At present, National Bureau of Standards (NBS) work in the Long Wave Infrared (LWIR) spectral region is performed using three calibration facilities. These facilities are: 1) a spectral calibration facility for "medium temperature" (350-1300 K) blackbodies, 2) a broadband facility for calibrating blackbodies (-50 - +150°C) against a room temperature background and 3) a facility for calibrating blackbodies (150-600 K) in a cryogenic environment. Two recent calibrations for the LWIR community will be described to illustrate the features and limitations of the present NBS facilities: for the first calibration [in facility (3)] the total radiant output at a source in a small, well-defined solid angle was measured using an electrically calibrated absolute radiometer; for the second [in facility (2)], a source was radiometrically compared with a high-quality, variable temperature blackbody using an IR radiometer with a variable wavelength filter. Future plans to enlarge and upgrade the cryogenic facility to provide increased sensitivity will be described. Systematic studies of the errors due to diffraction, polarization, and attenuation are planned. Future work to explore the possibility of basing calibrations on self-calibration techniques with LWIR detectors will also be described.

The Navy Primary Standards Department (NPSD) is charged with maintaining and disseminating the fundamental standards of measurement for the U.S. Navy. Measurement areas include Electrical (standard volt and resistance), Microwave, Temperature, Mass, Pressure, Dimensional, Flow and the topic of this paper, Radiometry, Photometry, and Electro-Optics. This paper is divided into four sections. In the first section we describe the primary measurements performed. These include blackbody calibrations, photometric lamp cali-brations, spectral detector, filter and source calibrations, laser power and energy measurements, and optical pyrometry calibrations. Also included in this section are state-ments of the measurement uncertainty. In the second section we describe the primary standards maintained by the laboratory, the process used to maintain traceability to the National Bureau of Standards (NBS), and the processes used to provide the measurement to lower level laboratories. In particular, we discuss fundamental reference standards such as zinc and tin freeze point blackbodies, Measurement Assurance Programs (MAPs), and the use of primary standards calibrated by NBS. We discuss measurement system development in the third section of the paper. Major topics include automation, system standardization, and development of system uncertainties. The principal systems described are a blackbody calibration system, a spectral calibration system to calibrate detectors, filters and sources, and a laser power and energy calibration system. Finally, we discuss the new primary measurements needed to support future Navy requirements. Main topics are near and far field laser beam profile measurements, low light level measurements, high energy laser measurements, fiber optics measurements, and the development of systems that will extend the measurement of electromagnetic radiation to wavelengths of up to one millimeter.

The recent development of a new long-wave infrared (LWIR) sensor test facility^l presented a number of interesting problems during the course of the optical system alignment. This facility, the Lockheed Sensor Test Facility (STF), was designed primarily for calibration and system testing of LWIR sensors in the 5 to 30 pm range. It combines a low background vacuum chamber with a unique wide field-of-view optical system. This paper will discuss the facility and its optical instrumentation with emphasis on assembly, metrology of alignment, optical characterization, tasks complicated by manufacturing oversights, and an aluminum support structure which shrinks over 1 in. when it is cooled 250 K to the cold operating temperature.

The Advanced Sensor Evaluation and Test (ASET) facility is used to characterize infrared sensor performance under conditions simulating those of outer space (viz., low radiant back-ground and high vacuum). One function of the facility is to provide to the sensor under test a well collimated beam having controllable irradiance covering seven (or more) decades of dynamic range, with spectral character approximating that of a 300K temperature Planckian source. The sensor response to these stimuli yields a sensor response function, provided that the stimulus (i.e., the test beam spectral irradiance) is accurately known over the wavelength region of interest (4 pm to 24 μM). To obtain this information a Spectroradi-ometer Assembly (SRA) was designed and built. It is based on a Czerny-Turner grating monochromator with beam collecting optics, and a Si:As photoconductive detector. The task of covering the dynamic range of interest translates to a dynamic range of nine (or more) decades of detector signal. The unavailability of a linear detector necessitates a complicated calibration procedure. Further, the only "standard" on which the calibration can currently be based is a simulated blackbody. Because of limitations imposed by the space available, the spectral irradiance provided by such a reference is much greater than that normally obtaining in the test beam, thus further increasing the complexity of the calibra-tion. The approach taken in the current calibration is to first calibrate the SRA using a blackbody as a "standard" source, taking steps to account for detector nonlinearity. Then, using the calibrated SRA, the spectral irradiance of the test beam is determined. The results are given as spectral transfer functions of the ASET optical system, and samples will be presented. In conclusion the results of a detailed error analysis will be given.

In recent years there has been a demand for increased accuracy and precision in the calibration and alignment of visual optical systems for military use. This has led to the development of a number of innovative techniques involving the application of basic optical metrology principles to the testing of these systems. This paper will deal with some of the basic instruments used in such tests, their capabilities and limitations, and will describe in some detail the procedures involved. A typical military instrument will be considered, its optical requirements will be outlined and the instrumentation and procedures employed to verify conformance will be discussed. Typical metrology instruments involved include; Microscopes, Collimators, Theodolites, Dioptometers and Dynameters. Performance characteristics evaluated using these tools and procedures include; magnification, field of view, entrance and exit pupil size, resolution and line of sight scan accuracy. While the level of detail to be discussed will be somewhat superficial, the concepts presented should prove useful in solving a variety of similar problems related to the testing of instruments of this general nature.

Considerable effort at The Eppley Laboratory, Inc., over the past decade has been directed toward the accurate measurement of solar irradiance and the design and development of low temperature blackbodies. Additionally, we have been involved with the design and production of thermopile-type sensors for special applications. The implications of refering all relevant measurements to a unified reference scale of radiation are always of concern. In this paper, self-calibrating radiometers with cavitated receivers which can be designed for a number of total irradiance measurement tasks will be described. Among these tasks are the measurement of solar irradiance, both direct beam and over larger fields-of-view, and the measurement of earth emitted flux (generally from satellites), for which the low temperature blackbodies act as simulators. The radiometer can also be a-dapted to a number of laboratory measurements such as the output of lamp or laser sources. The basic type of sensor employed in the Eppley radiometers is a circular, wirewound and plated thermopile, sometimes called toroidal thermopile. Self-calibration is performed using a heater on the cavity receiver which is employed in an electrical substitution mode. The difference in response produced by the heater to that produced by the radiation source is called the "nonequivalence." Determination and application of the correction factors including the components of the nonequivalence term will be discussed. Results of inter-comparisons with other self-calibrating radiometers will be presented.

Hewlett-Packard Company introduced a new laser system based upon a new HeNe laser tube in September 1982. The 5528A Laser Measurement System is finding wide spread use in industry for improving productivity in the numerically controlled machine shop, and is now being designed into machines as a basic positioning transducer. The new laser tube design is simpler and less expensive than that previously built by HP using 2-frequency techniques.

A new surface measuring concept developed under government contract at Itek Optical Systems has been previously reported by Allen Greenleaf. The method uses four steerable distance-measuring interferometers at the corners of a tetrahedron to determine the posi-tions of a retroreflecting target at various locations on the surface being measured. A small wooden breadboard had been built and tested, demonstrating the feasibility of the concept. This paper reports the building of a scaled-up prototype surface measuring machine to allow the measurement of large aspheric surfaces. A major advantage of the device is that, unlike conventional interferometry, it provides surface measurement in absolute coordinates, thus allowing direct determination of radius of curvature. In addition, the device is self-calibrating. Measurements of a 24-inch mirror have been made with the new machine, giving repeatability of 4 µ m peak sag in the curvature and accuracy of 0.7 μm rms in the surface figure at best focus. The device is currently being used in the production grinding of large aspheric mirrors at Itek. The device is potentially scalable to other industries where highly accurate measurement of unusual surfaces is required.

By using three noncoplanar coherent laser beams intersecting one another at angles larger than 30° three sets of interferometric grating are generated. By placing a specimen coated with a photosensitive material inside this grating field and exposing to it before and after load three distinctive moire fringe patterns can be obtained, from which principal surface strains can be calculated by using the strain rosette formula.

Departing from the conventional Doppler velocimetric method, we propose, in this paper, a simple yet effective holographic technique, based on the temporal filter concept, for the measurement of the velocity of a uniformly moving object. We note that such a technique can be implemented by the repetitive recording of two time-lapse Fourier Transform holograms of the moving object on the same plate in the successive time frames. The finished hologram can be reconstructed by either coherent or white light to reveal a set of high contrast Young's interference fringes located on the recording plane of the hologram. By measuring the spacing between fringes, we are able to determine the tranverse velocity of the moving object. This technique is complementary to the Doppler velocimetry in both the magnitude of the velocity and the direction of motion. In this paper, both theoretical analysis and experimental results will be presented.

The failure of IR systems to attain their design performance is usually attributed to four basic errors, namely decentration or tilt, form error, materials defects and transmission or coating defects. Until fairly recently it was difficult to determine what error or combination of errors was contributing to an unacceptable drop in performance of a system. The introduction of a semi-automatic high precision scanning far infrared interferometer has allowed the complete diagnosis of system defects, along with an accurate measure of any error and its effect on system performance in terms of wavefront error, MTF, etc. Results on a variety of systems which exhibit the above errors will be presented and their root cause and subsequent cure will be discussed. The value of such an instrument for both prototype building and production runs will be described.

The development of an efficient infrared prism for the 8 to 12 micrometer wavelength range required materials with the lowest bulk absorption possible and fabrication tolerances equivalent to the best available. Enlarged cadmium telluride ingots were grown to provide blanks over 14.0 centimeters long. Selection of the final material was accomplished by absorptivity testing via laser calorimetry of the candidate substrates. Optical fabrication of the prism presented special problems in both the polishing and the testing phases and required the adoption of specialized polishing and measurement techniques. The calorimetry tests determined the prism's bulk absorptivity to be equivalent to the lowest values achieved to date at Two-Six. The success of this project indicates that very low loss infrared prisms with tight optical tolerances can be manufactured from cadmium telluride with high reliability.

An Optical Fiber Measurement System has been designed and developed to characterize the optical bandwidth of a concatenated fiber in a field environment. This system utilizes a modified EIA/NBS frequency domain fiber measurement technique normally used within laboratory environments. A brief discussion of the band-width measurement system and system components are described. Methodology described for the characterizations and calibration include modal output from the scrambler, source stability, absolute frequency detector line-arity, frequency response flatness, and spectral linewidth. Areas of problems and uncertainties leading to measurement error are adressed along with critical measurement issues.

The value of the fundamental quantity in radiometry, the watt, is presently realized by electrical substitution in which the temperature produced in a blackened material due to absorption of radiant energy is balanced against that produced by electrical energy whose current and voltage can be accurately measured. A new method to measure optical radiation is being explored by the National Bureau of Standards in which photons are absorbed in a semiconductor and converted with an efficiency closely approaching the theorectical maximum - 100 percent. Other radiometric concepts, such as radiance, irradiance, and radiant intensity can easily be defined through simple geometric relationships. Photometry on the other hand, while sharing these identical relationships also introduces detector response modeled after human visual traits, new measurement unit names, and a reliance on source intensity as its practical fundamental quantity. The validity of assumptions often unconsciously made in measurement techniques will be examined from the view point of detector-based radiometry and photometry.

A 20-in.-diameter integrating sphere has been designed to provide a precision radiometric calibration source over a spectral range of 0.02 to 50 μm for the Earth Radiation Budget Experiment (ERBE) instruments and the reference active cavity radiometers (ACR$). The calibration source described herein has unique specifications to provide simulated earth albedo plus longwave background emittance to these instruments, in as near uniform an irradiance as possible, with fields of view ranging from 3 to 168 deg conical. The widest field of view simulates the limb-to-limb earth input with a "space-ring" of liquid nitrogen around the entrance aperture. This paper describes: (1) how the measurement of a distributed shortwave source of radiation has been used to calibrate the ERBE instruments in radiation regimes representing earth-reflected solar radiation, (2) the design of an integration sphere comparing the theoretical throughput irradiance levels to the measured levels using reference ACR radiation measurement devices, and (3) methods of controlling the shortwave intensity and wall temperature, as well as monitoring the changes in these levels due to changes in loading as measurement devices are moved into and out of the exit aperture.

The Portable Optical Sensor Tester (POST) is a low background, long wavelength infrared test and calibration chamber used for evaluation and calibration of developmental LWIR sensors. It is operated by Rockwell International for the Ballistic Missile Defense Advanced Technology Center (BMDATC). The POST system generates a collimated output IR beam from a working blackbody source for test and calibration of LWIR sensors. Internal scan mirrors are used to scan the output beam to simulate flight sensor scanning. The optical path has eleven reflective surfaces making a spectral calibration of the output beam necessary. This calibration is accomplished by utilizing an NBS calibrated blackbody with a calibration accuracy of 4.2% (la quadrature accuracy = 2.0%) as a reference standard. In situ calibration of the output beam is accomplished by sampling part of the output beam and comparing it spectrally, point by point, with the output from the reference blackbody. A grating cube spectroradiometer resident in POST is used to make the spectral comparison. By careful analysis of the diffraction effects at the reference blackbody source and the utilization of a single reflective optical element to direct the reference source energy to the spectroradiometer, the calibration uncertainties are minimized.

The reference active cavity radiometers (ACRs) form the heart of the Earth Radiation Budget Experiment (ERBE) calibration system. These instruments are designed to measure an absolute input irradiance level, independent of wavelength. As such, the reference ACRs can be configured to accept various input fields of view and measure sources of irradiance from the ultraviolet through infrared regimes without the problem of any spectral response corrections. The use of active cavity radiometers, with optimized duplex cones, provides a direct transfer between: (1) input energy and (2) that which is provided electrically to maintain the same temperature differential relationship between the two cones. Ray trace techniques, described in this paper, were used to design the cone geometry. An effective emittance of over 0.999 resulted from the geometric enhancement over input angles of 0 to 170 deg. The electronic bridge amplifier, coupled with the geometric design of the input aperture, makes these radiometers accurate to well within 0.5%, as shown by the repeatability of the calibration data and the design analysis referenced. This paper describes: (1) the design of these reference ACRs, (2) similarities to the ERBE flight configurations, (3) improvements in design made possible through the use of ground system electronics and heatsinks, and (4) performance accuracy as an absolute measurement of input radiation.

The Aerospace Chamber (7V) at AFDC is a radiometric calibration facility for cold back-ground space-based long-wavelength infrared sensors. A working standard, low-temperature blackbody (BB) has been developed for use in establishing radiometric calibrations that are directly traceable to the National Bureau of Standards (NBS). A description of the BB and the NBS calibration results are presented. This standa.rd source has been utilized to calibrate a phosphorous-doped, silicon bolometer which serves as a transfer device for the calibration of new blackbody sources. The electrically self-calibrating feature of this bolometer has been used to normalize variations in responsivity from one installation to anotter over a period of five years. For infrared (IR) sensor testing, the radiometric quantity of interest is beam irradiance at the sensor aperture. The calibration transfer process which is used to relate the working standard to attenuated sources, is described and the transfer devices are discussed.

Note to the Readers: The moderator (Clair L. Wyatt) has edited the verbatim transcripts of the Panel Discussion to remove the most obvious ambiguities and repetitions without altering the general conversational tone of each panelist's comments. The Panel Discussion was recorded using magnetic audio tape cassettes. Recording failures occurred during the audience question period and that portion had to be omitted from this document.

Honeywell's Low Background Calibration Facility (LBCF) is undergoing an extensive upgrade that will increase the overall testing flexibility while meeting all the performance requirements necessary to calibrate and to provide realistic mission simulations for state-of-the-art IR space sensors. The upgrade includes a new, off-axis optical collimator using a Cassegrain design with an effective focal length of 200 in.; an internal radiometer that will, in effect, calibrate a sample of the test beam while a sensor is under test; and a new source assembly similar to the AEDC Mark 7 design to improve source dynamic range and stability and to increase testing flexibility. Concepts are also being generated for a scenario simulator that will consist of a focal plane with a matrix of 4,000 discrete targets individually controlled by computer to emulate a typical threat scenario. The objective of these upgrades is to provide a facility with total sensor calibration and mission emulation capability without sacrificing the low operational cost that typifies this medium-sized chamber. It will be most useful both for rapid testing of production systems and for economical integration and functional checking of developmental sensors.

A low-background test chamber for testing infrared astronomical focal-plane arrays has been built and characterized. The spot-scanning chamber simulates the optics and orbital conditions of the Infrared Astronomical Satellite. The performance of the chamber was established through detailed calculations of the infrared flux on the array and through measurements with reference detectors. The chamber met its performance goals, and produced results consistent with measurements from other laboratories.

The development and operation of a ground-based facility designed for spectral and spatial simulation of long-wavelength infrared ) optical target scenarios under low radiant background and hard vacuum conditions required the integration of many engineering disciplines and the merging of associated science and technology. The unique features and capabilities of the Advanced Sensor Evaluation and Test (ASET) facility will be discussed. Single and multiple point target simulation sources with controllable irradiance cover a large dynamic irradiance range of nearly Planckian and complex spectral characteristics, as well as variable extended targets for optical background simulatio.. An overview of the collimator subsystem presenting the simulated targets optically at infinity and the scanning mirrors subsystem providing scan capability over several degrees across the field of view will augment the description. An overview of the cryogenic support subsystem will complete the ASET facility description. The ASET facility computer syste s will be described, including computer aided facility operation and test data acquisition and processing capability.

A blackbody calibration source designed for operation in a vacuum uses cryogenic coolant and a four-stage controller to provide 0.01 K stability over an operating range from 150 to 350 K, with an absolute uncertainty of less than 0.2 K. The blackbody radiator consists of a black anodized aluminum disk into which is cut concentric grooves that provide a cavity-like enhancement of the blackbody emittance. A gold-plated, temperature-controlled conical reflector with a 5-in. diameter aperture extends the apparent size of the blackbody so that sensors with wide fields of view up to 175 deg conical can be calibrated. A minicomputer-based temperature measurement system converts an embedded array of platinum resistance thermometer (PRT) resistance measurements (traceable to IPTS-68) into blackbody temperature and then calculates the radiant exitance. Data is presented from modeling predictions and actual performance measured during calibration of the ERBE instruments.

Infrared sensing equipment is often calibrated by using a blackbody and collimator to provide a standard source to simulate an infinitely. distant target. The calibration of these blackbody-collimators to reasonable accuracy is quite difficult, and therefore should be done by actually measuring the irradiance produced with a radiometer. The radiometer itself must be calibrated with a blackbody-collimator, but only this one standard source need be maintained to high accuracy. Among the characteristics that must be carefully understood are the goniometric uniformity of the standard blackbody radiance, the spatial and goniometric uniformity of the radiometer response and the independence of these variables from the wavelength of the radiation. The application of this idea to the calibration of a large number of infrared target simulator sources is described along with many of the practical problems this method makes apparent that would otherwise be undetected.

We describe the design and testing of a helium-cooled absolute radiometer (HCAR) devel-oped for highly reproducible measurements of total solar irradiance and ultraviolet flux, and for laboratory standards uses. The receiver of this cryogenic radiometer is a blackened cone of pure copper whose temperature is sensed by a germanium resistance thermometer. During a duty cycle, radiant power input is compared to electrical heating in an accurate resistor wound on the receiver, as in conventional self-calibrating radiometers of the PACRAD and ACR type. But operation at helium temperatures enables us to achieve excellent radia-tive shielding between the receiver and the radiometer thermal background. This enables us to attain a sensitivity level of 10-7 watts at 30 seconds integration time, at least 10 times better than achieved by room temperature cavities. The dramatic drop of copper specific heat at helium temperatures reduces the time constant for a given mass of receiver, by a factor of 103. Together with other cryogenic materials properties such as electrical superconductivity and the high thermal conductivity of copper, this can be used to greatly reduce non-equivalence errors between electrical and radiant heating, that presently limit the absolute accuracy of radiometers to approximately 0,2%. Absolute accuracy of better than 0.01% has been achieved with a similar cryogenic radiometer in laboratory measurements of the Stefan-Boltzmann constant at NPL in the U.K. Electrical and radiometric tests con-ducted so far on our prototype indicate that comparable accuracy and long-term reproducibility can be achieved in a versatile instrument of manageable size for Shuttle flight and laboratory standards uses. This work is supported at AER under NOAA contract NA8ORAC00204 and NSF grant DMR-8260273.

The requirement to verify that the Earth Radiation Budget Experiment (ERBE) radiation measurement instruments respond accurately to input radiation over large input angles is discussed in this paper. The use of active cavity radiometers (ACRs) in the reference and the ERBE flight instruments, with fields of view ranging between 50 and 145 deg conical angle, required considerable development and analysis to ensure that the radiometers were designed properly. Ray trace analyses, laser experiments in both visible and infrared regimes, test data of the instrument when subjected to distributed irradiance calibration sources and, finally, a cosine response test on the overall instrument, are also discussed. Cosine response is demonstrated within the included field of view with these large acceptance angles, while zero input response is verified for sources emanating outside the nominal cut-off angles for each sensor. The use of reflecting field-of-view limiters reduces the sensitivity of thermal input from the instrument front end, but enhances the stray radiation. These instruments are shown to be insensitive to stray radiation caused by the radiometric design. This paper describes the angular response analyses, tests, and associated hardware that enables the prospective users of the ERBE instruments to con-fidently measure earth radiation for input acceptance angles extending beyond the earth limb, without having to be concerned with solar or lunar stray radiation effects outside the acceptance angles.