Instrumentation and measurement techniques

• Pages: 13-16

5

Instrumentation and measurement techniques

In any discussion of the measurement of laser radiation for the purpose of

evaluating health hazards, one should first consider the necessity of measurement. As a

general rule measurements are required only of the laser output for the purpose of

determining the laser hazard classification. Routine monitoring is seldom considered

necessary and measurements are performed by the laser developer or manufacturer.

However, where exposure is intentional and in the outdoor environment, it is often

necessary to measure laser beam irradiance or radiant exposure down range.

5.1 Laser parameters to measure

One can calculate the irradiance or radiant exposure at any distance from a laser.

To do this the output power or energy, the initial beam diameter and beam divergence

must be determined.

One can use a calorimeter or other types of energy or power meters to measure

the output energy or power. The measurement of output beam diameter or divergence

can be more difficult. Measurement of the beam divergence is critical for determining

the potentially hazardous viewing distance (nominal ocular hazard distance (NOHD).

Power-through-aperture measurements may be employed to determine the effective

beam diameter and divergence.

From the standpoint of hazard analysis, it is necessary to know the maximal

output radiant exposure of a pulsed laser to determine if a diffuse reflection hazard

exists. The simplest technique for this purpose is the use of thermally or

photochemically reacting surfaces (beam-profile paper). In other cases where the beam

irradiance is insufficient to cause a surface change in the special beam-profile paper,

conventional photography or a radiometric instrument which has a sufficiently small

aperture must be used.

5.2 Types of radiometric instruments

Radiometric instruments of interest to this discussion generally consist of a

detector which produces a voltage, a current, a resistance change, or a charge which is

measured by a sensitive electronic meter.

The detector is the primary determining factor in selecting an instrument. Each

type of detector has certain characteristics which may pose either an advantage or a

disadvantage for measuring a certain level of optical radiation in a certain wavelength

range. No one type of detector can serve for measuring all types of laser radiation. A

very sensitive detector can be destroyed by a high-power laser beam.

13