Measurement of Optical Radiation to Assess the Blue Light Hazard

For the safety assessment of optical broadband radiation, separate limits are given for thermal and for blue light hazard to the retina by ACG1H and ICNIRP. For small sources, the blue light limit is given as a radiant exposure limit in J/m2, however for extended sources the basic limit is specified as radiance in J/m2 sr. For the measurement of radiance, the size of the field of view (FOV) of the measurement detector is an important factor, as the radiation is averaged over the FOV, even if the source size is smaller than the field of view. Limits given in radiance can be transformed into irradiance limits by multiplying the radiance limits by the corresponding measurement FOV. For photobiological limits it is important to note that the measurement FOV corresponds to the extent of eye movements and is therefore time dependent. In the latest draft for the 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. This paper will discuss the derivation of the irradiance blue light limits from the broadband radiance limits and will discuss the importance and applicability of a well defined measurement FOV.