I. Introduction
National Institute of Standards & Technology (NIST) began its low background infrared source calibration efforts in the early 1970s. Since that time many needs have changed and applications have expanded. With the changes that are occurring, it becomes appropriate to review how NISTs calibration efforts have expanded, the rationale behind the development sequence used, new requirements being solicited, and new capabilities being developed. To accomplish those objectives, a two-day workshop was held on December 6 and 7, 1994, at NIST in Gaithersburg, MD. The meeting was called "NIST IR Metrology Standards and National Needs - 1994." It included prepared presentations from both NIST and NIST users, a meeting of the Users Advisory Board, and facility tours. Workshop organizers were Raju Datla, Group Leader, Infrared Radiometry, NIST; Robert L. Hinebaugh, Program Manager, Ballistic Missile Defense (BMD) Metrology, Newark Air Force Base, OH; and Milton Triplett, Nichols Research, Huntsville, AL, and chairman of the Users Advisory Board.
Most of the papers presented described methods and equipment used to test infrared sensors. For this Workshop, the term "infrared sensor" refers to a system, which includes infrared-energy-capturing telescopes, detectors, signal conditioning, signal processing, and possibly data processing. These sensors can be subdivided into broad application categories such as air/ground-based, space-based, Earth-viewing, space-viewing, and combined. These applications also serve to identify the key issues involved in ground testing and calibration. How these categories set key measurement parameters is tabulated as follows:
Infrared sensors used to measure point-source targets are commonly tested in chambers equipped with blackbodies and collimators. Blackbodies give the desired broadband energy output. The collimator makes the target appear to the sensor as a realistically distant object. For accurate calibration, the flux entering the sensor must be known. There are two approaches to determining this flux in a calibration facility: (1) measure the output of the blackbody and any intervening optical transmission or reflection and (2) measure the flux in the region in front of the sensor. The flux level at the blackbody is the easiest (and sometimes the only feasible) measurement but leaves the most questions. Measuring the flux in front of the sensor involves much of the same technology and difficulty as building the sensor itself but is much more direct.
In the 1970s, the National Bureau of Standards (now NIST) built a low background chamber equipped with a power substitution calorimeter. This chamber was used to calibrate blackbodies from the Air Forces Arnold Engineering Development Center and from the Armys Portable Optical Sensor Tester (POST) chamber. Concurrently, engineers from the active sensor test chambers proposed a round-robin telescope for measuring the flux in each chamber. The uncertainty level that could have been achieved with detector technology of that era is unclear. However, funding was never available for such at attempt.
When the Strategic Defense Initiative (SDI) program started in the early 1980s, it became clear that an improved calibration capability was needed. New chambers to test both exoatmospheric and endoatmospheric sensors were being proposed. A meeting was held at Nichols Research in 1985 under the sponsorship of the US Army Strategic Defense Command (SDC) to examine calibration source status and requirements. It was learned that the calibration chamber at NIST had developed an internal leak that would be very expensive to repair. Further, it was felt that lower flux levels needed to be measured and a more rapid turnaround time for calibration was needed. These items resulted in SDC drafting a set of requirements for and obtaining funding from SDIO for the facility that has now become the Low Background Infrared (LBIR) facility.
The LBIRs requirements were set to measure the total energy output from a blackbody within a small solid angle around the centerline. This technique produces a NIST traceable blackbody source package. Another technique is to use a NIST traceable temperature transducer and to assume that the blackbody source package can be characterized by the planck function. Tests performed in the LBIR chamber to date have demonstrated that this is frequently not true. Warm housings, warm choppers, and improperly aligned apertures produce non-Planckian results not related to source temperature measurement error.
Many blackbody sources are used at flux levels lower than the original LBIR requirements. The reduction in output is typically produced by integrating spheres, output aperture area, or neutral density filters. Integrating spheres are subject to spectral output shifts from contamination. Small aperture areas are subject to total or partial plugging from outgassing. Small aperture areas also cause difficult in quantifing diffraction effects. Neutral density filters are frequently not neutral but are variable with wavelength. All these effects spawned the improvement plan that has been ongoing since the completion of the original LBIR chamber. Work has proceeded toward making spectral measurements, improved measurement sensitivity to support spectral calibrations and sources plus attenuators, and attenuators as a separate component. Concurrently, work has proceeded toward a detector calibration capability. This will support both detector and sensor test chambers. NIST is currently developing a radiometer for measuring the collimator output flux in sensor calibration chambers used in NASA's EOS program.
NIST is currently soliciting a definition of needs from all potential users (DoD, DOE, NASA, commercial, etc.). The results of this survey will be used to determine what test capabilities need to be developed. Potential users with identifiable needs are urged to contact Raju Datla at NIST (rdatla@micf.nist.gov).