An integrating sphere is a spherical shell that has its internal wall coated with a highly reflective, diffuse scattering material. It typically includes both entrance and exit ports where illumination sources and light monitoring sensors are added to produce a well-known uniform light source. Integrating spheres are used to calibrate radiometric instruments ranging from imagers to spectrometers. Sensors that need radiometric calibration used by NASA and the commercial aerial imaging community include aerial hyperspectral spectroradiometers, aerial multispectral cameras, and some moderate- and high-resolution satellite sensors. However, many of the larger sensors need large radiometric calibration integrating spheres, which can be costly and complex. Part of the issue is that a calibration source should simulate a solar spectra with high brightness levels. Achieving the spectral and brightness goals with traditional illumination sources, such as tungsten halogen and plasma arc lamps, requires a significant number of lamps. Traditional lamps are inefficient and generate a large amount of heat that must be dissipated. Another issue is that these calibration sources are typically manufactured using spun cast aluminum machining techniques, and because of this, a fairly thick coat of highly reflective Lambertian material must be applied to mask the manufacture-induced spun cast rings. These factors combined limit the widespread use of these radiometric calibration sources, especially for large-diameter optical sensors.
Therefore, a new lower-cost solar-simulated calibration integrating sphere was developed to absolutely radiometrically calibrate large camera sensors. The technology developed incorporates novel software-controlled, high-powered light emitting diodes (LEDs), and low-cost novel material spheres with highly reflective Lambertian coatings that can achieve National Institute of Standards and Technology (NIST) traceability calibration. Fiberglass, instead of spun cast aluminum, was used to design and manufacture the sphere. This material makes the sphere lighter, significantly reduces the production cost, and removes the possibility of imaging spun cast ring features that can appear in calibration imagery acquired with cast aluminum spheres. The sphere is lamped with white light LEDs, discrete color LEDs (blue, green, cyan, and far-red), and quartz-tungsten-halogen (QTH) lamps, in a unique combination that enables the ability to match the solar spectrum, thereby improving calibration. Advantages associated with using LEDs over incandescent lamps is that efficiency is much higher, fewer lamps are required to illuminate the sphere, and thermal management techniques are less complicated; if the LEDs are temperature controlled, they can be stable for thousands of hours. Taking advantage of the features associated with utilizing LED illumination makes this sphere easier to operate, and significantly reduces the frequency that the lamps need to be replaced. Software was developed to set the current to each LED so that, when combined with the QTH lamps, the resulting spectra optimally matches a given input solar spectra. By varying the input current to both the QTH and LED lamps, and if necessary, turning on and off selected QTH lamps, the overall illumination level within the sphere can vary by a factor of 10 to 20 while preserving a solar spectral shape. The lamps are electronically controlled — this electronic brightness control significantly reduces complexity and cost, while considerably increasing reliability (with no moving parts) and usefulness (with a large variation in illumination intensity).
In summary, the innovative technology developed is a large integrating sphere made with a fiberglass shell instead of a traditional aluminum shell, with electronically adjustable, high-power LED light sources producing a computer-controlled uniform light source that can be used to calibrate large optical systems over a large range of radiance levels. The technology has also been extended to smaller spheres, and can be applied to small-format aerial cameras, small satellites, and unmanned aircraft system (UAS) sensors.
This work was done by Robert E. Ryan and Mary Pagnutti of Innovative Imaging and Research for Stennis Space Center. For more information, contact Mary Pagnutti, (228-688-2452), or Innovative Imaging and Research, Building 1103 Suite 140C, Stennis Space Center, MS 39529. Refer to SSC-00451.