UNCERTAINTY BUDGET ASSESSMENT FOR PRACTICAL ASSESSMENT OF THE RETINAL HAZARD OF EXTENDED LIGHT SOURCES IN ACCORDANCE WITH IEC 60825 AND IEC 62471 GUIDELINES

摘要

The rapid pace of technology has produced devices that have some characteristics of both lasers and "non-laser" devices. Examples of these optical sources are high brightness light emitting diodes and supercontinuum laser sources. This has led to an explosion of interest in the practical evaluation of the photobiological hazard posed by broadband extended light sources. The various photobiological safety standards, such as CIE S009, IEC 62471 and IEC 60825, provide indicative diagrams and information concerning the practical evaluation of the photobiological hazard potential of these sources. These involve: 1. measurement of the spectral radiance and irradiance of the source under defined geometric conditions (these allow for factors such as the minimum size of image that can be formed on the retina and the effect of eye movements); 2. weighting the results with a defined action spectrum (which allows for the relative spectral effectiveness of optical radiation for the specified photobiological effect) to determine the exposure hazard value (EHV); and 3. comparison of the EHV with defined permissible limits (i.e. conditions under which it is believed that nearly all individuals in the general population may be repeatedly exposed without adverse health effects). However, although these standards describe the measurement procedures and processes for determining the EHV for the source in question, the determination of the uncertainty associated with these measurements is seldom discussed. This paper will present an approach for establishing an uncertainty budget for assessment of photochemical and photothermal retinal hazards, based on evaluation of the practical limitations of the measurement equipment and procedures used and their impact on the measurement results. In particular, a simple software based stochastic process will be described, which allows the influence of the various uncertainty contributors to be explored dynamically; this helps to ensure that attention is paid to the most important contributory factors (such as field stop placement, spectral resolution etc.) so as to minimise the uncertainty associated with the determination of the EHV. The use of this approach to determine the blue light hazard EHV for a white LED will be presented as an example. Based on this analysis, it will be shown that certain measurement uncertainties (such as the wavelength calibration of the spectroradiometer) have very little impact on the EHV, while others (such as the area of the field stop) are much more significant. This information could be helpful in preparing improved guidance for measurement procedures and for assessing 'typical' uncertainties for a given measurement set-up.