There are things to consider, principally:
-- Wavelenghts (nm)
The laser wavelength for deep, thick & dark hair roots should be 1064nm (Nd:YAG); but it won't be effective for shallower hair roots.
The laser with wavelength of 755 nm (Alexandrite) & 800-900 nm (Diode) work effectively with shallower, thinner & lighter hair roots.
Many IPLs & Lasers with other wavelengths do not work effectively to retard hair growth. So before you pay, ask the therapist what wavelength they use to remove your hairs? Because if your hairs root deeply, energy form lasers & IPLs with shorter wavelengths won't reach your hair roots; vice versa shallower hair roots won't respond well to lasers with longer wavelengths
-- Skin colours
Nd:YAG is suitable for all skin types, but Alexandrite & Diode are safe to be used for Fitzpatrick Skin Type 1, 2 & 3 only.
Very dark-skinned individuals need to have a laser with a longer wavelength. The longer wavelength is needed so as not to have the laser energy preferentially absorbed by the top layer of skin as would occur with the shorter wavelength laser. The longer wavelength penetrates deeper and will target the deeper dark pigment decreasing the risk of blisters or burns on the surface.
-- Natural Hair Colours
See => https://www.facebook.com/upweylaser/posts/586060421734589
-- Density & diameter your dark hair roots
The treatments are most effective on dense, thick, coarse & dark hair roots because they have a lot of pigment and can absorb a lot of heat. Sparse, fine hairs cannot absorb much heat. Thus, LHR lose efficacy on very thin or vellus hairs.
-- Pulse Duration
Laser devices operate as continuous wave or pulsed systems (e.g. single or repetitive‑pulsed lasers). Examples of multi-pulsed (in other word, repetitive-pulsed) laser devices are Short Pulsed, Long Pulsed and Q-switched for different treatment purposes. But long-pulsed lasers in 755, 800-900 & 1064 nm have proved to be effective to reduce hairs permanently.
Q-switched & other short-pulsed lasers which emit much shorter pulses won't do anything to your hairs except bleaching hair colors. Both the amount, density & size of your hairs will remain exactly the same. Remember! Once hair roots get bleached, IPL & Laser won't work on lighter hairs 😭.
-- Fluence (J/cm^2), Spot Size (mm) & Repetition Rate (Hz) to be applied are according to different recommendations among different brands.
In general, the smaller the spot size the higher the fluence.
Settings can be increased or decreased depending on the previous results and clients' feelings during different sessions.
Rule of thump: Look for perifollicular edema which should appear few minutes after firing