The technological innovations embodied in the instrument described in this paper can significantly improve the performance of black bodies and their associated thermometry and electronics as used for on-board calibration in many space-borne infrared radiometer instruments, whilst simultaneously reducing mass and power requirements. The same innovations can provide similar advantages for terrestrial based calibrations for both pre-flight and validation activities.
Traditional spectrophotometry, including regular and diffuse scattering measurements of reflected and transmitted light, employs lamp-based sources of radiation which are spectrally filtered by a spectrometer or by individual optical filters. Lasers offer distinct advantages to the traditional sources, most notably high spectral power and the possibility of very small irradiated area. NPL has established a new laser-based facility for reflectance and transmittance measurements which utilises NPL’s National Laser Radiation Facility, a suite of lasers covering a broad spectral range from the UV to the IR. This paper will highlight the capabilities of this new facility, discuss several of its unique design features, and present an analysis of the factors which influence its ultimate accuracy.
KEYWORDS: Diffraction, Black bodies, Sensors, Radiometry, Temperature metrology, Roads, Near field diffraction, Tolerancing, Geometrical optics, Far-field diffraction
Estimating the effects of diffraction is essential in modern
radiometric experiments. The majority of the tools used for this
date back to the pioneering work by W. Blevin and W. Steel. These
were analytical in nature, obtained by aggressive use of approximate techniques applied to the Fresnel diffraction integral; further, blackbodies were treated as uniform sources that could be described by a single characteristic wavelength, enabling diffraction effects to be determined through a single monochromatic calculation. This requires diffraction effects to change linearly with wavelength. The domain over which this is satisfied to the error tolerance required by contemporary radiometry is unclear. This paper investigates the single wavelength technique and establishes criteria for its use.
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