Product Code: ILSC2007_1506

Using Broadband Detectors for Measuring the Output of Broadband Sources such as White Light LEDs
Authors:
Woody Strzelecki, TUV Rheineland of North America; Newtown CT USA
Nikolay Stoev, Valkom Laser Consulting; Toronto ON Canada
Presented at ILSC 2007

Broadband sources can be measured in different ways. Monochromators are used for measurements within narrow spectral intervals (typically a few nm). Bandpass filters are used for measurements within larger spectral windows, usually tens of nm. Special detectors, calibrated against the appropriate action spectrum may be used to directly measure the effective radiation values, i.e. the source emission weighted using the hazard functions. Each method has some drawbacks. Monochromators are subject to stray light problems and may be difficult to use for absolute measurements. Bandpass filters are difficult to use when the Condition 2 measurement distance is short. Additionally, filter transmission curves need to be considered for re-calculating the measurement results. Broadband hazard sensors attempt to match the weighting spectra by special filters, but the matching is never exact and leads to some amount of error.

Straight forward measurements are possible when both the relative spectral intensity of a source and the detector's responsivity are known. This approach is mentioned in IEC 62471/CIE S009, but no technical details are provided. The presented method allows the use of common laser power meters. A measurement is performed at a preset wavelength and the reading is multiplied by a correction factor based on the source's relative intensity and the detector's responsivity. As a result, the absolute spectral intensity is determined. The correction factor calculation is explained. Tests at several wavelength settings were performed to validate the method including assessment of its accuracy. The method can be used to measure radiant power, energy, irradiance, radiant intensity, radiance and other radiometric parameters of broadband sources.