Spatial uniformity of the spectral radiance by white LED-based flat-fields
Marco Talone, Giuseppe Zibordi
Abstract
The spatial uniformity of the spectral radiance by four commercial flat-field sources relying on white Light-Emitting Diode (LED) technology were investigated in the 400–800 nm spectral range using a hyperspectral radiometer. Results showed that for most of the flat-fields the spatial non-uniformity defined by the standard deviation of radiance differences across the source area in a given spectral interval, is typically lower than 1% within 5 cm from the center of the source and not exceeding 5% within 10 cm. The corresponding impact on radiance spectral distribution indicated non-uniformity values generally lower than 1.5% within 5 cm from the center of the sources, but increasing to several percent beyond the 5 cm distance. Finally, the spectral noise of radiance measurements defined by the variation coefficient across repeated acquisitions of the same target area on the sources exhibited mean values between 0.02% and 0.1% for most of the flat-fields in the 420–700 nm interval, but reached several percents outside such a spectral region as a result of the low flux from white LEDs. The above non-uniformity features of white LED-based flat-fields were shown to generally outperform those of a custom flat-field built of a lamp illuminating various diffusers. These findings indicate that flat-fields based on white LED technology may be suitable radiance sources for applications whose measurement requirements are satisfied by the spectral distribution of the radiant flux by white LEDs.