NPL’s Radiometric calibration and characterisation facility provides direct access to SI traceability at the highest accuracy possible for a range of quantities in the optical domain (UV, 200 nm through to TIR, ~100 μm) both pre-flight and post-launch. The facility is constructed so that it consists, where possible, of transportable systems that can travel to other sites in the UK and elsewhere to facilitate delivery of measurement services at point of need e.g. at a vacuum facility, or in a desert. The measurement capabilities are underpinned by a cryogenic radiometer as the primary standard, which can measure the power of an intensity stabilised laser beam to better than 1 part in 104 or 0.01%. A version of the cryogenic radiometer measures total solar irradiance and is deployed at the World Radiation Centre in Davos to provide the reference for the World’s solar radiometers.
Radiometric Calibration Facility detail
The NPL facility provides calibration of: detector properties (responsivity, linearity, uniformity etc.) at ambient to cryogenic temperatures, high and low valued reflective and transmissive properties (angular, BRDF, polarised) of materials and surfaces (including land/snow surfaces in the field) spectral radiance/ irradiance/brightness temperature. The services can be delivered in a variety of ways through a range of systems, tailored to meet the user requirements be it a, sub-system or end-to-end calibration of a full instrument. The capabilities are all state-of-the-art, and are maintained and improved through resources of the national measurement system programme of BIS. They include the novel use of spectrally tuneable laser radiation capable of providing high spectral resolution for wavelength/band shape and radiance simultaneously and a variety of transfer standards, instrumentation, and infrastructure that can be loaned or utilised on an as-needs basis. As part of its work with ESA it has established, and will soon operate, a test site in Namibia for the post launch radiometric gain calibration of solar reflective imaging sensors such as Sentinel 2.