How Laser Hair Removal Works

The core principle: selective photothermolysis

Every medical laser used for hair removal exploits a principle called selective photothermolysis. The idea is simple: pick a wavelength of light that is preferentially absorbed by one thing (the pigment inside the hair shaft) and much less absorbed by everything around it (the surrounding skin). Deliver that light in a pulse short enough to heat the target without heating the neighbors.

Do it correctly and only the hair follicle receives a therapeutic dose of heat. The rest of the skin stays cool. Do it incorrectly — wrong wavelength, wrong pulse, wrong cooling — and you either heat the surrounding tissue or fail to reach the follicle at all. This is why device selection is a clinical decision, not a marketing one.

Why wavelength matters (and why one device isn't enough)

Different wavelengths of light penetrate the skin to different depths and are absorbed by pigment with different efficiency. In modern medical practice, two wavelengths dominate:

• ·Alexandrite (755nm) — highly absorbed by melanin, ideal for lighter skin (Fitzpatrick I–III).
• ·Nd:YAG (1064nm) — penetrates deeper and bypasses surface melanin, making it safe for medium to dark skin (Fitzpatrick IV–VI).

A clinic that only has one wavelength cannot safely treat every skin type. This is one of the most common reasons patients get poor results (or worse, pigment changes) from med-spas that use a single generic device.

The role of cooling

The heat that disables the follicle also wants to warm the surface of the skin. Modern systems solve this with integrated contact cooling — a sapphire or metal tip chilled to just above freezing that presses against the skin immediately before, during, and after each pulse. It draws heat out of the surface while the pulse does its work below.

Why results build over sessions

Hairs cycle through three phases: growth (anagen), regression (catagen), and rest (telogen). Only anagen hairs contain enough active melanin to fully absorb the laser and respond to treatment. At any moment, roughly 10–30% of the hairs in a given area are in anagen — the rest are dormant to the laser.

This is why a six-week interval exists. It's calibrated to catch new hairs entering anagen on each subsequent visit. Read the full mechanism in our guide on hair growth cycles.

This is not the same thing as IPL

Intense Pulsed Light (IPL) devices — common in spas and at-home tools — emit a broad spectrum of light rather than a single laser wavelength. Because the light is unfocused, it delivers less energy to the follicle and more to the surrounding tissue, which is why IPL is generally less effective and less predictable across skin types. It is not classified as a laser.