Lasers, Lasers

Light Amplification by Stimulated Emission of Radiation

I underestimated both the depth and breadth of this topic. It started as a self-serving interest in how laser hair removal works, but it ended up as a review of Physics 101. Anyways, here's what I've got so far...
Lasers produce a ocused beam of light through the synchronized release of photons. They are named for the substance that is being activated (usually through electrical energy). As the electrons fall back to their original energy levels, they release energy. Most is released as heat (lasers are very inefficient) and some is released as light. Laser light is coherent (waves trave in a synchronized fashion), collimated (no light divergence... hence, the laser pointer), and monochromatic.

The most popular laser types used in medicine are:
1. Argon laser: it targets pigmentation... so the hemoglobin in red blood cells and the melanin in hair follicles and skin are all damaged by this laser
2. Carbon dioxide laser: excites both intra- and extracellular water
3. Nd:YAG laser: it targets pigmentation (just like the argon laser), BUT it has a 1064 nm wavelength, causing deeper penetration

Laser properties and their applications:
1. Wavelength- determines depth of penetration and selects what biologic component will absorb it--- characteristic of the specific type of laser
2. Irradiance (power density) = laser energy (watts) x 100/ surface area of laser beam (cm^2), literally the density of your energy source--- you need to know this to achieve the same effect using different manufacturer lasers
3. Energy fluence (energy per pulse) = irradiance x exposure (sec) = joules /cm^2--- you need to know this to achieve the same energy over a large lesion


So the laser has to do two things for it to zap the hair follicle: penetrate to the dermis (requiring a wavelength between 630-100 nm) and target the melanin in the follicle. Because it is unknown which part of the follicle needs to be removed to prevent recurrent hair growth, the whole thing is zapped right now. Shorter wavelength lasers will also affect pigment in the epidermis. For fair skin, this is not a problem, but for pigmented skin there is a significant risk for permanent hypopigmentation. Using a longer pulse also helps target laser energy to the follicle, as melanosomes respond better to shorter pulses.

Here's my brief summary:
The closest to 1100 nm wavelength lasers without going over will give the least epidermal side effects (hypopigmentation, fibrosis, vesiculation).
The longer pulses will help target follicles over melanosomes.
Ideal conditions are pale skin with dark hair.
Argon and Nd:YAG lasers are used for photothermolysis.


Alster TS, Bryan H, Williams CM. Long-pulsed Nd:YAG laser-assisted hair removal in pigmented skin: a clinical and histological evaluation. Arch Dermatol. 2001 Jul;137(7):885-9.

Lasers in Skin Disease by Ronald Wheeland