Environmental Remote Sensing
Spectralon® integrating spheres used in airborne atmospheric measurements
Integrating spheres were first developed in 1894, not long after the invention of the electric light bulb. Today, integrating spheres are widely used in the measurement of light sources, optical radiation and for testing the optical properties of materials. One of the more interesting recent applications we have encountered was the use of integrating spheres mounted on a jet aircraft flown by the UK’s Met Office as part of a programme to improve weather and climate predictions.
Scientists within the Met Office’s Observations Based Research (OBR) group undertake field campaigns to gather research-quality observations of key physical processes within the atmosphere and at the Earth's surface. These studies provide the necessary physical insight required to develop an improved understanding of these physical processes which in turn is used for weather forecasting and climate prediction. The Met Office has been involved in airborne research since 1942. In 2001 the Met Office entered a new partnership with the Natural Environmental Research Council (NERC) for the jointly-funded provision of a state-of-the-art atmospheric research aircraft. The BAe-146 aircraft is operated out of the airfield at Cranfield University, Bedfordshire and is known as the Facility for Airborne Atmospheric Measurements (FAAM).
For the Met Office, it is important to know the emissivity of the Earth's surface if the infrared satellite irradiance measurements recorded over land are to be used for weather predictions or climate studies. The emissivity defines how much of the upwelling irradiance at the Earth's surface is due to thermal emission from the surface and how much is reflected irradiance from the sky. Land surface emissivity is a very variable quantity, but unfortunately there are no global maps of emissivity. There are techniques for estimating it from space, but these always require assumptions about the intervening atmosphere. Instead, the Met Office exploits the ability of the FAAM aircraft to fly low over the land and measure the emissivity directly. The technique involves taking measurements of both upwelling and downwelling irradiance. By recording both surface and sky irradiance, the Met Office can determine how much of the sky irradiance is reflected by looking at the surface irradiance and comparing it to the sky irradiance. These ratios enable them to derive the spectral emissivity of the region of the Earth’s surface of interest.
The Spectral Hemispheric Irradiance Measurement System (SHIMS) instrument flown on the aircraft employs two, custom-made Labsphere Spectralon integrating spheres, one mounted on the upper and one on the lower fuselage. The integrating spheres provide for a near-perfect cosine angular collection of spectral irradiance in the wavelength range 300-1700nm. For accurate irradiance measurements, it vital that a radiometer correctly accounts for the reduction in irradiance from light that is received at high angles of incidence. The irradiance from a light source decreases with the cosine of the angle to the normal. It has been established that the most accurate means of imparting a cosine spatial response onto an optical detector is through the use of a correctly designed integrating sphere.
Ian Rule of the Met Office worked closely with Pro-Lite to develop a custom integrating sphere design capable of providing the required optical performance whilst being able to withstand the rigours of use on an aircraft. Labsphere’s Spectralon was chosen as the optimum sphere coating material for this application due to its high reflectance, extreme durability and environmental stability. The SHIMS sphere is shown below installed in its ruggedised housing.
In a related development, Pro-Lite also supplied a custom Labsphere uniform light source to the Met Office. This is used to calibrate the Met Office’s Short Wave Spectrometer which measures radiance in the 300 to 1700nm band. The uniform light source is based upon a ½ meter integrating sphere equipped with four tungsten halogen lamps which provides a spatially and angularly uniform output of known spectral radiance. In order to support the use of the light source outdoors, Labsphere coated the integrating sphere with its special, water-proof integrating sphere coating, Duraflect™.
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