Linearity and spectral characteristics of integrating sphere sources

In order to calibrate long wavelength infrared (LWIR) sensors, low temperature sources which approximate blackbody radiators are required. To calibrate a sensor over its entire operating range, the dynamic range of these sources must be large. One technique for extending the dynamic range of such sources and attaining the low irradiance levels required involves the use of integrating spheres as attenuation devices. Aperture wheels are used with these sources. Questions concerning linearity performance and spectral qualities arise when these types of sources are employed. This study addresses these issues by examining the performance of two different types of integrating sphere sources as monitored during several calibration efforts. The two types of sources considered were a Unisphere (a cavity radiator with a single integrating sphere) and a capsule (a dual integrating sphere arrangement with a capsule heater mounted inside one of the spheres).

A radiation monitor telescope (RMT) was used to calibrate the integrating sphere sources. This device contains a reference cavity source which has been calibrated and is traceable to the National Bureau of Standards. The detector in the RMT is a photoconductor (Ge:Cu) which can detect sources over seven orders of magnitude. A filter wheel in the RMT is used to provide spectral characterization of sources.

The responsivity of the RMT detector is not as constant with source temperature as desired, even when the relative spectral response of the detector is included in the calculations. It appears that the relative spectral response characteristics of the detector are suspect. It is recommended that these values be checked and the true spectral response characteristics determined.

Several methods of linearity characterization were evaluated. All of the methods examined were found to be valid. The preferred method was determined to be the one based on signal attenuation ratios. The results obtained at the different aperture positions were highly repeatable. Thus, linearity characteristics can be accounted for in source output calculations. It was determined that a single calibration constant can be determined and applied to all apertures associated with a particular source without greatly increasing the source output uncertainty.

Based on theoretical considerations, the RMT source has an effective emissivity greater than 0.99. The RMT source spectral ratios obtained with the filter wheel do not agree as closely with predicted blackbody ratios as hoped, even when the spectral response of the detector is accounted for. The Unisphere and capsule values agree reasonably well with the RMT source spectral ratios. When a large aperture is used with the Unisphere source, the spectral ratios obtained are closer to those predicted for a blackbody. All of the sources examined follow the Stefan-Boltzmann relationship over the temperature range investigated. The calibration constants determined did not vary significantly with source temperature. Although no indication of any major spectral problems associated with the integrating spheres was found for the conditions evaluated, several significant improvements need to be made in order to provide accurate spectral calibrations of such sources.