Clair L. Wyatt

Infrared spectrometer: liquid-helium-cooled rocketborne circular-variable filter

An ir spectrometer used to obtain atmospheric emission spectra from a rocket at altitudes between 45 km and 200 km in the ICECAP Auroral Measurements Program is described. The instrument continuously scans the 6.75-23.2-microm spectral region at the rate of 2 scans/sec. The spectral resolution of the instrument, which employs a circular-variable interference filter (CVF), ranges from 3% to 4%. The entire optical subsection, silicon-arsenic detector, CVF, Irtran 6 lens, baffle, and removable cold cover are cooled below 10 K in a high-vacuum Dewar system. The noise equivalent spectral radiance (NESR) of the spectrometer at 22 ,microm is better than 1 x 10(-11) (W cm(-2)sr(-1)microm(-1)). The dominant high altitude atmospheric emission features observed on two rocket flights were 9.6-microm O(3) and 15-microm CO(2).

Irradiance Linearity Corrections for Multiplier Phototubes

The principal causes of nonlinearity in vacuum multiplier phototubes when used with practical circuits are investigated. This investigation was made because the wide application of phototubes as irradiance measurement devices requires a linear detection system or a system of known nonlinearity. There are two principal causes of nonlinearity associated with multiplying photoemissive devices as used in practical circuits: (1) A dynamic plate impedance which is finite compared with the load impedance; (2) a responsivity which is a function of anode current, due to changing interelectrode voltages. The first cause of nonlinearity gives a measurement of irradiance which is less than the actual irradiance, whereas the second cause gives a reading which is too high. The effects are, therefore, compensating to a degree which depends on the specific tube and circuit parameters. An expression is derived to apply as a correction to irradiance measurements obtained with a particular multiplier phototube circuit configuration.

CIRRIS-1A interferometer: radiometric analysis

The CIRRIS-lA spectroradiometer system is designed to obtain spectral and spatial airglow data on the shuttle platform over a global extent. The sensor system includes a Michelson interferometer which exhibits a noise equivalent sterance [radiance] (NER) of 2 x 10(-13) W cm(-2) sr(-1) Hz(-1/2); and at a resolution of 0.964 cm(-1) (scan time 9.1 s) exhibits a noise equivalent spectral sterance [radiance] NESR of 7 x 10(-14) W cm(-2)sr(-1)/cm(-1). The entire optical subsystem, silicon-arsenic focal-plane, off-axis high-rejection telescope, and removable cover are cooled below 20 K. The measured performance is shown to agree with theory to within a few percent. Noise-equivalent-sterance [radiance] per root-Hz is recommended as a figure of merit for interferometers that is independent of the specific operating conditions; i.e., mirror velocity, scan time or transform size.

Terrestrial Measurement of the Performance Of High-Rejection Optical Baffling Systems

Spatial rejection is critical when exo-atmospheric or in situ radiometric measurements of faint sources, such as air glow emissions, are made in the presence of relatively intense sources such as the sun, moon, or earth. Scattering from atmospheric molecules and aerosols during evaluation in an earth-bound laboratory makes the high-rejection baffle appear to be less effective than it actually is in the measurement situation. A Monte Carlo computer program SCAT was written to predict the effects of atmospheric scattering upon field-of-view calibration measurements of optical baffling systems. It was found that Mie scattering could be reduced in a clean room environment and Rayleigh scattering was determined to be the limiting mechanism in a special chamber designed for off-axis measurements. The background flux at 40° off axis was found to be 1 x 10-9 of the on-axis incident flux for a 5° full-angle baffle when illuminated by a 10.2 cm diameter collimated beam. A two-step process was used to measure a baffle response at 40° off axis down to 1 x 10-11 of the on-axis incident flux.

Multiplexed Dispersive Spectrometers Using Reduced Background Infrared Detectors

The application of multiplex spectrometry to cryogenically cooled LWIR extrinsic photodetectors is limited by system noise. This noise limitation results in a detector NEP that is directly proportional to bandwidth. Therefore, multiplex schemes that require increased bandwidth are not productive of real advantage. However, doubly encoded systems that are based on 2n - 1 or n + N - 1 measurements have the potential to provide a real throughput gain proportional to the number of elements used on the throughput matrix.

Portable Compact Multifunction IR Calibrator

New infrared calibration test facilities are required to provide for the calibration of sensors that exhibit resolution in the microradian range and that utilize a large number of detectors in linear and area arrays. Simple collimators with low divergent beams are typically physically large, costly to cool, and provide only a partial calibration. Other capabilities such as multiple point sources, bar patterns and an extended-area source are needed. A compact portable multifunction calibrator is designed for future sensor systems that enables a linearity calibration for all detectors simultaneously using a near small area source ( Jones source), a high resolution mapping of the focal plane with 10 prad setability and with a blur of less than 100 prad, system spectral response calibration (radiometer) using a Michelson interferometer source, relative spectral response (spectrometer) using high-temperature external commercial blackbody simulators, and an absolute calibration using an internal low-temperature extended-area source. In addition, a scatter plate is available to provide a diffuse full-field and full-aperture but attenuated high-temperature radiation source, bar pattern reticles to provide direct evaluation of modulation transfer function and and bandpass filters to provide system parameter evaluation at selected wavelengths. The portable system is made compact through the use of a folded Gregorian collimator design and includes an extended-area source, scatter plate, and Jones source which are stowed in the system and can be switched into the beam.

Dark-Noise Analysis

This chapter provides an overview of the dark-noise analysis. The first step in the calibration of an electro-optical sensor is to qualitatively and quantitatively evaluate the dark-noise signal. Such an evaluation can be obtained when the sensor aperture is blocked with a light-tight cover. The noise content of the dark signal provides for one measure of the sensor precision, or reproducibility, and for a measure of the false signal content of the output. The properties of the dark-noise output are a function of wavelength, or spectral scan position, of a spectrometer. A radiometer is a degenerate case of the spectrometer for which there is only one wavelength. The precision, or reproducibility, of the sensor is limited by the system noise. The variance of the dark-noise signal is a measure of the uncertainty in terms of either the output signal or the input flux. The statistical characteristics of the dark-noise output are approximated by the sample mean and the sample variance based on a set of measurements. The mean value and the standard deviation of the dark signal of a spectrometer are obtained by processing many scans over a relatively long time. The mean of the dark-noise output is frequently referred to as the offset error. The chapter also highlights dark-noise data processing.

Short Wavelength Rocketborne Infrared Spectrometer

An ir spectrometer, used to obtain measurements of faint atmospheric emission spectra from a rocket carrier, is described. The sensor scans the 2.07 to 5.47 μm spectral region at the rate of 2 scan/s. The spectral resolution of the sensor, which employs a circular variable filter (CVF), ranges from 1.0 to 3.0%. The entire optical subsection, indium anti-monide detector, CVF, silicon lens, baffle, and removable cover are cooled below 80° K in a high-vacuum Dewar system. The noise equivalent spectral sterance [radiance] of the spectrometer at 5.3 μm is 1.93 x 10-10 (W cm-2sr-1μm-1).

Theory And Methods Of Radiometric Calibration

The theory and techniques of radiometric calibration, as applied to instruments used to characterize remote target sources, are not well understood by many engineers and physicists engaged in measurement programs. This invited tutorial lecture is designed to show that the correct interpretation of field data is dependent upon an understanding of the theory of calibration. The instrument must be qualified in four nearly independent domains: (1) field of view, (2) spectral bandpass, (3) time, and (4) polarization.

Calibration of a helium-cooled infrared spatial radiometer and grating spectrometer

Methods used by the Space Dynamics Laboratory of Utah State University (SDL/USU) to calibrate infrared sensors are described, using the Infrared Background Signature Survey (IBSS) spatial radiometer and grating spectrometer as examples. A calibration equation and a radiometric model are given for each sensor to describe their responsivity in terms of individual radiometric parameters. The calibration equation terms include dark offset, linearity, absolute responsivity, and measurement uncertainty, and the radiometric model domains include spatial, spectral, and temporal domains. A portable calibration facility, designed and fabricated by SDL/USU, provided collimated, extended, diffuse scatter, and Jones sources in a single cryogenic dewar. This multi-function calibrator allowed calibration personnel to complete a full calibration of the IBSS infrared radiometer and spectrometer in two 15-day periods. A calibration data system was developed to control and monitor the calibration facility, and to record and analyze sensor data.