Advanced Calibration Source for Planetary and Earth Observing Imaging

Planetary and Earth imaging requires radiometrically calibrated and stable imaging sensors.  Radiometric calibration enables the ability to remove or mitigate sensor-specific and environmental anomalies from the data and to convert a sensor’s output to common engineering units (e.g. radiance or reflectance).  Additionally, radiometric calibration provides the means to compare image sets acquired by multiple sensors at different times and geometries. 

Sensor radiometric calibration and stability is typically accomplished by initially calibrating an imaging sensor in a laboratory before it is fielded or integrated onto a spacecraft and launched. National Institute of Standards and Technology (NIST) traceable integrating sphere based radiometric calibration sources have been the laboratory work horse technology for several decades.  However, with a few exceptions, many calibration systems are from vendors that sell designs and technology similar to those that were introduced into the market a decade or more ago even though advances in digital imagers and spectroscopic systems would benefit from several improvements.  In addition, the push for higher spatial resolution imagery is causing scaling issues with traditional designs.

A systems engineering approach was used to develop an advanced calibration source for imaging systems.  To accomplish this, a novel large integrating sphere, made from a fiberglass shell instead of a traditional aluminum shell, along with electronically adjustable high power LED light sources capable of producing a computer controlled uniform light source, was developed.  The resulting LED illuminated integrating sphere can be used to calibrate large optical systems over a large range of radiance levels. 

Radiometric calibration is critical to many NASA activities.  At NASA SSC, imaging cameras have been used in-situ to monitor propulsion test stand activities.  Radiometric calibration can help analyze the illumination produced by a test article during firing in order to detect defects or stop a test if required. 

Laboratory calibrations are typically performed using stable NIST traceable integrating sphere-based calibrated sources.  After launch, calibration is maintained using on-board calibration sources, ground-based vicarious calibration campaigns, and sometimes stellar and/or lunar calibration approaches. 

NIST traceable integrating sphere based radiometric calibration sources have been used for several decades. An integrating sphere source is a spherical shell coated with highly reflective diffuse material that is illuminated either internally or externally.  These spheres have ports that allow light to enter or exit. Sometimes the integrating sphere source will have a detector or spectrometer to monitor its output. An observatory class calibrator (~1.2-1.5 m in diameter) can radiometrically calibrate imaging systems over the 360-2500 nm (UV-SWIR) spectral range by incorporating tungsten lamps into the design.  In this project, LEDs were combined with tungsten lamps to produce a UV-SWIR source.  Operating the tungsten lamp at temperatures lower than their normal limits extends their lifetime while only slightly reducing their SWIR output.  Since operating at lower temperatures also lowers illumination in the UV and visible spectrum, LEDs were used to fill in that part of the spectrum.  In this way, the light source was made stable for better than a few percent for 1000 hours or more as compared to the typical 50 hours for traditional light sources.  This feature improved the robustness of the calibration source and potentially reduces future maintenance costs.