Apparent Source Size Dependence upon Measurement Optics (Applying IEC 60825-1 AEL Classification to Unusual Sources)

摘要

Most European countries have optical radiation safety regulations based on the International Electrotechnical Commission (IEC) Standard 60825-1, Safety of laser (& LED) products. Having started as a laser safety standard, the standard included light emitting diodes (LEDs) in 1993. Unfortunately, the manner of inclusion greatly overstated the potential hazard of most LEDs. While efforts are underway to continue the work to correct this problem, there are some aspects of LEDs that make them quite unlike lasers. Apparent source size is a critical parameter for determining Accessible Emission Level (AEL) limits according to the IEC standard. For some range of this parameter, doubling it would allow a fourfold increase in emitted power or energy. The apparent source is defined as the real or virtual object that produces the smallest possible retinal image. While, for many sources, this is well defined and well behaved, for some light emitting diode (LED) structures it is not. For some of these structures, the apparent source size is a function of viewing conditions. Thus, the safety standard and derived regulations cannot be simply applied. In this paper, a method of determining effective classification of these sources is presented. Rather than calculating a single limit, a range of limits is determined from the range of device apparent source size, which in turn, depends upon viewing conditions. The limits are presented as irradiance of the cornea, and include the effect of light gathering optics. Measured powers or energies for the range of viewing conditions are compared to the limits. Satisfying the constraint that the measured powers or energies does not exceed the calculated limits for that class of operation over the viewing condition range is interpreted to mean the emitter does not exceed that class of operation.