Thermal Damage Time (TDT): The Pulse-Duration Generalisation

Status: draft compiled 2026-04-20.

The original selective photothermolysis framework required pulse durations shorter than the target's thermal relaxation time (TRT). In hair removal, this produced a tension: the target is not the pigmented shaft (which regrows) but the bulge stem cells and dermal papilla (which do not contain melanin). The original TRT logic implicitly assumed the target is the structure that absorbs the beam; when the target and the absorber are physically separated, the TRT model is an incomplete guide. Altshuler, Anderson, & Manstein 2001, Lasers Surg Med 29:416-432, PMID 12030874 introduced the thermal damage time (TDT) concept to close this gap, and modern long-pulse laser hair removal depends on the TDT generalisation rather than on strict TRT-bounded pulse durations.

The distinction

The TRT of a structure is the time over which that structure loses half its heat to surrounding tissue. The TDT of a target is the time required to induce irreversible damage to the target assuming it receives heat diffused from a nearby absorber. For hair, the absorber is the pigmented shaft (and matrix melanocytes) and the target is the bulge stem cells and papilla. The two are 2-5 mm apart in the dermis. Heat must conduct from shaft to target, and that conduction takes time. A pulse short enough to be below the shaft's TRT deposits heat in the shaft but does not hold the shaft hot long enough for useful conduction to target structures. A longer pulse — up to 170-1000 ms — heats the shaft to a temperature profile that sustains heating of surrounding tissue long enough for the bulge and papilla to receive a thermal dose sufficient for irreversible damage. Sources: Altshuler 2001; Sun 2022, PMID 35289409. Confidence: C1.

What modern long-pulse devices do

The practical consequence of TDT is that modern clinical hair-removal lasers run longer pulses than the strict TRT-bounded window would allow: 10-100 ms on light skin (comfortable inside the shaft TRT), 40-170 ms on medium skin, 100-400 ms on darker skin where epidermal cooling windows are wider and the beam can be held on longer without epidermal injury. The "SHR" (super-hair-removal) modality on Alma Soprano devices takes this further: low fluence (5-10 J/cm²) at high repetition rate (~10 Hz) with continuous sliding motion across the skin, so that cumulative heating raises bulk dermal temperature into the follicular destruction range while never exposing any single spot to an injuriously high instantaneous fluence. The SHR technique is a TDT-framework approach; it is safer on Fitzpatrick V than a single high-fluence pulse on the same skin because it avoids the peak fluence that would burn the epidermis while accumulating the total thermal dose required for follicular destruction. Sources: Altshuler 2001; Grunebaum et al 2020 Soprano Titanium, PMID 32484620; Bhat & Handog 2021 Soprano Ice Platinum, PMID 33567152. Confidence: C2.

Why this matters for patient reasoning

The TDT framework gives a reader the tools to reason about a device's pulse-duration parameters. A long pulse setting (40-100 ms) is appropriate for coarse dark hair on light skin, where the shaft TRT is long enough to accommodate long pulses without exceeding it, and the bulge and papilla need the extended conduction window. A shorter pulse (3-20 ms) is appropriate for fine hair where the shaft TRT is short. Very long pulses (170-400+ ms) are appropriate for darker skin types where epidermal cooling and slow heat delivery produce the best safety profile. Pulses much shorter than either the shaft TRT or the epidermal TRT are appropriate only for specific non-hair indications (tattoo removal, pigment toning) and are actively wrong for hair removal. Confidence: C2.

Clinics that use device presets without understanding the TDT reasoning behind them still usually produce adequate results because manufacturers have tuned the presets for typical cases. Clinics that adapt parameters for specific patients — shortening pulse for fine hair, lengthening for darker skin, reducing fluence with extended pulse for sensitive skin — outperform preset-only operation particularly in edge-case patients. Confidence: C3.

What TDT does not rescue

TDT cannot rescue SPTL where the chromophore is missing. Blonde, red, grey, and white hair remain invisible to the beam regardless of pulse duration. TDT cannot rescue SPTL on heavily tanned skin, where epidermal competition dominates. TDT cannot convert a mismatched wavelength into a matched one; an alexandrite beam on Fitzpatrick V skin is wrong regardless of pulse duration. TDT is a generalisation of the temporal parameter, not a workaround for the chromophore-selectivity problem.