Measuring instruments for determining skin color have a variety of classifications. Telespectroradiometers (TSRs) and spectrophotometers (SPs) are two classifications for instruments that are widely used for measuring skin color. For example, in the cosmetic industry, TSRs are used for measuring color appearance for developing skin color charts and evaluating skin products. In contrast, SPs are mainly used for diagnosing skin disease symptom, such as erythema (superficial skin reddening) and irritation.
Instruments for measuring skin color (pigmentation and erythema) are designed based on two main principles: reflectance spectrophotometry and tristimulus colorimetry. With both techniques, light reflectance data from the skin are converted into indices or colorimetric values for the estimation of chromophores (including melanin) in the skin.
Spectrophotometers measure the spectral reflectance or transmittance of an object across the full spectrum of visible wavelengths, from 400 mm to 700 mm, and are suitable for more complex color analysis.
Tristimulus colorimeters have three color filters red, green and blue (RGB). The instrument’s light source directs a beam of light to an object , which then bounces back, passing through the colored filters.
Measurement techniques can either be contact or non-contact. For example, the Konica-Minolta CM700d is a contact spectrophotometer and Photo Research’s SpectraScan Colourimeter PR650 is a non-contact telespectroradiometer. Other instruments provide insight into the different implementations of the classifications.
Rather than the full light spectrum, colorimeters, like the Nix Mini, focus on certain wavelengths, making them smaller and less expensive than spectrophotometers.
Cortex Technology ApS Colorimeter DSM-4 uses advanced software for automatic classification of skin phototype (ITA) in 6 groups: very light, light, intermediate, tan, brown, and dark. It provides a portable colorimeter for accurate 4-in-1 measurements of color, pigmentation, erythema and gloss.
The Mexameter MX18 from Courage + Khazaka electronic GmbH is a portable narrow-band simple reflectance meter with a small and lightweight probe that provides 1 second measurement results for melanin index and erythema index.
Delfin Technologies’ SkinColorCatch® is a portable, battery-operated all-in-one colorimeter. Using direct skin contact, it calculates ITA degree automatically and provides skin color L*a*b, L*c*h, RGB, melanin and erythema indices outputs.
White LEDs arranged circularly inside the measurement chamber of the SkinColorCatch illuminate the skin in a 45-degree angle to minimize gloss. The light reflected from the skin is detected with an RGB color sensor. Due to the direct skin contact, measurements are not affected by ambient lighting conditions.
The Search for Consistent Results
In 2022, the Google Research Center for Responsible AI and Human-Centered Technology partnered with a Harvard professor to address bias and diversity issues in the company’s products. Ellis Monk, a professor at Harvard University, created the Monk Skin Tone or MST Scale. Designed to be more representative and inclusive of a wider range of different, especially darker, skin tones, the MST Scale is 10-point skin tone scale. It provides more details than the narrower six categories of the Fitzpatrick scale and avoids the large number of choices that other skin tone scales offer that can lead to inconsistent results.
Google’s skintone.google website is dedicated to explaining the research and best practices for using the MST scale in artificial intelligence (AI) analysis.