Laser Safety Manual

8 Laser Safety Manual regions of the electromagnetic spectrum. The energy emitted by laser radiation can be transmitted, absorbed, or reflected, depending upon the characteristics of the material with which the laser light comes into contact. Materials that transmit laser beams are said to be transparent. Conversely, opaque materials either absorb or reflect the laser energy. Transmission and Absorption Laser transmission and absorption depend not only on the chemical and physical makeup of the target material, but also on the wavelength of the laser being used. At visible wavelengths, laser radiation impinging on the eye is focused on the retina and, if sufficient energy is absorbed, can cause cell destruction. At longer and shorter wavelengths, such as the far infrared and ultraviolet regions, radiation striking the eye is absorbed by the cornea and the lens rather than the retina. Although these structures are less easily damaged than the retina, excessive energy absorption can still cause cell damage and vision impairment. Reflection Reflection is primarily a function of the physical characteristics of the surface of the target material. A smooth, polished surface generally provides a high-quality or specular reflection, whereas rough, uneven surfaces are usually poor reflectors and produce a diffuse reflection. A reflector such as a flat mirror changes the direction of an incident beam with little or no absorption. A curved mirror or surface will change the divergence angle of the impinging laser beam as well as its direction. For a diffuse reflection, the reflected energy is scattered in multiple direction thereby reducing the energy or power density. Generally, diffusely reflective surfaces are favored when designing a laser experiment, since their use reduces the likelihood of a specular reflection and thus increases the safety of the experiment. Classes of Lasers To provide a basis for laser safety requirements, all lasers and laser systems in the United States are classified according to the ANSI Z136.1 Standard and the Federal Laser Products Performance Standard. The laser classification is most often supplied by the manufacturer. Custom-built and modified lasers shall be classified by the builder and verified by the LSO. The standards are enforced by the Occupational Safety and Health Administration (OSHA). The Laser Products Performance Standard is enforced by the Centers for Devices and Radiological Health, a division of the Food and Drug Administration.