The fact that lasers give off radiation may be a surprise to some. Let's clarify this point right away.
Laser light, x rays, sunlight, microwaves and broadcast radiowaves are all similar radiations that move in wave-like patterns. Each of these have different wavelengths. The radiations range from x rays, with waves less than 1 billionth of an inch long, past radiowaves with waves several miles long. Those with short waves are more energetic than those with longer waves. The difference in wavelength accounts for important differences in the physical properties and in the biological effects of the various types of radiations.
So, although laser light is part of this family of radiations, it should not be con- fused with the others. For example, un- like x rays or radioactivity, visible light radiation has not been associated with causing cancer or genetic damage.
Light radiation falls on the spectrum of radiations ranging from ultraviolet through infrared (or heat). Within this range, only a small band of wavelengths is visible to the human eye. Each color that we see is actually light radiation of a particular wave 1ength. Visible light spans from violet, with short waves, to red, with longer waves. Lasers generally give off visible or optical radiation; some lasers can also give off radiation in the ultraviolet or infrared ranges that we can't see. Of course, lasers used in light shows give off visible radiation.
Essentially, the laser light that creates such spectacular and exotic effects, is the same radiation that comes from an ordinary light bulb ... but it has some important differences. Laser light can have the purest and brightest of colors. And it can be thousands of times more intense than the light by which we read.
The light from a light bulb radiates in all directions. If you were able to separate and trace the waves of light, you'd see a jumble of different wavelengths, and directions. In the light from a light bulb all the colors of the spectrum (i.e., the various wavelengths) are present and add to each other so that the light appears white.
Because the light from a light bulb spreads out, its power falls off or decreases as you move farther away. This is because of a property called "divergence." Think of a flashlight, whose light beam spreads out as you move farther away from it. This divergence or spreading out of the beam means that the power of the light is spread over an in- creasingly larger area as you move farther away.
Laser light is quite different. All the light waves in a laser beam can have the same wavelength. Furthermore, they are in phase with each other. They travel in locked-step or synchronized pat- terns. This unique property of laser light is called "coherence." Again, color depends upon the wavelength. Since a laser beam is composed of light of the same wavelength, it has an extraor dinarily pure color.
And, most important, unlike the flashlight, a laser beam does not diverge or spread out very much. The laser light can travel in a very narrow beam even over long distances. Because of this, its power can be extremely concentrated. In fact, some lasers can produce a beam of light that, even miles away, can be thousands of times brighter than the sun's surface appears from earth. The fact that a laser beam can retain such high power, even over long distances, partly accounts for its use in light shows and many other applications. But this same fact also accounts for its potential hazard.
A laser beam loses very little power when it is reflected off a smooth, shiny surface. When the light from a light bulb is reflected off a mirror, it continues to diverge and spread its energy over even larger areas. When a laser beam is reflected off a mirror or other smooth, shiny surfaces, such as water, glass, metal beams or a glossy floor, it still does not diverge very much. So a reflected laser beam can have almost the same power and potentially the same hazard as a direct laser beam.
Ordinary light reflects off a mirror and its beam continues to diverge.
Laser light reflects off a mirror and still does not diverge very much.
Mirror balls are frequently used in light shows to separate and reflect the laser beam into many rays of laser light. When done properly, this can significantly reduce the power and, therefore, the potential hazard of a laser beam. If the beam is reflected off enough facets on the mirror ball, the resulting rays will go off in many directions. Although the individual rays still do not diverge very much, each has only a fraction of the power in the direct beam. Obviously, the degree of safety that this can produce depends upon the power of the direct laser beam, and the number of rays and directions into which the beam is split.
The more rays into which the beam is split, the smaller the fraction of power each reflected ray will have. A scanning device is usually used to sweep the beam back and forth across a broad section of the mirror ball so that the beam is broken up by several facets on the ball. Rotating the mirror ball can provide even more safety because the movement of the reflected rays reduces any exposure time. Without a scanning device, or without a properly designed scanning system, the beam is broken by the mirror ball into fewer rays, each having a larger fraction of the power in the direct beam. This means that even with a mirror ball there could still be a potential for harm.
If a laser beam is reflected off a rough or irregular surface, like a concrete wall or even some "walls" of smoke, the irregulari- ties in the surface scatter the beam in many different directions. The beam is forced to diverge and therefore lose some of its power. However, a very high powered laser beam can still retain enough of its intensity after reflecting off a rough or irregular surface to cause injury. In addition, some rough surfaces may have shiny spots that allow for a mirror-like reflection of part of the beam.
Source: Excerpted from Laser Light Show Safety: Who's Responsible? PDF June 1980; Revised May 1986