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Human Skin in Colour Space Value
Key words: Colour space, CIE L*a*b*, RGB, Hue, Human skin
The phenomenon of colour has been intriguing scientists and philosophers for centuries. Various theories have been proposed about the nature of colours . Sir Isaac Newton who was a pioneer in this field first proposed the Colour circle made up of seven primary colours. The choice of seven colours should be seen here for convenience, rather than a precise description of the visible spectrum. Millions of colour tones originate from the narrow visible part of the ElectroMagnetic wave spectrum. The dimensions of colour appearance are brightness ,luminance, lightness, colourfulness, chroma, saturation, and hue.
Colour of the skin is one of the main phenotype traits of humans and hence is one of the most thoroughly discussed topic in various fields. Even within a particular subpopulation there are significant differences in the tones of skin colour . Skin colour of an individual is decided by the amount of melanin, oxyhemoglobin and carotenoids present in the skin. Light reflectance, refraction within the skin layers also plays a role in deciding the skin colour. Subjective description of colours and various colour palettes are incomplete. So there is a need for objective assessment of skin colour which may be useful for diagnosis and treatment purposes.
Since colour is not a physical phenomenon like light it cannot be expressed in any units. Colour circle of Newton is two dimensional and could not account for all the shades of colour. Colour space is an imaginary gamut usually three dimensional in which the colours are represented in different axes. The landmark CIE was proposed in 1931 and had undergone many modifications the popular one is the CIE L*a*b* as far as medical research is concerned. The latest modification of CIE is the CIECAM02 . L*a*b* space consists of two axes a* and b* which are at right angles and represent the hue . The third axis is the lightness L*. It is perpendicular to the a*b* plane. Within this system, any colour can be specified with the coordinates L*, a*, b*. This colour space with modifications like L*u*v* , LCH covers the colours perceived by our visual system.
The need for expressing the skin tone in absolute values was felt with advances in colour science. So skin colour can be expressed in numerical figures with the help of Colour Spaces. These values will also be useful in comparing two colour tones for research, industrial, cosmetic uses. Difference in colour values can be expressed in terms of CIE deltaE*ab . Usually colour communication is done in terms of differences rather than absolute values.The total change of colour, âˆ†E*, is commonly used to represent a colour difference. So only when the CIE âˆ†E* exceeds a minimal value it will be perceived as a difference by the human eye. Other models of colour spaces are RGB, HSV, CMYK etc. There are some advantages and drawbacks in each of these colour spaces.
Transformation of one colour space unit to another can be done 1 . Images which were recorded in an unknown colour space can be transformed to a known or standard colour space like sRGB or CIE L*a*b*. These type of standardized images can be passed via electronic media to the other observer and calibrated without any change in colour data.
Colour space values can be extracted from the skin by specialized instruments like Spectrophotometers, Tristimulus colorimeters which displays the value in the mode we desire. Cheaper ways to extract the colour space values by digital cameras are also available.2 This expression of colour in numerical values has many applications in Medicine. Colour space information can be used to estimate Erythema Index, Melanin Index 3.The Melanin Index is an excellent indicator for pigmentation whereas the Erythema index indicates the state of erythema of the skin. Determination of Minimal Erythema Dose (MED) of Ultravoilet B radiation is important in the treatment protocol of Psoriasis and Vitiligo. The L* value and MED value were shown to be inversely correlated, and as the L* value was decreased, the MED was increased.4 In calculation of Psoriasis Area and Severity Index before and after treatment CIE L*a*b* can be used . The relationship of Sun Protection Factor (SPF) to MED was assessed in Japanese patients. 5
Colour spaces can be used to compare the change in colour over a period of time like in injuries, ulcers etc 6 .It can be used to estimate the effect of certain cosmetics7 ,drugs 8 , to predict the risk of skin cancers9 to help in diagnosis of melanomas10 .For anthropological11 and epidemiological studies colour space may be useful 12 .In Postmortem state estimation of the colour was to done to differentiate carbon monoxide poisoning from cold exposition induced pink discoloration13 .
Skin colour was measured in sun exposed and covered areas of Chinese women in CIE colour space.14 . Facial attributes - wrinkles, skin gravity effects, feature lines, and skin optical appearance determine aged appearance . The colorimetric and optical diffusion properties of aged skin have been compared in LCH space15 .
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