Chromophores: Melanin, Haemoglobin, Water, and the Blonde/Red/Grey/White Problem

Status: draft compiled 2026-04-20.

SPTL depends on a chromophore inside the target that absorbs the chosen wavelength preferentially relative to surrounding tissue. For hair removal, that chromophore is melanin, but three other absorbers in skin — haemoglobin, water, and tattoo ink — become relevant as competing absorbers at specific wavelengths. Understanding the chromophore geometry is what lets a reader make sense of why particular wavelengths work for particular skin and hair combinations, and why the field has an essentially unsolved problem with pale and colourless hair.

Melanin's absorption spectrum

Eumelanin — the dark brown-black pigment of dark hair and of brown/black skin — has a broad absorption spectrum that peaks in the UV and declines through the visible and near-infrared. At 694 nm (ruby), absorption is high. At 755 nm (alexandrite), absorption is moderately high. At 810 nm (diode), absorption is moderate. At 1064 nm (Nd:YAG), absorption is low but nonzero. The absorption is at all wavelengths enough for clinically useful heating of concentrated follicular melanin, but the proportional difference between short and long wavelengths is the reason the clinical use cases differ so dramatically across the laser classes. Sources: Anderson & Parrish 1981 review of tissue optics; Sun 2022, PMID 35289409. Confidence: C1.

The critical observation is the ratio of follicular melanin to epidermal melanin absorption, not the absolute absorption. At 694 and 755 nm, epidermal melanin absorbs strongly, so in Fitzpatrick V-VI the beam deposits significant energy superficially before reaching the follicle; the result is epidermal burns and PIH. At 1064 nm, epidermal melanin absorbs much less, so more of the beam passes through the epidermis to the follicle, and the ratio of follicular-to-epidermal energy deposition is more favourable. This is the fundamental reason Nd:YAG is safer for darker skin. Confidence: C1.

Pheomelanin and the red-hair failure

Pheomelanin — the yellow-red pigment of blonde and red hair — absorbs much less efficiently in the 600-1100 nm range than eumelanin. Follicles containing pheomelanin heat to some extent under laser exposure but not to the temperatures required for selective photothermolysis to reliably damage the papilla and bulge. Clinical experience with red-haired patients on standard laser protocols is that the response is inconsistent, often requiring more sessions for less reduction, and that a substantial fraction of patients see no meaningful reduction at all. Pheomelanin-dominant hair is a physiological boundary of laser hair removal, not a technical problem that a different wavelength or pulse could solve. Confidence: C2.

The grey/white hair problem

Greying hair loses its melanin entirely as melanocytes in the matrix cease producing pigment. A pure grey or white shaft has essentially no chromophore in the SPTL wavelength range; the beam passes through without significant absorption. This is a hard boundary, not a sensitivity problem — no currently-available laser or IPL device works reliably on grey or white hair. The only method that does is electrolysis, which uses chemical (galvanic) or thermal (RF diathermy) destruction applied to the follicle directly rather than depending on absorption of light by hair pigment. Richards & Meharg 1995 JAAD, PMID 7673501 is the canonical expert observation on this; every subsequent review confirms it. The clinical consequence is that any hair-removal programme for a patient with significant grey or white hair (elderly patients; patients with post-inflammatory leukotrichia; patients in the grey fraction of their beard) must either accept no result from laser or plan for electrolysis as the definitive method. Confidence: C1.

The nanoparticle workaround

The attempted engineering answer to the grey/white problem was to introduce exogenous chromophores into the follicular infundibulum that would absorb light at a clinical wavelength and transfer heat to the surrounding follicle structures. Sebacia Microparticles (silica-core, gold-shell, PEG-coated) and Sienna SNA-001 (silver particles) were both in this family. The mechanism was mechanistically sound — particles massaged into follicular openings, excess wiped off, particles activated by laser, heat delivered to the papilla regardless of shaft melanin — but neither product produced convincing hair-removal efficacy data, and both companies wound down by 2020-2021 (Sebacia via Gerbsman Partners sale November 2020; Sienna bankruptcy 2019 with Sebacia acquisition of assets). The nanoparticle-assisted approach is effectively shelved as of April 2026 and does not represent a current treatment option. Sources: FDA 510(k) K181518; Gerbsman Partners Sebacia sale notice November 2020; Practical Dermatology Sebacia/Sienna coverage. Confidence: C3.

Photodynamic therapy for hair

A second attempted workaround is photodynamic therapy (PDT) with topical 5-aminolevulinic acid (ALA) or methyl aminolevulinate (MAL) and red or blue light activation. ALA is metabolised in situ to protoporphyrin IX, which accumulates selectively in pilosebaceous units during anagen. Activation with 630 nm red light or 415 nm blue light generates reactive oxygen species that damage the follicle. The mechanism is porphyrin-selective rather than melanin-dependent, so PDT should in principle work on blonde, red, grey, and white hair. In practice, PDT for hair removal has never moved beyond small case series; burn risk, prolonged photosensitivity, pain during illumination, and inadequately-standardised protocols have kept it out of mainstream clinical use. Grossman et al 1995 Lasers Surg Med Suppl 7:44 was the original proof-of-concept. Subsequent work (Shin et al 2016 Lasers Surg Med, PMID 27504592; Qscience 2013 review) has confirmed feasibility without moving PDT toward mainstream adoption. No major device maker offers PDT hair removal as a primary indication in 2026. Confidence: C4.

Haemoglobin as a competing chromophore

Haemoglobin absorbs strongly in the 530-600 nm range (Q-band), which is why pulsed dye lasers at 585-595 nm are used for vascular lesions. At hair-removal wavelengths (694-1064 nm), haemoglobin absorption is much weaker than melanin absorption, so vascular competition is not a primary concern. An exception is strongly vascular skin (rosacea, telangiectasia, keloid scars with prominent vasculature) where some vascular heating occurs as a side effect of hair removal and may produce transient erythema or, rarely, bruising. Confidence: C2.

Water as a competing chromophore

Water absorption in tissue rises sharply above 1300 nm and becomes a major factor at wavelengths used for skin resurfacing (2940 nm erbium:YAG, 10,600 nm CO₂). At hair-removal wavelengths, water absorption is low enough to be essentially irrelevant: at 1064 nm Nd:YAG, water absorption is non-negligible but small compared with follicular melanin absorption, and it contributes slightly to overall dermal heating without compromising SPTL selectivity. Clinically, water is not a problem for hair removal; it becomes a problem only when SPTL-class lasers are misused at resurfacing-class wavelengths. Confidence: C2.

Tattoo ink

Tattoo ink is a near-ideal chromophore at almost every wavelength used in medical dermatology, which is why Q-switched lasers remove tattoos. For hair removal, this means tattoos in the laser field are a contraindication: the beam will heat the ink fragment at fluences that produce ink fragmentation, superficial burns, and permanent change to the tattoo appearance. Standard practice is to mask tattoos with opaque tape before laser hair removal delivery or to avoid the tattooed region entirely. Electrolysis is unaffected by tattoo ink and is the correct method for hair removal in or adjacent to a tattoo. Confidence: C1.

What the chromophore framework means for method selection

For a patient with dark terminal hair on Fitzpatrick I-III skin: alexandrite 755 is the traditional optimum because follicular melanin absorbs efficiently and epidermal competition is minimal.

For a patient with dark terminal hair on Fitzpatrick III-IV skin: diode 810 balances depth and melanin specificity; Nd:YAG 1064 is a safer choice when tanned or sun-exposed.

For a patient with dark terminal hair on Fitzpatrick V-VI skin: Nd:YAG 1064 is standard of care; SHR-mode diode at low fluence is acceptable; alexandrite and high-fluence diode are contraindicated.

For a patient with blonde, red, grey, or white hair, of any skin type: no laser reliably works; electrolysis is the only effective definitive method.

For any patient with tattoos in the proposed field: mask tattoos or avoid the region; electrolysis for hair in the tattoo.

Sources: Husain 2022, PMID 35634805; Dorgham 2020, PMID 31587390; Kao 2023 NMA, PMID 37493187; Richards & Meharg 1995, PMID 7673501. Confidence: C1.