Concepts in dosimetry pertinent to hazard evaluation of optical radiation and specifically laser radiation are discussed. The basic units of power, energy, irradiance, exposure and radiance will be reviewed, as well as the relation of retinal exposures and experimental data given as intra-ocular energy to exposure limits specified in exposure at the cornea or time integrated radiance. Averaging apertures and field of views are specified with the exposure limits to be used when exposure values are measured or calculated which in turn are compared to exposure limits for laser radiation or broadband optical radiation. The size of the averaging aperture for irradiance measurements or the size of the averaging field of view for radiance measurements is closely linked to biophysical effects and dimensions such as the diameter of the pupil of the eye or the angular extent of eye movements. In some cases, the specified size of the averaging aperture and FOV result in measured irradiance and radiance values, which are much smaller than the real physical values. In the latest revision of the international laser safety standard, IEC 60825-1, and in the revised ICNIRP laser limits, blue light limits are split from the thermal limits and are given in irradiance, specifying corresponding measurement criteria for the measurement FOV. The derivation of the irradiance limit from the basic radiance limit as it is specified for the broadband blue light hazard (for instance by ACGIH and ICNIRP) can be understood on the basis of the specification for the measurement FOV.