Selective Photothermolysis

Status: draft compiled 2026-04-20.

Selective photothermolysis (SPTL) is the theoretical foundation of every laser hair-removal device in commercial use, laid down in a single 1983 paper and elaborated in a supporting 2001 generalisation. Understanding SPTL in enough detail to reason about device selection, fluence setting, cooling, and failure modes is the one piece of physics the rest of this deep dive refers back to. The alternative — treating laser hair removal as a black box where fluence and pulse width are parameters to be accepted from the manufacturer — is how clinics produce burns, PIH, paradoxical hypertrichosis, and failed courses. Most of what separates a competent clinician from a negligent one at the bedside is their ability to reason about the SPTL parameters for the patient in front of them.

The 1983 Anderson & Parrish paper

The paper in question is Anderson RR & Parrish JA, "Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation," Science 220:524-527 (1983), PMID 6836297. The argument is three conditions:

1. A pulsed light source with a wavelength preferentially absorbed by a target chromophore inside the target structure.
2. A pulse duration shorter than the target's thermal relaxation time (TRT — the time required for the target to lose half its heat to surrounding tissue).
3. Sufficient fluence to produce the desired thermal effect.

When all three are satisfied, the beam's energy is preferentially absorbed by the target chromophore, the target heats faster than surrounding tissue, and confined thermal injury to the target is achieved without significant collateral damage. The paper's worked example was port-wine stain treatment (oxyhaemoglobin as chromophore, dermal blood vessels as target); the framework translated directly to hair removal once the relevant parameters were worked out. Confidence: C1.

Chromophore: melanin in hair

For hair removal, the chromophore is melanin — specifically eumelanin, the black-brown form — located in the hair shaft (the outer cuticle, the cortex, and the medulla) and in the matrix cells at the follicle base. Pheomelanin (red / blonde hair) absorbs much less efficiently at 600-1100 nm wavelengths, and follicles containing only pheomelanin or very little melanin (red hair) produce weak target heating. Grey and white hair, which have essentially no melanin, are invisible to SPTL entirely; no laser works on them. This is the biological floor on laser hair removal efficacy and the reason electrolysis remains the only definitive method for pale and colourless hair. Sources: Anderson & Parrish 1983; Sun 2022, Photochem Photobiol Sci, PMID 35289409. Confidence: C1.

The target, however, is not the pigmented shaft itself — the shaft regrows after being severed, so destroying it accomplishes nothing permanent. The target is the bulge stem cells (the follicular stem-cell niche roughly midway up the outer root sheath) and the dermal papilla (the mesenchymal signalling condensate at the follicle base). Heat generated by absorption in the pigmented shaft must conduct from the shaft to these structures for the laser to produce durable reduction. The bulge and papilla themselves have little or no melanin and do not absorb the beam directly. Sources: Richards & Meharg 1995 JAAD, PMID 7673501; Sun 2022; Dierickx 2000, Dermatol Clin, PMID 10903915. Confidence: C2.

Thermal relaxation time and pulse duration

The TRT of a terminal hair shaft is about 40-100 ms depending on diameter. The TRT of epidermal melanin (distributed as pigment in keratinocytes across the stratum basale) is about 3-10 ms depending on phototype. For SPTL to work on hair without damaging epidermis, the pulse duration must be longer than the epidermal TRT (so epidermis can cool off between pulses, or during the pulse if slow enough) but shorter than the shaft's TRT (so the shaft retains its heat long enough to transfer to the bulge and papilla). That window — roughly 10-100 ms for a typical combination — is where most modern long-pulse devices operate.

In practice, the shaft is not the only thing that needs to be heated; the bulge and papilla are 2-5 mm from the pigmented shaft, and heat has to conduct that distance by thermal diffusion. This requires the shaft to remain hot for longer than its own TRT, which is where the Altshuler & Anderson TDT generalisation (see thermal-damage-time.md) extends the pulse-duration window up to 170-1000 ms. Modern "long-pulse" and "super long-pulse" devices run in this extended window and are designed to heat the shaft long enough that the bulge and papilla both receive the thermal dose needed for destruction. Sources: Altshuler & Anderson 2001, PMID 12030874; Sun 2022. Confidence: C1.

The skin-type problem

Epidermal melanin is the competing absorber that complicates SPTL in darker skin. In Fitzpatrick I-III (light skin), the fraction of beam energy absorbed by epidermal melanin is low, and most of the beam reaches follicular melanin at useful depths. In Fitzpatrick IV-VI (dark skin), a much larger fraction is absorbed by epidermal melanin on the way in, which both reduces the energy available for follicular heating and creates a superficial heat deposit that can produce burns, blisters, and post-inflammatory hyperpigmentation. The solution is either to use a wavelength for which epidermal melanin is a weaker absorber (Nd:YAG 1064 nm — melanin absorbs less at longer wavelengths, so the beam penetrates deeper before losing energy) or to use a lower fluence with longer pulse and aggressive epidermal cooling so the skin has time to clear the superficial heat. Both strategies are employed in clinical practice; both are the reason Fitzpatrick IV-VI patients should be treated with Nd:YAG or SHR-mode diode rather than alexandrite or short-pulse diode. See ../wavelengths/ndyag-1064.md and ../populations/fitz-iv-vi.md. Confidence: C1.

Fluence

Fluence — the energy per unit area per pulse, measured in joules per square centimetre — is the parameter that determines whether the beam deposits enough energy to produce the desired thermal effect. Too low a fluence and the shaft heats but the papilla never reaches destruction temperature, leaving the follicle to recover and regrow; this is the mechanism of paradoxical hypertrichosis (sub-therapeutic stimulation activating dormant follicles) and of the end-of-course plateau where further sessions produce diminishing returns. Too high a fluence and the epidermis burns before the beam has time to clear through the skin; this is the mechanism of laser burns. The fluence-selection problem is always a balance between these two failure modes, and the right value depends on skin type, hair colour, body region, and device specifics. Typical clinic-level fluences: alexandrite 20-50 J/cm², diode 10-40 J/cm², Nd:YAG 24-60 J/cm²; home-device fluences 3-10 J/cm². Sources: Dierickx 2000; Husain 2022; Haedersdal 2006 Cochrane. Confidence: C2.

Spot size

The beam's spot size matters because photon scattering in tissue makes a narrow beam lose more of its energy at superficial depths than a wide beam does. A 3 mm spot has effective penetration 1-2 mm shallower than a 12 mm spot at the same fluence. Modern devices use 10-18 mm spots for bulk-area work on limbs and trunk, 5-7 mm spots for smaller precision areas (face, bikini line), and 3 mm spots for very small or contoured areas. The spot-size-penetration relationship is why clinic devices generally outperform home devices at the same nominal fluence: home device spots are smaller and the effective penetration is correspondingly lower. Confidence: C2.

Cooling

Selective photothermolysis benefits from active epidermal cooling for two reasons: it allows higher fluences to be used safely (by protecting the epidermis at the moment of pulse delivery) and it reduces the pain profile of the session. Three main cooling approaches are in use. Sapphire contact cooling (built into most modern diode and alexandrite heads) continuously chills the skin surface against a thermally conductive sapphire window. Dynamic cooling device (DCD) cryogen spray (Candela devices) delivers a 30-100 ms spurt of liquid R134a cryogen immediately before each pulse. Forced cold air (Zimmer Cryo 6) blows -30 °C air across the treatment field throughout the session. Ice pre- and post-session is a low-tech but widely used adjunct. All are covered in more detail in ../protocol/cooling.md. Confidence: C2.

What SPTL does not do

SPTL does not work without a chromophore; no laser destroys blonde, red, grey, or white hair reliably, because selective absorption cannot occur without selective absorption. SPTL does not work without a target; a follicle that has had its shaft plucked has no chromophore in the beam path, which is why pre-laser plucking and waxing must be avoided (see ../../shortterm-deep/interactions-with-permanent-methods.md). SPTL does not work selectively on tanned skin; epidermal melanin competes for absorption and the beam's selectivity collapses, which is why tanning is a contraindication. SPTL does not work on tattoos as hair removal; the tattoo ink is a chromophore at almost every clinical wavelength and the beam damages the tattoo rather than the follicle. SPTL does not work with 100% efficiency; bulge stem cells can survive sub-lethal heating and re-epithelialise the follicle, which is why clinical results plateau around 70-90% reduction. These are not detail failures; they are the boundaries of the method. Confidence: C1.

The gold/silver nanoparticle workaround

One attempt to extend SPTL beyond its melanin dependency was to introduce exogenous chromophores into the follicular infundibulum. Sebacia Microparticles (silica-core, gold-shell, PEG-coated particles) and Sienna SNA-001 (silver particles) were both FDA 510(k) trials aimed at this problem. Sebacia was cleared for acne in 2018 but the hair-removal indication never produced convincing data and the company wound down in November 2020. Sienna filed bankruptcy in 2019 with Sebacia acquiring its assets for $1.7M before Sebacia itself collapsed. Neither product is commercially available in 2026 and the nanoparticle-assisted SPTL idea is not currently being pursued by any major device maker. The workaround failed; the only practical way around SPTL's melanin dependency remains electrolysis. Sources: FDA 510(k) K181518; Gerbsman Partners Sebacia sale notice, Nov 2020; Practical Dermatology, Sebacia acquires Sienna. Confidence: C3.

What to take from this chapter

The SPTL framework compresses hair-removal laser practice to five parameters — wavelength, pulse duration, fluence, spot size, cooling — all adjusted for the patient's skin-melanin and hair-melanin profile. Every subsequent chapter in this deep dive is a specific instance of these parameters being tuned for a particular clinical goal: wavelength choice for skin type, pulse duration for hair diameter and TRT, fluence for therapeutic window, spot size for body region, cooling for safety envelope. A reader who has internalised this framework can read the individual chapters as engineering tradeoffs rather than as device-specific recipes.