Phototherapy—using ultraviolet (UV) or visible light to treat skin conditions—is widely used for issues like psoriasis, eczema, vitiligo, and neonatal jaundice. Your skin color plays a key role in how well this therapy works. In darker skin, extra pigment acts like a natural filter, absorbing more light and reducing how much reaches deeper layers. This affects the treatment’s effectiveness and safety.
Here, we explain clearly why skin color matters in phototherapy and what tweaks doctors make in real life to match light doses to different skin tones. All points are backed by reliable research.
Why Skin Color Matters
Melanin is the pigment that gives skin its color. Everyone has a similar number of pigment-producing cells (melanocytes), but in darker skin these cells pack more and bigger melanosomes, spread out individually—making skin better at soaking up and scattering light. According to Setchfield et al., darker-skin types (Fitzpatrick V–VI) have light absorption up to 74 % greater than lighter types in the 400–1,000 nm range. That means less light reaches the deeper cells, affecting the dose needed for therapy.
Melanin is also protective. People with darker skin are about 70 times less likely to get UV-induced skin cancers, because melanin absorbs significant amounts of UV: only about 7 % of UVB and 18 % of UVA reaches the lower layers in dark skin, compared to 24 % UVB and 55 % UVA in light skin. That is great for sun protection—but for therapeutic skin treatments, it means that more light is absorbed superficially, requiring higher doses to reach the target area.
How Light Penetration Varies by Skin Tone
Shorter wavelengths (400–600 nm) are absorbed more, especially in darker skin, while near-infrared light (>940 nm) penetrates deeper and more evenly across all skin types. In practice, visible and UV light need dose adjustments based on skin tone, whereas infrared or red light may be used when deep penetration is needed, and melanin interference is smaller.
Adjusting Phototherapy: What Studies Say
A Seoul study on narrowband UVB (NBUVB) for psoriasis measured minimal erythema dose (MED)—the amount of light needed to lightly redden skin. Darker skin (lower L* value) needed higher UV doses. A 2011 review on phototherapy in darker-skinned patients noted that more melanin means you generally need higher doses to treat skin diseases effectively, but these patients are also more prone to post-inflammatory hyperpigmentation, so careful monitoring is important.
A New Zealand study treating mostly light-skinned children with NBUVB for eczema and psoriasis reported an average dose of 886 mJ/cm² per treatment, with 36 % getting mild redness. For darker-skinned children, doctors would start higher and escalate more carefully.
Beyond UV: Visible and Red Light
Blue light activates pigmented cells via a light-sensitive receptor, increasing pigment production, while red or yellow light can sometimes reduce pigment. In darker skin, blue/visible light may trigger pigmentation more strongly, while red light could help normalize pigment—but exact dose guidance is still emerging.
Practical Guidelines for Clinicians
1. Assess your patient: Use Fitzpatrick phototype or L* colorimetry.
2. Set the initial dose: Lighter skin starts lower; darker skin starts higher.
3. Escalate carefully: Increase dose by 10–20% per session, watching for redness or dark spots.
4. Monitor side effects: Watch for pigment changes; pause or reduce dose if needed.
5. Use the right wavelength: UVB/NBUVB is standard; PUVA and visible light need more caution in darker skin.
Conclusion
Skin color significantly affects how much therapeutic light penetrates skin and how it responds. In darker skin, more light is absorbed near the surface by melanin—requiring higher doses to treat deeper tissues safely. But higher doses also risk pigment changes, so clinicians must monitor reactions carefully.
By using objective measures, adjusting initial dose, escalating thoughtfully, and watching for side effects, phototherapy can be adapted to be both effective and safe for all skin types.
