Characteristics of laser radiation

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Characteristics of laser radiation

2.1 Radiometric quantities and units and terminology

The physical (radiometric) terms and units used in the optical region of the

electromagnetic spectrum are standardized by the Système International d'Unités (SI).

The International Commission on Illumination (Commission Internationale de

l'Eclairage, CIE) together with the International Electrotechnical Commission (IEC)

publishes a standardized vocabulary of lighting terminology that includes definitions of

radiometric and photometric terms, quantities, and units (CIE, 1989) which are used in

this document. There are only a few terms, quantities, and units in widespread use when

specifying exposure limits for the health protection of workers, and these are discussed

in detail in Appendix A.

2.2 Types of lasers

There are a number of methods to group or categorize lasers depending upon

wavelength, pulse characteristics, active medium or pumping process. Lasers may be

pulsed or continuous-wave (CW) depending upon the duration of excitation of the active

medium by pumping. The duration of a pulse may vary from femtoseconds (10-15 s) or

picoseconds (10-12 s) to larger fractions of a second. If the laser emits pulses of duration

less than 1 nanosecond (10-9 s), it will normally be a "mode-locked" laser. If the laser

emits pulses of the order of several nanoseconds (ns) to several 100 ns it is referred to as

a "q-switched" laser. If the emission of an optically pumped laser follows the normal

pulse emission of a flashlamp, then the laser is normally referred to as a "long-pulse" or

"normal-pulse" laser. For safety purposes, lasers which emit continuously for periods

greater than 0.25 s are referred to as "CW lasers." Lasers which emit groups or "trains"

of pulses are referred to as "repetitively pulsed" and the frequency of pulses is referred

to as the "pulse repetition frequency" (PRF). By contrast with conventional light

sources, the laser is coherent, normally collimated and monochromatic, as shown in

figure 1.

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