Product Code: ILSC2007_1507

Method to Measure Apparent Source Location and Angular Subtense of Extended Sources with Additional Optics
Authors:
Clark Venzke, Banner Engineering Corp; Minneapolis MN USA
Presented at ILSC 2007

In using the international laser safety standard, IEC 60825-1, to classify a laser or light emitting diode (LED) source, the angular subtense (α) may need to be determined to calculate the accessible emission limits (AELs). The angular subtense is used to calculate the extended source correction factor (C6) and to determine the measurement geometry over much of the retinal hazard wavelength range. The easiest way to find a value for the angular subtense of the apparent source is to assume a conservative default value of 1.5 mrad (αmin). For many laser sources this is good assumption, but for extended sources such as LEDs, this overly conservative assumption can lead to underestimating the AEL by more than an order of magnitude compared to the value calculated from an actual measurement of the angular subtense. For LEDs or diffused beams without modifying optics, the location and size of the real source are the same as for the apparent source and the angular subtense can be easily determined. With the addition of optics (integral lens, projection lens or reflector) to the real source, the apparent source location and size are changed and the measurement of these features can be more challenging. This paper presents a straightforward method to optically locate the apparent source under these conditions of additional optics with an imaging system (such as a CCD camera) and to calculate the angular subtense based on a calibrated image. With information derived from this method, a more realistic calculation of the AEL and an improved measurement geometry for an extended source can be found.