Laser versus Noncoherent Light Sources

Individual photons from laser or noncoherent light sources are identical, and a tissue does not know how photon was born. The choice between laser and noncoherent light sources is strictly dependent on the particular application. In therapeutic applications, a laser is almost just one choice for short pulse (<0.1 ms), small spot size or microbeam (<1 mm), fiber delivery, narrow absorption spectrum bandwidth of chromophore (<50 nm), and focusing beams in tissues. In other cases, noncoherent sources can be very competitive with laser, especially if cost and size of equipment are important parameters. For example, for light – based hair removal systems the target is the melanin in hair follicles. For deep penetration into the skin, very broad spectrum of wavelengths can be used, from 600 to 1200 nm. Melanin absorbs light in this entire spectrum. For greater safety, the ratio of irradiance of melanin in the hair bulb (0.5-1.5 mm depth) and bulge (2-5 mm depth) areas to irradiance of melanin in epidermis (0.05-0.1 mm depth) must be maximal. This condition can be achieved just for a large beam size on the surface of the skin, when diffusion of light in the skin due scattering in the follicle depth is smaller than the initial beam size. Based on these criteria, the beam size on the skin surface must be about 2 cm. At the same time, for per­manent hair removal the fluence on the skin surface must be in the range 30-100 J/cm2. The total energy of the light pulse must be of 100-300 J. A laser with such high energy is very expensive. At the same time, a Xe arc lamp with a filtered 600-1200 nm spectrum has an efficiency of about 15 %; it is small and low-cost.