Diode vs alexandrite vs Nd:
YAG laser hair removal: wavelength, skin type and the trade-off between speed and safety

What diode, alexandrite and Nd:YAG lasers actually are

The names describe the material that produces the light, and the material fixes the wavelength. An alexandrite laser uses a chrysoberyl crystal and emits at 755 nm. An Nd:YAG laser uses a neodymium-doped yttrium aluminium garnet crystal and emits at 1064 nm. A diode laser uses a semiconductor and can be built for several wavelengths; the hair-removal diodes in most Singapore clinics emit at 808 or 810 nm, and some newer systems combine 755, 808 and 1064 nm diode emitters in one handpiece, which blurs the categories but does not change the physics of each wavelength.

All three work by selective photothermolysis. Light is absorbed by melanin in the hair shaft and bulb, converts to heat, and the heat conducts to the structures in the follicle that make the hair. A long pulse lets the heat spread through the follicle while the surface is cooled. The three differ in two properties that move together with wavelength: how strongly melanin absorbs the light, and how deep the light penetrates before it is scattered. Shorter wavelengths are absorbed more avidly and penetrate less; longer wavelengths are absorbed less and penetrate further. The principles are set out on the laser hair removal page; what follows is how the three compare.

Melanin absorption and skin type: why the same property is a strength and a risk

Melanin absorbs light across the visible and near-infrared range, and the absorption falls steadily as wavelength rises. At 755 nm absorption is strong, which is why the alexandrite is so effective on dark hair, including finer hair that carries less pigment. At 1064 nm absorption is a fraction of that, which is why the Nd:YAG needs more energy to heat a follicle and is a weaker match for fine or light hair. The diode at 808 to 810 nm sits between them.

The catch is that the melanin in the skin absorbs by the same curve. On Fitzpatrick I to III skin, where dark hair contrasts with lighter skin, the strong absorption of 755 nm goes mostly to the hair. As skin pigment increases, the epidermis takes a larger and larger share of the energy at any wavelength, but the share is largest at the shortest wavelength. That is why pigment change, blistering and burns on darker skin are reported more with alexandrite and diode than with Nd:YAG. Dierickx's review of hair removal lasers set out the logic twenty years ago, Battle and Hobbs described the long-pulsed Nd:YAG as the system of choice for darker skin types, and later reviews of adverse effects have reported pigment change in a markedly higher share of patients treated with shorter wavelengths on darker skin than with 1064 nm.

None of this makes one wavelength better in the abstract. It means that each answers a different skin, and that the question to ask of any clinic is which wavelength will be used on your skin, not which acronym appears on the menu.

“Every hair removal laser is absorbed by melanin. The only question is whose melanin, the hair's or the skin's, and the wavelength is what decides the share.”

Dr Sin YongOn choosing a hair removal wavelength

Speed vs safety: what diode systems trade for coverage

Diode lasers dominate hair removal in salons and chains for a practical reason. The semiconductor source is compact and inexpensive, the handpieces carry large spot sizes, and many systems run in-motion at low fluence with high repetition, sweeping the handpiece across the skin so that bulk temperature in the follicles rises gradually. A full leg can be covered quickly, the sensation is milder than single high-energy pulses, and the operator needs less training. For Fitzpatrick I to III with dark hair, that is a reasonable way to treat a large area.

The trade-off is that a fixed wavelength leaves only one lever for darker skin: fluence. On Fitzpatrick IV and above, a diode at 808 nm is still absorbed by surface melanin at a meaningful rate, so the energy is turned down to protect the skin, and turning it down also reduces what reaches the follicle. In-motion delivery averages the heating across the pass, so whether a given follicle reached a damaging temperature depends on how evenly the operator moved. The alexandrite has the same single-wavelength limitation at the other end; the long-pulsed Nd:YAG has the margin on darker skin but is a poorer match for fine hair.

The third column is the platform used here: R2 Glow, a long-pulsed laser carrying both 755 nm alexandrite and 1064 nm Nd:YAG with epidermal cooling. The point of two wavelengths is that the operator is not forced to compromise at one end of the skin-type range; the wavelength changes before the fluence does. Dr Sin Yong does not use a diode system, and this page does not claim that a diode cannot treat darker skin, only that it does so with one lever rather than two.

Which laser fits which person in Singapore

Singapore's population sits largely in Fitzpatrick III to V, which is the range where wavelength choice stops being academic. Fitzpatrick III is a borderline for the alexandrite and the diode, and the borderline moves with a tan and with the area: the underarm, bikini line and genital skin are a shade or two darker than the arm in almost everyone. Fitzpatrick IV and V is Nd:YAG territory.

In practice

• Fair skin, dark or medium hair, including finer hair: the alexandrite is the strongest match, and a diode is a reasonable alternative; Nd:YAG would under-treat the finer hair
• Fitzpatrick III, untanned, dark coarse hair: alexandrite at conservative settings or a diode on the arms and legs; Nd:YAG on the underarm and bikini area where the skin is darker
• Fitzpatrick IV to VI, or any tanned skin: long-pulsed Nd:YAG with cooling and a test patch; a diode only with fluence reduced, which costs follicular heating
• Fine, light, white or grey hair on any skin: no wavelength targets it reliably; electrolysis, by referral, is the alternative
• Started on a diode or IPL elsewhere: the remaining hair is usually finer and lighter, and on lighter skin the 755 nm is often the better match for what is left

These are the reasons Dr Sin Yong chooses the wavelength area by area rather than once for the whole body, and why a person whose arms are treated at 755 nm may have their underarms treated at 1064 nm on the same visit. The underarm, brazilian, legs, face and beard each have a page explaining how that choice is made for that area.

Where IPL and SHR fit in this comparison

Two other terms appear on the same menus and are neither of the three lasers. IPL is not a laser: it is a flashlamp emitting a broad band of wavelengths, typically 500 to 1200 nm, filtered to remove the shortest. Because energy is spread across wavelengths that skin also absorbs, the margin between treating the follicle and heating the epidermis is narrower than with any single-wavelength laser, and that margin is what matters in Fitzpatrick III to V. SHR, super hair removal, is a delivery method rather than a device: low fluence in rapid repeated passes with in-motion technique, run on diode platforms and on some IPL systems. SHR alone says little about which wavelength is being used on you.

The question to ask of any clinic is the same whichever name is on the door: which wavelength, at what fluence, with what cooling, matched to which skin type, and chosen by whom. What decides how long a course runs, on any of these systems, is set out on what decides the number of sessions.