Radiometric terminology and physical characteristics of lasers

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Appendix A – Radiometric terminology and physical

characteristics of lasers

1 Radiometric quantities and units

1.1 Irradiance, radiant exposure and radiance

Irradiance is used to describe the concepts of flux density in units of watts per square

metre (W.m-2) incident upon a surface such as the skin, cornea or retina. The photobiological

term for exposure dose is radiant exposure which refers to the surface exposure in units of joule

per square metre (J.m-2). Here the surface area being irradiated is the defining area and it differs

from the orthogonal cross-sectional area of the incident radiation by the cosine of the angle of

incidence. The volumetric dose concepts are not generally used in the optical spectrum since

the radiation penetration is normally only superficial, and if it penetrates, as in the case of light,

it may still be absorbed at the surface, such as the retina in the case of visible light penetrating

the ocular media of the eye. Radiance is a physical concept for brightness, i.e. the radiation

emitted from an object per unit area and per unit solid angle. The unit area here is taken normal

to the axis of propagation of radiation. This concept is widely used for specifying exposure

limits for extended sources (large areas). The units or radiance are watt per square metre per

steradian (W m-2, sr -1), where the steradian is the unit of solid angle. The CIE standard symbols

for irradiance, radiant exposure, and radiance are: Ee, He, and Le respectively. These and related

quantities and units are presented in table Al.

1.2 Photometric quantities and units

If the above radiometric quantities are mathematically weighted against a spectral

sensitivity function, such as V (), the photopic (daylight) sensitivity function of the eye, new

quantities are formed: in this case photometric quantities. Normally, the photometric quantities,

such as luminance Lv, illuminance Ev, and luminous exposure Hv, are not used extensively in

considering the health effects of optical radiation, except in the special case of evaluating the

potential for flashblindness, or the production of after images by laser light.

1.3 Spectral definitions

Optical radiation is located in the electromagnetic spectrum between the softest ionizing

radiations on one side and microwave radiation on the other. According to this definition, the

short wavelength boundary is not very precise, but is generally accepted to lie between 10 and

100 nm. The value of 100 nm corresponds to a photon energy of approximately 12 eV, and is

generally accepted as the limit of production of single-photon ionization in biological systems.

The optical spectrum is divided into spectral bands based upon different physical and

biological effects. The various bands will differ according to the criteria used. For example, the

band below 180 nm is the region of vacuum ultraviolet radiation (UVR) which is absorbed by

air to such an extent that no biological effects can be realized. The remaining UVR spectrum is

then frequently divided by optical engineers into the far-UVR region between 180 nm and 300

nm and the near-UVR region between 300 nm and 380 nm. A somewhat different scheme used

by the CIE takes some of the biological effects into account and divides the UVR spectrum into

three bands: UV-A, UV-B and UV-C (CIE, 1987). These CIE divisions are used throughout

this overview.

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