Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 73

Directionality is an unusual property of laser light and is not found in regular light sources. It means that little divergence is present (Figure 2) in the beam output and the laser light beam will travel considerable distances with little widening or spreading.

FIGURE 2 Directionality is possible because of the low divergence of a laser beam. A typical laser divergence is of the order of 1 mrad, which means that a beam spreads by one millimeter in one meter or by one meter in each kilometer.

This is one of the reasons why laser light is so hazardous. Unlike light from regular lamp sources, which rapidly spreads from the source, laser light maintains its brightness by having very little beam spread or divergence. The measure of beam spreading is called “divergence,” and usually measured in units of milliradians. A beam with a divergence of 1 milliradian (mrad) expands one meter every kilometer. Low divergence arises from the long path length created by the multiple photon reflections within the cavity. Very small laser cavities, e.g. diode lasers have initially high divergences. Ordinary light sources emit photons in many directions; a laser produces a collimated beam of high brightness. This collimation and brightness are why a laser beam is hazardous over long distances.