A large, uniform, laser-based spectrally tunable irradiance field for optical sensors calibration

Abstract

Highly reflective diffuse boards or integrating spheres illuminated with lamps are routine laboratory equipment for 2D radiometric calibration. The traceability chain to validate the measurement results is from absolute cryogenic radiometer (spectral power and then spectral power responsivity) through filter radiometer (spectral radiance at single wavelength), standard blackbody radiation sources (spectral radiance over a wavelength range), and calibrated spectroradiometers (spectral radiance responsivity over a wavelength range), to the large area spectral radiance sources for large area optical sensors and imaging spectroradiometers. However, due to (1) the fairly long traceability chain, (2) the non-uniformity of lamp illuminated diffuse boards and integrating spheres, (3) the relatively low spectral power from the broadband source for high spectral resolution, and so on, the calibration uncertainties for spectral radiance and irradiance responsivity are above the level of a few per cents. In order to improve the calibration performance, tunable lasers have been widely adopted to generate a large, uniform radiometric calibration source. For instance, an integrating sphere can be illuminated with a wavelength tunable laser to form a good spectral radiance field. Though very high laser power is necessary if the exit port of the integrating sphere is large. An alternative way is to directly unify and expand the laser beam for a good spectral irradiance field. In this work, we built a large, uniform, spectrally tunable irradiance field based on a Ti-Sapphire laser. First, two double-convex lenses were employed as a beam expander; second, a rotary diffuser was placed at the common focal point of the two lenses; thirdly, the expanded optical beam was then delivered onto and through a micro lens array to form a highly uniform spectral irradiance field with a size of 30 mm by 30 mm. The non-uniformity was measured to be 0.24% over a 20 mm by 20 mm area. The total optical losses through the beam expansion and unification system would be significantly reduced compared with integrating sphere method. The traceability chain is from the absolute cryogenic radiometer (spectral power and then spectral power responsivity) and the effective radiometric aperture area to the photodetector with a precision aperture (spectral irradiance responsivity), and then to the large, uniform, laser-based spectrally tunable irradiance field (spectral irradiance) for the calibration large size optical sensors and imaging spectroradiometers. The traceability chain is short, the non-uniformity is low, and the signal-to-noise ratio is high, compared to that based on the lamp illuminated boards and integrating spheres. Furthermore, the laser-based spectral irradiance field can be easily converted to a large, uniform spectrally tunable radiance field by simply placing a large-size, highly reflective diffuse board in the irradiance field. The laser-based irradiance and radiance field would play an important role for radiometric calibrations with low uncertainties.