The light produced by a laser has a unique combination of spatial coherence (all waves in "step" or in "phase"); being monochromatic and at a high level of collimation (low beam divergence). Some lasers can only be operated in a pulsed mode to produce short bursts of coherent radiation. Other types of lasers can be made to produce a continuous output and are known as continuous wave (CW) lasers.
Harm from laser radiation results from the absorption of energy from a beam by biological tissue. The narrow, almost parallel beam of coherent monochromatic laser light means that, in the absence of absorbers, the power delivered by the beam does not diminish significantly with distance. In addition, if such a beam is focused by a lens down to a spot, the power intensity is significantly increased. Because of the eye's ability to focus visible radiation (and some near IR) onto the retina, and the irreversible nature of the injury that may result, the eye is recognized as the critical organ for damage by lasers. High safety standards are essential when using lasers. The nature of the precautions needed may vary considerably depending on the type and power output of the laser in use. This manual defines responsibilities and precautions that are relevant to all work with lasers.