Ultraviolet (UV) light, an invisible part of the electromagnetic spectrum, interacts with human skin in unique ways. Unlike visible light, which our eyes perceive as colors, UV radiation causes skin to appear dramatically different, often revealing features not discernible under normal illumination.
The Science of Skin’s UV Interaction
Human skin interacts with UV light through several physical processes, including absorption, reflection, and fluorescence. Different molecules within the skin, known as chromophores, selectively absorb UV light. Melanin, the pigment responsible for skin color, is a primary absorber of UV light, particularly in the UV and blue regions of the spectrum. Hemoglobin, found in blood, also absorbs some UV wavelengths.
While some UV light is absorbed, a portion is reflected off the skin’s surface. Certain skin molecules absorb UV light and then re-emit it at a longer, visible wavelength, a process called fluorescence. Molecules like collagen, elastin, and porphyrins are natural fluorophores in the skin. This re-emission of light causes these skin features to “glow.”
UVA and UVB are two types of UV radiation that interact differently with skin. UVA light penetrates deeper into the skin, reaching the dermis, while UVB primarily affects the epidermis, the outermost layer. The specific molecules present in each layer and their individual absorption and fluorescence characteristics dictate how they respond to these different UV wavelengths.
What UV Light Reveals on Skin
When human skin is viewed under UV light, various features and conditions become apparent or enhanced. Accumulations of melanin due to sun exposure appear darker or more prominent under UV light because melanin absorbs UV radiation. This technique can reveal underlying sun damage that has not yet surfaced as visible spots.
The fluorescence of healthy collagen and elastin, which contribute to a blue or blue-green glow, can be altered by aging or sun damage. Damage to these structural proteins, such as solar elastosis, can lead to changes in their fluorescence, causing compromised skin to appear altered.
Certain fungal and bacterial infections also exhibit distinct fluorescence under UV light. For example, tinea versicolor, a common fungal infection, appears yellow-green, while some bacterial infections like Corynebacterium minutissimum (causing erythrasma) may show a coral-pink fluorescence. Acne-related bacteria, specifically Cutibacterium acnes, can produce an orange-red glow due to porphyrins.
Cosmetics, lotions, and other residues on the skin may fluoresce. Scars and bruises can also appear altered under UV light due to changes in skin structure and the breakdown products of blood.
Practical Applications of UV Skin Examination
Examining human skin under UV light has several practical applications across various fields. In dermatology, a specialized device called a Wood’s lamp is used for diagnostic purposes. This lamp helps dermatologists identify and monitor skin conditions, including fungal and bacterial infections like tinea capitis or tinea versicolor. It also diagnoses pigmentary disorders like vitiligo and melasma by highlighting areas of altered melanin concentration.
In cosmetic analysis, UV skin examination is employed to assess skin health, quantify sun damage, and evaluate the penetration of skincare products. This allows skincare professionals to develop personalized treatment plans based on the damage revealed by UV light. The technique can also track the progress of treatments and the effectiveness of products over time.
UV light also finds utility in forensic investigations. It can help identify bodily fluids, fibers, or other trace evidence on skin or clothing. Forensic experts use both reflective and fluorescent UV imaging to reveal wounds or patterns not discernible to the naked eye, aiding in crime scene reconstruction and evidence analysis.