB (based on mixing of 3 additive primary color red, dark-green in addition to bluish produced past times the phosphors of the display unit of measurement inward cubic space) in addition to HSB (hue, saturation in addition to brightness inward cylindrical form).
For instrumental color measure of an object, additive primaries are used to predict the quantity of subtractive colours required to reproduce that colour. This entails the utilisation of touchstone source of light, viewing geometry in addition to a touchstone observer. The human face of color as numerals has been standardized past times Commission Internationale de l’Eclairage (CIE) laid upwards inward 1931 for standardisation of measure of colour. Thus the instrumental measure of color is helpful in, communication of color across the entire render chain as good as comparison samples in addition to setting pass-fail criterion for sourcing in addition to render of products. In this shape it is a useful tool for character control.
It is rattling convenient, easily understandable in addition to memorable when the colours are called past times names similar to mutual objects orange, ultramarine, olive, malachite green, bottle-green, peanut-green, sea-green etc. But such color names are rattling approximate, unreliable in addition to temporary. Their important also changes with observer, time, place, style, technology, language, civilization etc.
When nosotros bargain with a reasonable issue of specimen, say a few thousands, to cover the whole arrive at of possible colours (1millions or more), the specimen must live selected according to a scheme or plan. It is good known that the colours are three-dimensional. However, the dimensions of color are expressed inward diverse ways inward unlike fields. For systematic arrangements, the dimensions should live independent of each other.
Visual Description of Colour
Wright (1984) identified 2 sets of visual attributes namely:
- Group Influenza A virus subtype H5N1 attributes are lightness, hue in addition to chroma
- Group B attributes are whiteness, blackness in addition to chromaticness.
A color monastic enjoin scheme is a systematic in addition to rational method of arranging all possible colours or subsets past times agency of stuff samples. Once the colours are arranged systematically they are named inward some descriptive damage and/or are numbered (Graham, 1985).
The color monastic enjoin systems are of 3 types (Wyszecki, 1986):
- Colorant-mixture scheme based on subtractive mixture of colorants e.g. Pantone
- Colour-mixture scheme based on additive mixture of color stimuli e.g. Ostwald system.
- Colour appearance scheme based on the principles of color perception or color appearance e.g. Munsell.
Pantone Colour System (Figure 3) is basically a colourant mixture system. The Pantone scheme (www.pantone.co.uk) began life inward 1963 inward the USA, for defining colours for printers, but expanded into other fields later, e.g. textiles inward 1984, plastics inward 1993, in addition to architecture in addition to interiors (1925 colours) inward 2002, each of which has a 6-digit numerical notation (e.g. # 19-1764) in addition to an inspirational color name. This is widely used inward graphic fine art in addition to also inward textile manufacture mainly because of its depression cost, though the colours are non as spaced. The shades are prepared on newspaper using printing inks. It is non a color monastic enjoin scheme since it does non include a continuous scale. It is to a greater extent than appropriately considered a color naming system.
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| Fig-3: Pantone color guide |
The principal emphasis of appearance-based systems is the uniform visual spacing. The systems thus allow tardily interpolation betwixt the samples represented in addition to extrapolation of colours non illustrated inward a given collection. The collections of samples are mostly represented inward pages of constant hue.
Six pop color monastic enjoin systems, province of root in addition to their respective color attributes are as follows (Roy Choudhury, 2010) :
- Munsell (USA) – Hue, Value in addition to Chroma
- Natural Colour System (Sweden) – Hue, Blackness in addition to Chromaticness
- Ostwald scheme (Germany) – Hue, Lightness in addition to Saturation
- DIN scheme (Germany) – Hue, Saturation plane in addition to Darkness plane
- OSA-UCS (USA) - no separate scaling of 3 attributes
- Coloroid System (Hungary) – Hue, Saturation in addition to Lightness.
- Hue (H), measured along circumference of the horizontal circles
- Chroma (C) or purity of colour, measured radially outward from the neutral (grey) vertical axis
- Value (V), measured vertically from 0 (black) to 10 (white).
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| Fig-4: Munsell color system |
The Munsell atlas is usually available on painted newspaper inward glossy (1488 chips) in addition to matt forms (1277 chips). Influenza A virus subtype H5N1 method for specifying opaque object colours such as textiles, painted panel etc. past times Munsell color scheme has been described past times ASTM (1980).
SCOTDIC, a textile version of Munsell created past times fusion of 2 quite unlike systems - Standard Colour of Textile (Japan) in addition to Dictionnaire Internationale de la Couleur (France), is adopted past times over 8,000 companies worldwide. Textile touchstone colours of the SCOTDIC color scheme are widely used as color tools past times fashion color professionals. The scheme has 3 versions - glossy (2468 colours on polyester crepe fabric), matt (2038 colours on cotton wool poplin fabric) in addition to yarn (1100 colours on wool yarns). It has incorporated many brilliant colours in addition to the issue of constant hue nautical chart has been increased to 54 (20 for Wool). SCOTDIC scheme uses a vi digit code for each touchstone color – the get-go 2 digits for hue, the 2nd 2 digits for value in addition to the tertiary 2 digits for chroma. The prefix corresponds to the stuff of textile bases – P for Polyester, C for Cotton, due west for Wool. Therefore, the notation C-155010 agency cotton wool Standard sample having hue = 15, value = 50 in addition to chroma = 10 (Roy Choudhury, 2008).
Most of the stuff based atlases are forthwith available inward digitised shape e.g. NCS Digital Atlas (www.ncscolour.com), Digital Colour Atlas 3.0 (www. dtpstudio.de) etc.
Figure 5 shows 2 pages from the Munsell Book of Colours. Figure 5 (a) shows hue circle having samples of 100 hues inward 2 depths (moderate in addition to night shades) (samples of some hues are missing). Figure 5 (b) shows constant hue page of hue = 05. In these pages, rows stand upwards for constant values samples, piece columns stand upwards for constant chroma samples.
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| Fig-5: SCOTDIC mass of color (a) Hue circle (b) Constant Hue page (Hue = 05) |
Newton said that (light) rays are non coloured, but but has the mightiness to imitate sure sensations inward the heed of the observer. Human optic is a highly versatile detector of calorie-free in addition to colour. An observer tin perceive chromatic attributes in addition to diverse geometric factors (direction, texture, shape in addition to many others) simultaneously. An musical instrument till appointment is far behind inward versatility. It tin mensurate only ane attribute at time. In other words, nosotros demand several instruments to mensurate diverse aspect of visual perception.
Basically at that spot are 3 types of colorimetric instruments inward utilisation - colorimeter, spectrophotometer in addition to spectroradiometer. They are available inward the marketplace with varying degrees of sophistication in addition to specialisation. While the spectroradiometer measures inward illuminant-mode, the other 2 mostly mensurate inward object mode. The recent tendency on instrumental procedure command has resulted inward utilisation of on-line instruments. However bulk of the colorimetric instruments till appointment are off-line in addition to mostly used inward laboratories. Laboratory instruments should live highly accurate in addition to standardised, piece on-line instruments should live rugged nether diverse environments in addition to should have got proficient precision in addition to firmness.
The colorimeters mensurate color inward damage of the quantities of the 3 primaries required to fit the colour. On the other hand, spectrophotometer measures per centum reflectance or transmittance of the object plotted against wavelength at regular intervals of 1 nm, 5 nm, 10 nm, twenty nm throughout the visible arrive at of calorie-free i.e. 380-750 nm or for practical purposes 400-700 nm.
Colour past times Numbers!
The color of an object depends on the relative quantity of the calorie-free reflected at unlike wavelengths inside the visible arrive at (400-700 nm), but our color sensation is non analytical inward nature. We cannot jurist the existence of lights of unlike wavelengths individually. We larn the sensation from cumulative effect. As this cumulative quantity tin live matched past times mixing 3 primary lights it is proved that our eyes have got 3 types of colour-detecting cones, the stimuli generated past times them are mixed before reaching to brain. Various other phenomena of color have got also Pb to the conclusion that our eyes have got 3 types of cones only. Each object color is sensed past times each type of cone separately in addition to each type sends a stimulus to the brain.
So, for each object colour, the encephalon receives 3 separate stimuli. Keeping similarity with color mixing experiment, nosotros tin consider the 3 types of cones as red-sensitive r, green-sensitive g in addition to blue-sensitive b equivalent to the 3 additive primaries. The spectral sensitivity of the 3 colour-detecting cones has been measured in addition to named as color matching information r(λ), g(λ), b(λ) (bar stands for statistical average information of a issue of color normal observers) in addition to afterwards transformed into to a greater extent than usable CIE touchstone observer functions, x(λ), y(λ), z(λ). The expanse nether the functions signifies the amounts of 3 stimuli to live transmitted to the encephalon for the incidence of calorie-free having ane unit of measurement of unloosen energy at each of visible wavelength. These 3 stimuli are represented past times 3 numbers called CIE tristimulus values (X, Y, Z) which may live calculated as follows:
Where E(λ) is the relative spectral unloosen energy distribution of the illuminant, R(λ) is the spectral reflectance factor of the object in addition to x(λ), y(λ), z(λ) are the color matching functions of the CIE touchstone observer. K is a normalising constant.
A calorie-free source is an essential factor of visualisation in addition to measure of colour. Various calorie-free sources, such as daylight (D65), tungsten light (A), fluorescent light (F1 to F12), departmental light (TL84) etc., emit unlike amounts of unloosen energy inward the visible part of the spectrum that tin live expressed inward the shape of its relative spectral mightiness distribution (SPD) curve. An illuminant is an ideal shape of a calorie-free source with defined SPD. The SPD of calorie-free source may vary, but that of illuminant is constant or defined in addition to hence, it is used for quantification of color as mentioned above.
In the visual observing situation, the observer is the human optic that receives the calorie-free reflected from or transmitted through an object in addition to the encephalon which perceives the vision. Since unlike human perceive color inward unlike ways, subjectively, attempts have got been made to standardise the human observer as a numerical representation of what the average someone sees. This touchstone observer could so live used inward lieu of a human observer when assessments are made instrumentally. In 1931 CIE published the 2° CIE Standard Observer constituent based on color matching past times viewing through a hole of 2° champaign of view. Later it came to know that cones introduce inward a larger expanse of the eye. Hence, inward 1964, the 10° Standard Observer constituent was developed which is forthwith universally used.
When 2 objects have got equal tristimulus values nether a item illuminant, they volition await alike inward color nether the said illuminant. If their reflectance curves are same they volition await alike inward color nether whatever illuminant (universal match). Otherwise they may or may non differ inward color when the illuminant is changed. On the other hand, 2 alike colours with unlike lightness may have got unlike tristimulus values. To limited colours inward two-dimensional infinite independent of lightness, chromaticity coordinates may live calculated as follows:
However, none of the chromaticity coordinates is correlated with whatever meaningful visual attribute of colour.
When the chromaticity coordinates of spectral colours are plotted inward such diagram, a horse-shoe shaped bend called chromaticity diagram is obtained. Chromaticity diagram is of cracking assist inward finding colours generated past times additive color mixing. If 2 lights are represented past times 2 points on the chromaticity diagram, whatever additive mixture of the 2 volition correspond to a dot on the straight delineate joining the 2 points. Since the locus of spectral colours is concave, all existent colours must autumn inside the expanse bounded past times the spectrum locus in addition to joining the ends.
Figure 6 shows CIE 1976 chromaticity diagram along with location of unlike spectral colours (i.e. monochromatic lights of unlike wavelengths inward nm). The figure also shows the regions of locations of different surface colours viewed nether daylight. Approximately inward the centre of the bend is the neutral point, which represents the chromaticities of white, gray or blacks. The illuminant C having chromaticity co-ordinates x = 0.310 in addition to y = 0.317 also lies at the centre of the bend (point C). The locations of other illuminants depend on their color temperatures. The chromaticity diagram is closed past times delineate indicating the locations of non-spectral royal colours.
 |
| Fig-6: Location of Spectral Colors inward 1976 CIE Chromaticity Diagram |
The principal limitation of CIE scheme is its visual non-uniformity. Equal changes inward Yxy color infinite do non correspond to equal color perception. In other words, the distribution of colours inward CIE color infinite is non-uniform with regard to visual perception.
Uniform Colour Scales
CIE tristimulus values or chromaticity coordinates are non rattling convenient for identifying color of the objects, because these were designed for color stimuli of unlike modes. None of the values are straight correlated with whatever visual attributes of colour. Only Y value has high correlation with luminance in addition to object lightness. The spacing of colours inward chromaticity diagram is non visible uniform. Influenza A virus subtype H5N1 issue of uniform color scales are, therefore, developed which tin stand upwards for colours with equal visual spacing in addition to are straight related to meaningful attributes of color appearance.
In 1976 CIE recommended CIELUV in addition to CIELAB uniform color spaces. Colorant industries were inward favour of a formula similar to Adam-Nickerson (AN40) formula, pop at that time. The CIELAB formula was acceptable as colour-difference values were close 1.1 times those produced past times AN40 formula. On the other hand, television set industries preferred a color infinite (CIELUV) associated with a chromaticity diagram because of its uncomplicated way of presentation of additive mixture which also occur inward television in addition to other display devices. No uncomplicated relation exists betwixt the 2 color scales.
Both CIELUV in addition to CIELAB formulae are plotted on rectangular coordinates. Lightness L* constituent is same for both color spaces in addition to is represented past times the formula,
Subscript n stand upwards for nominally white object color stimulus given past times a perfect reflecting diffuser as reference surface illuminated past times touchstone illuminant. For touchstone daylight illuminant D65, the values are: Xn = 95.047, Yn = 100.000, Zn =108.883. White object has been taken into delineate of piece of occupation concern human relationship because nosotros perceive colours inward relation to surrounding colours.
In recent years efforts have got been made to define CIE correlates for perceptual attributes similar lightness, chroma in addition to hue. Hence, 2 novel attributes corresponding to visual attributes have got been derived from a* in addition to b* values namely:
CIELAB color infinite is shown inward the Figure 7. Lightness L* is represented inward vertical axis with white (L* = 100) at the top in addition to dark (L* = 0) at the bottom. Chromatic colours are represented past times 2 opponent a* in addition to b* axes. Red in addition to dark-green are represented past times a* axis – the positive values are for ruby-red in addition to negative for green. Similarly positive b* values are for yellowish in addition to negative b* values are blue.
Colour Difference
Measurement of departure inward color betwixt 2 objects is ane of the most complicated aspects of color vision. The color discrimination may live general/overall or of specific psychophysical attribute similar hue, chroma or lightness. For colorant users similar textile, leather, newspaper or pigment industries, the departure inward color of 2 specimens namely a touchstone in addition to a sample or of unlike portions of a coloured specimen may live to a greater extent than of import than the measure of absolute color (Luo, 1986). The prime number difficulty is that the perception of colour-difference past times an private is non a precise phenomenon in addition to may vary on successive assessment (Zeller in addition to Hemmendinger, 1978). Colour-difference perception in addition to evaluation may also vary widely alongside individuals.
The colour-difference evaluation is necessary for solar daytime to solar daytime color command in addition to for color matching inward colouration industries similar textile, pigment etc. Colour-difference formulae have got accelerated the instrumental pass-fail device a success.
The color differences are calculated past times subtracting values of the touchstone from the respective values of the sample. The full colour-difference (ΔE) is intended to live unmarried issue metric for pass/fail decisions in addition to inward the CIELAB scheme ΔE is given past times the next equation:
ΔE = [ (ΔL*)² + (Δa*)² + (Δb*)² ]1/2
In improver to the overall color departure (ΔE), the departure inward private parameters of the touchstone in addition to a sample are also estimated e.g. ΔL = L (sample) - L (standard).
These may holler for some specific visual departure such as
if ΔL < 0 or > 0, the sample is darker or lighter respectively,
if Δa* < 0 or > 0, the sample is greener or redder respectively,
if Δb* < 0 or > 0, the sample is bluer or yellower respectively.
ΔE (CIELAB) values are non ever reliable inward predicting perceptible differences betwixt object colours, peculiarly when the variations are inward unlike visual attributes. This is due to the fact that the visual spacing along L, a* in addition to b* axes are unequal.
The formulae based on surface-mode color discrimination information mainly aimed at unmarried issue shade-passing. Much of the available visual information related to physical samples are supplied past times the textile in addition to dye industries, where prime number criteria is that whether the colours volition live acceptable against the respective standards.
Main argue for pitiable correlation with visual information of the before formulae was equal weighting of the color parameters. The weighted values of lightness, chroma in addition to hue showed important improvement inward functioning of colour-difference equations. The weights tin live determined past times empirical plumbing equipment to experimental data-sets. These formulae are optimized past times visual acceptability/ perceptibility scaling. They stand upwards for most closely the average visual results of judgments of color departure of textile in addition to other physical samples nether normal evaluation weather (Kuehni, 1984)
Colorimetry is the scientific discipline of quantitative measure of colour. Even though report on color scientific discipline started as far dorsum as the Newtonian age, query continues fifty-fifty today. Colour Science is a vast field. Hunt (1977) identified 3 phases of evolution of colorimetry - color matching, color departure evaluation in addition to lastly, prediction of color appearance. It is forthwith possible to predict color appearance of an object nether a seek out illuminant from the color appearance information nether a reference illuminant with the assist of complex mathematical transformations.
References:
CIE tristimulus values or chromaticity coordinates are non rattling convenient for identifying color of the objects, because these were designed for color stimuli of unlike modes. None of the values are straight correlated with whatever visual attributes of colour. Only Y value has high correlation with luminance in addition to object lightness. The spacing of colours inward chromaticity diagram is non visible uniform. Influenza A virus subtype H5N1 issue of uniform color scales are, therefore, developed which tin stand upwards for colours with equal visual spacing in addition to are straight related to meaningful attributes of color appearance.
In 1976 CIE recommended CIELUV in addition to CIELAB uniform color spaces. Colorant industries were inward favour of a formula similar to Adam-Nickerson (AN40) formula, pop at that time. The CIELAB formula was acceptable as colour-difference values were close 1.1 times those produced past times AN40 formula. On the other hand, television set industries preferred a color infinite (CIELUV) associated with a chromaticity diagram because of its uncomplicated way of presentation of additive mixture which also occur inward television in addition to other display devices. No uncomplicated relation exists betwixt the 2 color scales.
Both CIELUV in addition to CIELAB formulae are plotted on rectangular coordinates. Lightness L* constituent is same for both color spaces in addition to is represented past times the formula,
L* = 116(Y/Yn)1/3 - 16 if Y/Yn = 0.008856
For CIELAB Colour Space, Red-green attribute, a* = 500[f(X/Xn) - f(Y/Yn)]
Yellow-blue attribute, b* = 200[f(Y/Yn) - f(Z/Zn)]
Subscript n stand upwards for nominally white object color stimulus given past times a perfect reflecting diffuser as reference surface illuminated past times touchstone illuminant. For touchstone daylight illuminant D65, the values are: Xn = 95.047, Yn = 100.000, Zn =108.883. White object has been taken into delineate of piece of occupation concern human relationship because nosotros perceive colours inward relation to surrounding colours.
In recent years efforts have got been made to define CIE correlates for perceptual attributes similar lightness, chroma in addition to hue. Hence, 2 novel attributes corresponding to visual attributes have got been derived from a* in addition to b* values namely:
Metric chroma, C*ab = [(a*)2 + (b*)2] 1/2
Hue angle, h = tan -1 (b*/a*).
CIELAB color infinite is shown inward the Figure 7. Lightness L* is represented inward vertical axis with white (L* = 100) at the top in addition to dark (L* = 0) at the bottom. Chromatic colours are represented past times 2 opponent a* in addition to b* axes. Red in addition to dark-green are represented past times a* axis – the positive values are for ruby-red in addition to negative for green. Similarly positive b* values are for yellowish in addition to negative b* values are blue.
 |
| Fig-7: CIELAB color space |
Measurement of departure inward color betwixt 2 objects is ane of the most complicated aspects of color vision. The color discrimination may live general/overall or of specific psychophysical attribute similar hue, chroma or lightness. For colorant users similar textile, leather, newspaper or pigment industries, the departure inward color of 2 specimens namely a touchstone in addition to a sample or of unlike portions of a coloured specimen may live to a greater extent than of import than the measure of absolute color (Luo, 1986). The prime number difficulty is that the perception of colour-difference past times an private is non a precise phenomenon in addition to may vary on successive assessment (Zeller in addition to Hemmendinger, 1978). Colour-difference perception in addition to evaluation may also vary widely alongside individuals.
The colour-difference evaluation is necessary for solar daytime to solar daytime color command in addition to for color matching inward colouration industries similar textile, pigment etc. Colour-difference formulae have got accelerated the instrumental pass-fail device a success.
The color differences are calculated past times subtracting values of the touchstone from the respective values of the sample. The full colour-difference (ΔE) is intended to live unmarried issue metric for pass/fail decisions in addition to inward the CIELAB scheme ΔE is given past times the next equation:
ΔE = [ (ΔL*)² + (Δa*)² + (Δb*)² ]1/2
In improver to the overall color departure (ΔE), the departure inward private parameters of the touchstone in addition to a sample are also estimated e.g. ΔL = L (sample) - L (standard).
These may holler for some specific visual departure such as
if ΔL < 0 or > 0, the sample is darker or lighter respectively,
if Δa* < 0 or > 0, the sample is greener or redder respectively,
if Δb* < 0 or > 0, the sample is bluer or yellower respectively.
ΔE (CIELAB) values are non ever reliable inward predicting perceptible differences betwixt object colours, peculiarly when the variations are inward unlike visual attributes. This is due to the fact that the visual spacing along L, a* in addition to b* axes are unequal.
The formulae based on surface-mode color discrimination information mainly aimed at unmarried issue shade-passing. Much of the available visual information related to physical samples are supplied past times the textile in addition to dye industries, where prime number criteria is that whether the colours volition live acceptable against the respective standards.
Main argue for pitiable correlation with visual information of the before formulae was equal weighting of the color parameters. The weighted values of lightness, chroma in addition to hue showed important improvement inward functioning of colour-difference equations. The weights tin live determined past times empirical plumbing equipment to experimental data-sets. These formulae are optimized past times visual acceptability/ perceptibility scaling. They stand upwards for most closely the average visual results of judgments of color departure of textile in addition to other physical samples nether normal evaluation weather (Kuehni, 1984)
A few colour-difference formulae based on surface-mode color discrimination information are:
- JPC79 colour-difference Formula
- CMC (l:c) colour-difference Formula
- BFD (l:c) colour-difference Formula
- CIE 94 colour-difference Formula
- CIE 2000 colour-difference Formula
Colorimetry is the scientific discipline of quantitative measure of colour. Even though report on color scientific discipline started as far dorsum as the Newtonian age, query continues fifty-fifty today. Colour Science is a vast field. Hunt (1977) identified 3 phases of evolution of colorimetry - color matching, color departure evaluation in addition to lastly, prediction of color appearance. It is forthwith possible to predict color appearance of an object nether a seek out illuminant from the color appearance information nether a reference illuminant with the assist of complex mathematical transformations.
References:
- Judd D.B. in addition to Wyszecki G. (1975). Color inward business, scientific discipline in addition to industry, 3rd Ed., John Wiley & sons, New York, p388.
- Kuehni R. G. (1984). Colour Technology inward the Textile Industry, AATCC, U.S.A., pp.123.
- Kuehni R. G. (2005), Colour: an introduction to practise in addition to principles, New Jersey, Wiley-Interscience.
- Leblon C. Jacob (1756). Coloritto or the harmony of colouring inward painting, English linguistic communication in addition to French, Edition reprinted inward Paris.
- Luo M.R. (1986). New Colour Difference Formula for Surface Colours, Ph.D Thesis, University of Bradford, Bradford, UK
- Jordan G. in addition to Mollon J. D. (1993). Influenza A virus subtype H5N1 Study of Women Heterozygous for Colour Deficiencies, Vision Research Vol. 33, No. I I, pp. 1495-1508.
- Optical gild of America (OSA) (1953). The scientific discipline of color, Committee on Colorimetry, Thomas Y. Cromwell, New York.
- Rhodes P Influenza A virus subtype H5N1 (2002), Colour notation systems. In Colour Engineering, P Green in addition to L MacDonald (ed.), pp 307-331, John Wiley, Chichester, England.
- Roy Choudhury A.K. (2000). Modern concepts of color in addition to appearance, Science publishers (USA) in addition to Oxford & IBH, New Delhi,.
- Roy Choudhury A.K. (2008), ‘Colorimetric Study of SCOTDIC Colour Specifier’, Color. Technology, 124, 273–284.
- Roy Choudhury A.K. (2010). Chapter 2. Scales for communicating color inward Colour Measurement: Principles, advances in addition to industrial applications. Edited past times M L Gulrajani, Woodhead Publishing Series inward Textiles No. 103, ISBN 1 84569 559 3.
- Sobel Michael I. (1989). Light, University of Chicago Press. pp. 52–62. ISBN 0226767515.
- Wright due west D (1984), ‘The basic concepts in addition to attributes of color monastic enjoin systems’, Col. Res. Appl., 9, 229-233.
- Wyszecki G (1986), Colour Appearance, Chapter ix inward Handbook of Perception in addition to Human Performance, New York, Wiley.
- Zeller R.C. in addition to Hemmendinger H. (1978). Evaluation of Color-difference equations: a novel approach, AIC Color 77, Bristol: Adam Hilger.