The Role of Subtractive Color Mixing in the Perception of Blue Nevi and Veins-Beyond the Tyndall Effect

Letters the same authors. Telephone consultation with the Western Institutional Review Board (WIRB), Puyallup, Washington, de- termined this research to be exempt and institutional review board review was waived. RESEARCH LETTER The Role of Subtractive Color Mixing in the Perception of Blue Nevi and Veins—Beyond the Tyndall Effect The original study 1 that proposed the “Tyndall effect” as the explanation behind the coloration of blue nevi drew from spec- trophotometric data on cadaveric skin and not the direct study of blue nevi. Since then, the moniker of the Tyndall effect has been applied to a variety of blue phenomena in the skin de- spite a lack of confirmatory data. We hypothesized that other light-skin optical characteristics may provide a better expla- nation for the visual phenomenon associated with why blue nevi appear blue. To briefly review the optics of the skin, perceived color is produced by light that strikes the skin and is remitted (a com- bination of light reflected and scattered back to the eye). The epidermis plays a minimal role in scattering, responsible pri- marily for the baseline reflectance of 5% to 7% of light from the skin surface. 2 The Tyndall effect originally described the preferential scattering of shorter wavelength blue light through particulate matter in air and fluids, and its application to solid tissue optics is based on extrapolative hypotheses. Accord- ingly, such scattering would occur in a homogenous manner diffusely throughout the skin, not only in blue nevi, which led us to believe that the Tyndall effect does not fully account for the optics associated with blue nevi and veins. Methods | We used a spectrophotometer (Konica-Minolta CM 2600d) to measure reflectance from (1) a normal and blue ne- vus and (2) a vein and comparable normal skin—each pair from Results | Both types of nevi showed reflectance spectra similar to known melanin curves—with high absorption of blue light and decreased absorption of red light (Figure 1). 2 The shorter wavelengths (approximately 400-500 nm) demonstrated the expected baseline reflectance off of the surface of the skin (8%), illustrating that the role of blue light scattering is minimal, and the amount of red-spectrum light is the main determinant of color. The spectral curves between vein skin and normal skin demonstrated similar decrease in red in the skin overlying a vein. Discussion | The decrease in red light that yields the blue ap- pearance of nevi and veins is accounted for through a con- cept termed “subtractive color mixing,” analogous to remov- ing red out of purple paint. 3 While the distinction between preferential blue scattering (Tyndall effect) or less red reflec- tance initially appears semantic, it recognizes that there are ad- ditional reasons beyond light scatter that can lead to de- creased red reflectance, including differences in vasculature, hemoglobin levels, or characteristics of melanin (small struc- tural differences can lead to changes in color, as seen in oxy- hemoglobin vs deoxyhemoglobin). In veins, the decrease in red color prompts a change in color perception. 4,5 In this situation, the eye uses a type of chro- matic induction called “simultaneous contrast” to enhance the contrast between 2 colors, leading the brain to perceive and Figure 1. Reflectance Spectra of Normal Skin, Skin Overlying a Vein, a Normal Nevus, and a Blue Nevus Normal Vein Reflectance, % Nevus Blue nevus Wavelength, mm jamadermatology.com This graph shows the reflectance of a blue nevus, nevus, normal skin, and a vein across a spectrum of visible light ranging from 400 nm to 700 nm. The blue nevus shows a decreased red light reflectance compared with a normal nevus, and the vein shows a decreased red light reflectance compared with normal skin. Through the concept of subtractive color mixing, this decrease in red light contributes to a perceived blue hue. (Reprinted) JAMA Dermatology Published online July 13, 2016 Copyright 2016 American Medical Association. All rights reserved. Downloaded From: http://archderm.jamanetwork.com/ by University of California - Los Angeles, Melissa Shive on 08/18/2016 E1