PI-WIKI-OPT-23 · Optical Physics · reviewed 2026-10-31
Optical Physics, Skin Optics and Pigment Appearance
Critical numbers
Epidermal thickness (typical)50-150 μm; up to 600 μm on palms/soles
Tattoo pigment locationInside cells in the dermis; no study has measured an optimal depth in millimetres
Melanin absorptionNo peak: rises steadily toward the UV; about 6× higher at 400 nm than at 700 nm [15]
Melanosome volume fraction, light skin1.3-6.3% of the epidermis [15]
Melanosome volume fraction, darkly pigmented skin18-43% of the epidermis (moderately pigmented: 11-16%) [15]
Scattering wavelength dependenceRayleigh λ⁻⁴: blue (450 nm) ~4× red (650 nm); dermis model with Mie: ~3× [15]
Dermal reduced scattering coefficient (μs')1-5 mm⁻¹ at 633 nm; about 2.5 mm⁻¹ in the Jacques dermis model [15]
Dermal refractive indexn ≈ 1.4 (water-dominated); collagen fibrils higher
Titanium dioxide refractive index (white ink)n ≈ 2.5-2.9, strong backscatter; reads bright even under melanin
Healed colour desaturation vs freshHealed colour is less saturated than fresh; no study quantifies the loss
Epidermal melanin absorption at 500 nmAbout 1 mm⁻¹ (light skin) to 30 mm⁻¹ (darkly pigmented), computed from [15]
Skin surface reflection4-7% of incident light (Fresnel reflection at air-stratum corneum interface)
Do not
- Selecting pigment based on bottle colour alone
- Using pure white for highlights on Fitzpatrick V-VI
- Applying the same colour palette identically across all skin types
- Overworking the dermis to increase saturation
- Placing cool colours (blue
- Ignoring anatomical variation in skin thickness
Applicable standards
- EU / UK — EU REACH Annex XVII Entry 75 (2020/2081)
- United States — FDA: Tattoo inks are regulated as cosmetics under the Federal Food
- ASEAN / AP — ASEAN Cosmetic Directive (ACD)