aSee the detailed discussion on hazard classes in the section on hazard evaluation measurements and classification.
Lasers operate at discrete wavelengths within the optical spectrum, and although most lasers are monochromatic (emitting one wavelength, or single color), it is not uncommon for one laser to emit several discrete wavelengths. For example, the argon‐ion laser emits several different lines within the near UV (e.g. near 350 nm) and several lines in the visible spectrum, but this laser is frequently designed to emit only one green “line” (wavelength) at 514.5 nm and/or a blue line at 488 nm. Some small laser pointers designed to emit a visible (e.g. green) wavelength may also emit an incompletely blocked, near‐IR pump beam (11) that can pose an unexpected hazard.
Although several thousand different laser lines (i.e. discrete laser wavelengths characteristic of different active media) have been demonstrated in the physics laboratory, perhaps only 20–30 have been developed commercially to the point where they are routinely applied in everyday technology (9, 12–16). Guidelines for human exposure have been developed and published which basically cover all wavelengths of the optical spectrum in order to allow for currently known laser lines and future lasers (2, 3, 5).
2 WHY IS A LASER HAZARDOUS? SPECIAL PROPERTIES
As noted earlier, the high collimation potential of a laser can project a hazard over considerable distance – even to kilometers. The special properties of the light beam, produced by the laser are that: laser light is highly monochromatic, coherent, directional, and extremely bright. Some of these factors are very important from a hazard standpoint – others not so.
2.1 Directionality
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.
2.2 Coherence
Coherence describes the uniform spatial relationship between all portions of the electromagnetic wave. Monochromatic refers to the highly purified color (i.e. one wavelength) produced by most lasers and the extremely narrow spectral band of radiation. Both of these concepts are illustrated in Figure 3.
FIGURE 3 Coherence. The coherent nature of laser light relates to both spatial and temporal coherence. Spatial coherence means that the light waves are in phase.
2.3 Radiance
Since the rays emitted from a laser are relatively parallel and do not diverge as the laser moves through space, the light energy remains concentrated and retains its characteristic brightness. This concentration of the beam causes the laser brightness to be very much greater than the brightness of any other man‐made light source. A small 1.0‐milliwatt (mW) helium‐neon laser or diode laser pointer used in a lecture hall is typically ten times brighter than the sun. The high brightness means that high concentrations of energy can be achieved when the laser beam is focused to a small spot. The resulting concentration of light energy, if absorbed, (Figure 4) can elevate the temperature at the focus to extremely high levels which can burn or melt materials. It is this phenomenon that makes the laser both a useful surgical or material processing device and a hazardous source of light.
Under certain circumstances, when the laser light is concentrated to an extremely high level, the atoms in the focal zone of the laser beam can be ionized because the electromagnetic energy field is sufficiently intense to strip the electrons from outer atomic shells and directly ionize matter. This forms a spark referred to as an “optical plasma” which can be used to cut normally transparent structures including biological tissues (e.g. as used in the Nd:YAG ophthalmic laser photodisruptor for eye surgery) (10).
FIGURE 4 Radiance. The light from a laser can be totally collected by a lens and focused on a far smaller spot than can the light from a conventional optical source. This smaller focal spot of the laser contains far more concentrated light (a higher irradiance) than the focal spot of a conventional light source. Laser materiel processing and surgical applications, as well as fiber communications, rely on this property.
2.4 Wavelength
The optical radiation emitted by a laser can be in the UV, the visible, or the IR portion of the optical spectrum. Because of the quantum nature of the stimulated emission within the laser, only one or, in some cases, a few wavelengths of light are emitted. For example, the familiar argon laser emits most of its light in two wavelengths: 488 nm (blue) and 514 nm (green). The Nd:YAG laser emits most of its energy in the near‐IR portion of the spectrum at 1064 nm (1.064 μm) and a slightly weaker output at 1334 nm (1.334 μm). The mirrors and other optical components of the laser's resonant cavity are designed to favor a certain wavelength to enhance output power and suppress other wavelengths to aid in the production of a truly monochromatic output beam. Thus, it is always essential to specify the wavelengths at which a given laser is operating rather than to rely on naming the active medium to identify the laser system.
2.5 Pulsed and CW Operation
Many types of lasers have been produced that vary in their wavelengths and temporal patterns of output. Hundreds if not thousands of different active laser media have been discovered, but only a few types have the characteristics and properties that favor widespread use and have properties suitable for industrial, scientific, or medical applications.
Depending