Current ocular safety standards associated with the application of light emitting diodes (LED), and other intermediate sources, cite the angular subtense of the apparent source as an essential quantity from the apparent source size of the LED package and the specified most hazardous viewing distance. However, an international standard for the measurement of the apparent source size parameter does not yet exist. This report describes the results of a study that provide rigorous practical support for a technique proposed for the measurement of apparent source size when observed from the most hazardous viewing distance. The results of this study allow, for the first time, an estimate of the potential optical significant step in reducing the ambiguity that currently exists in the application of these optical safety standards. The results also verify earlier numerical modelling of an improved method for the estimation of the critical angular subtense parameter for extended sources, such as LEDs and intermediate sources.
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