Colour is a hugely informative aspect of skin – it tells us about irritation (looking at redness), presence of dirt or makeup and how well your cleanser works (colours from things other than the skin), and gives us information about tanning both UV induced and from sunless tanners. However its measurement is a complex subject – the colour of something depends on a number of things including what light it is being observed under. You’ll probably have noticed how early morning and evening light is often very flattering when taking photographs – it adds a certain quality of light to a picture. What’s going on here? In the morning and evening, the sunlight has to pass through more of the atmosphere, as the sun is lower to the horizon. The short wavelengths of light (the blue end of the spectrum) are scattered off dust in the atmosphere, and the resultant light is ‘warmer’ in colour, i.e. it is more red and orange. Another example is the difference between tungsten light (older style light bulbs), fluorescent strips lights, and daylight. The differences here are so striking that when film was being used to capture photographs different films were used under the different lighting conditions to correct for these different lighting conditions.
This is where it gets interesting. While it is possible to observe this when you are actually there the effects can be quite subtle, and it becomes more obvious when you look at a photograph in isolation from the situation. The brain has a certain degree of inbuilt compensation when looking at colours, so that irrespective of the lighting it can make sense of the world. For instance, grass is green, clear skies are blue, sunflowers are yellow etc, so when you are immersed in the environment the brain is experiencing the light directly and can perform to basic correction to make things look how they should do. However when you look at a photograph you are separate from it, and as such it is harder to make that correction, so often the colours look more exaggerated. It also helps explain why it is very difficult to objectively assess colour by eye, especially over a long period of time and under different conditions.
Given the colour that something appears can be changed depending on the nature of the light hitting it when you want to accurately determine changes in colour it is necessary to standardise the lighting conditions being used to make the observation. Not only that, but the angle which the light hits the subject, and the relationship between the person observing and the light source and the subject also has an impact, as such needs to be standardised. In skin assessment it is common to use lighting known at D65. This refers to Daylight (the D) at a colour temperature of 6500K (colour temperature refers to how hot something would have to be to glow at that colour). Basically this means that it mimics the type of daylight experienced at noon in the northern hemisphere.
When it comes to measuring the colour of skin, typically something like a colourimeter or chromameter is used. This applies standardised light to the skin, and measures the reflected light at a standard angle (typically 2 or 10 degrees). This standardisation allows changes in skin over time to be looked at, and even some degree of comparison between different studies. Although it should be stressed that no two devices are never precisely alike and it is difficult to draw many meaningful conclusions looking at studies performed with different devices. It is therefore common practice to use the same device throughout a given study.
These devices can be used to measure the skin colour directly (for instance when looking at irritation induced by washing with harsh soaps and surfactants), and also for measuring the colour of things which are present on the skin (such as make up or dirt) and how efficiently cleanser can remove these. To do this the colour measured is broken down into a set of numbers and parameters which can be used to describe the colour. There are a number of scales which have been defined to describe colour, however one which is typically used is the L*a*b* scale (pronounced L star, a star, b star). This breaks down the colour into three distinct groups – L* is a measure of lightness (white to black), a* a measure of redness and greenness (as a becomes more positive the skin is redder, and as it becomes more negative it becomes greener), b* is a measure of the yellow and blue components of the skin. The a* reading is very important being a direct measure of redness, it can split out the redness from all the other colours so this is very useful for irritation assessments. If a* is reduced after a given treatment then the skin would be less red. Conversely if a* has increased then it has become more red.
There is a term known as ΔE (pronounced delta E) which is an absolute measure of difference in colour between two samples. This takes into account changes can be used to measure the difference between a treated and untreated skin sample, for instance when looking at how well a cleanser has removed makeup. With this you would measure the colour of the skin before the makeup is applied, apply to makeup in a standardised way and the then remove it with the cleanser and measure the colour again. The ΔE between the skin at the start and after cleansing is a measure of how much difference in colour there is – the higher the ΔE, the greater the difference in colour from the untreated skin. Although ΔE gives you a difference in colour is does not tell you what the colour is – it cannot for example tell you whether the skin is more blue or more red. For that type of information you would need to look at the changes in L*, a* and b*.
There are watchouts when measuring skin colour. If the device is pressed against this skin too hard during the measurements it can blanch the skin (make it look whiter) or even make it redder. Training the operator and using the same person to take the measurements throughout the study is the best way to deal with this. Also as the measurement is done over a certain area (depends on the device but typically about 1cm diameter circle) anything in that area will have an impact on the colour measured. So it is important not to do the measurement over moles, scars or tattoos as these will impact the average measured colour. As with all skin measures, the panelist need to be acclimatised in the room where the measurements are done so that they become used to the temperature. Oh, and very important, try not to do your measurements in a room at the top of 3 flights of stairs, peoples skin tends to be slightly redder when they have walked up there.