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Laser Color
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The color of a laser is determined by the wavelength of the light. The usual wavelength for a red laser is 650 nanometers (nm). A nanometer is unit of measure equal to 10 angstroms, which is one billionth of a meter.
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Brightness
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The human eye has different sensitivities to different wavelengths, so that a 635 nm laser seems about four times as bright as a 670 nm laser.
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Laser Power
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Laser power is in units of watts and indicates the “strength” of a laser beam. A watt is one joule of energy per second. This does not necessarily correspond to “brightness” which is wavelength dependent (see above). A green laser will be brighter than a red laser of the same power because the human eye picks up green better.
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Classifications
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The Center for Devices and Radiological Health (CDRH), part of the Food and Drug Administration (FDA), has laser safety classifications.
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Class I
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Lasers that are not hazardous for continuous viewing or are designed in such a way that prevent human access to laser radiation. These consist of low power lasers or higher power embedded lasers (i.e. laser printers).
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Class II
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Lasers emitting visible light, which because of normal human aversion responses, do not normally present a hazard, but would if viewed directly for extended periods of time. (like many conventional light sources).
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Class IIa
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Lasers emitting visible light not intended for viewing, and under normal operating conditions would not produce an injury to the eye if viewed directly for less than 1000 seconds. (i.e. bar code scanners).
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Class IIIa
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Lasers that normally would not cause injury to the eye if viewed momentarily but would present a hazard if viewed using collecting optics.
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Class IIIb
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Lasers that present an eye and skin hazard if viewed directly. This includes both intrabeam viewing and specular reflections. Class IIIb lasers do not produce a hazardous diffuse reflection except when viewed at close proximity.
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Class IV
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Lasers that present an eye hazard from direct, specular and diffuse reflections. In addition such lasers may be fire hazards and produce skin burns.
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