Colorimetry - Pigmentation of the skinThere are many pigments that have an effect on the colour of the skin. Of particular importance in human skin is the quantity of melanin it contains. The amount of melanin in the skin is determined by genetic factors but it is also increased by the impact of the sun’s radiation. There are two variant forms of melanin: eumelanin (brown to black) and pheomelanin (red to yellow).
The second factor that affects skin colour, in particular in the case of light-coloured skin, is the amount of red tone, determined by the blood vessels that lie beneath the skin. These can become subject to short-term constriction or dilation, resulting in the skin appearing paler or redder.
Measuring colour; quantitative determination of colour changes
The L*, a*, b* colour space was developed as a method of expressing colours in terms of numbers. The colour space corresponds with the perceived characterisation of colour. L* stands for lightness (100 corresponds with white and 0 with black), a* denotes red-green coloration and b* denotes blue-yellow coloration. By determining the L* and b* values of a colour, it is possible to calculate the so-called ITA° (individual topology angle). The ITA° was developed as a system for determining overall pigmentation or skin colour. The lighter the skin, the higher the ITA°.
|55° < ITA°||Very light|
|41° < ITA° < 55°||Light|
|28° < ITA° < 41°||Intermediate|
|10° < ITA° < 28°||Tanned|
|-30° < ITA° < 10°||Brown|
|TA° < -30°||Black|
Increased browning of the skin is indicated by a rising b* value and a falling L* value and ITA°. Conversely, a lightening of the skin is indicated by a falling b* value and rising L* value and ITA°.
If the skin is irritated by a product, the degree of reddening increases and the a* value rises. If the product is designed to have an alleviating effect, a decrease in skin reddening and in turn a* value is expected. On the other hand, an increase in the b* value indicates a browning effect, accompanied by a lowering in the lightness value, L*.
To minimise the effect of colour deviations in the measuring field, all measurements are taken three times, and the mean values calculated and recorded by the measuring equipment.