Radiometry mistakes are made throughout industry and academia with many of them being a result of misapplication of fundamental principles. Since we are all, in one way or another, students of Professor Bill Wolfe, this paper continues his example to educate at every opportunity and mitigate propagation of these errors. Based on the author’s observations, the top “seven deadly” radiometry mistakes are described with explanations and examples of the proper applications and interjections of Bill’s teaching concepts and wit.
Particulate contamination scatter is often modeled using Bidirectional Scatter Distribution Functions (BSDFs) based
upon Mie scattering by a distribution of spherical particles. Starting with the basic model described in P. R. Spyak and
W. L. Wolfe [1,2,3,4], we improve upon it by adding multiplicative geometrical form factors. These factors prevent the
Total Integrated Scatter (TIS) from exceeding unity and ensure that reciprocity is always obeyed. Preventing the TIS
from exceeding unity is necessary for energy to be conserved in the raytrace, and obeying reciprocity is necessary to
obtain consistent results between forward and backwards raytraces. As will be shown, this improved model fits
measured data better than the previous model.
Optical instruments are normally calibrated with incandescent irradiance or radiance sources. Recently, accurate calibrations using solar radiation have been demonstrated in the visible and near-IR regions (VNIR). The solar-radiation based calibration (SRBC) has major advantage in that the calibration source is the same source used on-orbit by earth- viewing remote sensing sensors such as the ASTER, MODIS, and Landsat 7 ETM+ sensors. In this paper, such a radiometer calibration covering the region between 740 and 2400 nm is presented and compared with lamp-based laboratory calibrations. This work extends the spectral range over which a calibration using solar-radiation has been made.
EOS satellite instruments operating in the visible through the shortwave infrared wavelength regions (from 0.4 micrometer to 2.5 micrometer) are calibrated prior to flight for radiance response using integrating spheres at a number of instrument builder facilities. The traceability of the radiance produced by these spheres with respect to international standards is the responsibility of the instrument builder, and different calibration techniques are employed by those builders. The National Aeronautics and Space Administration's (NASA's) Earth Observing System (EOS) Project Science Office, realizing the importance of preflight calibration and cross-calibration, has sponsored a number of radiometric measurement comparisons, the main purpose of which is to validate the radiometric scale assigned to the integrating spheres by the instrument builders. This paper describes the radiometric measurement comparisons, the use of stable transfer radiometers to perform the measurements, and the measurement approaches and protocols used to validate integrating sphere radiances. Stable transfer radiometers from the National Institute of Standards and Technology, the University of Arizona Optical Sciences Center Remote Sensing Group, NASA's Goddard Space Flight Center, and the National Research Laboratory of Metrology in Japan, have participated in these comparisons. The approaches used in the comparisons include the measurement of multiple integrating sphere lamp levels, repeat measurements of select lamp levels, the use of the stable radiometers as external sphere monitors, and the rapid reporting of measurement results. Results from several comparisons are presented. The absolute radiometric calibration standard uncertainties required by the EOS satellite instruments are typically in the plus or minus 3% to plus or minus 5% range. Preliminary results reported during eleven radiometric measurement comparisons held between February 1995 and May 1998 have shown the radiance of integrating spheres agreed to within plus or minus 2.5% from the average at blue wavelengths and to within plus or minus 1.7% from the average at red and near infrared wavelengths. This level of agreement lends confidence in the use of the transfer radiometers in validating the radiance scales assigned by EOS instrument calibration facilities to their integrating sphere sources.
KEYWORDS: Radiometry, Calibration, Temperature metrology, Black bodies, Sensors, Mirrors, Signal detection, Transmittance, Remote sensing, Signal to noise ratio
A comparison of spectral diffuse reflectance between different national standards laboratories is being planned under the direction of the Comite Consultatif de Photometrie et Radiometrie (CCPR). A similar comparison of bidirectional reflectance distribution factor among laboratories in the United States in support of optical remote sensing measurements is nearing completion. Since this comparison provides valuable lessons for the one organized by the CCPR, pertinent results and their implications are presented.
Errors can occur in laboratory measurements when the response of a bandpass-filtered radiometer extends into an atmospheric absorption region. Atmospheric models, such as MODTRAN3, can be valuable tools that allow optical measurement in these regions to be accurately analyzed. Comparisons of MODTRAN3-predicted and laboratory-measured atmospheric transmittance have been made to help establish the validity of MODTRAN3 for use in modeling short-path length, low resolution, optical effects over the absorption band near 1380 nm. MODTRAN3-predicted transmittance is shown to be within 4 percent of the measured data and well within 2 percent foremost of the water band. The spectroradiometric measurement of the water-vapor absorption band, its description, and its comparison to the MODTRAN3 prediction are presented. Also presented are examples of errors that can occur when an instrument response extends into this region.
KEYWORDS: Radiometry, Calibration, Black bodies, Temperature metrology, Optical filters, Sensors, Signal detection, Signal to noise ratio, Mirrors, Electronic filtering
A four-band, prototype, thermal-IR radiometer with a built- in radiance reference has been fabricated by CIMEL Electronique, Paris, France, for use as a field instrument. This paper briefly describes the instrument and discusses laboratory characterization measurements and results, including spectral response, linearity of better than 0.8 percent, field of view of 9.5 degrees, noise-equivalent temperature difference of 0.06-0.2 degrees C for temperatures of 0 to 75 degrees C, signal-to-noise ratio greater than 1100 for the broad band and greater than 400 for the other bands for temperatures between 10 and 80 degrees C, nonrepeatability of less than 0.35 percent after four field campaigns, and absolute calibration.
A spectral polarimeter with an autotracking mount to obtain atmospheric parameters required for the vicarious calibration of satellite sensors has been modified to work with anew computer and electronic components. The instrument has 12 bands covering the visible through the short-wave IR. There are 9 bands from 400 nm to 1100 nm which use a silicon detector, and 3 bands from 1100 nm to 2500 nm which use a temperature-stabilized, lead-sulfide detector. The instrument's operation was verified by using it as a solar radiometer and collecting Langley plot data. These were compared to data taken concurrently by a well-characterized, manually-pointed radiometer with 10 visible and near-IR channels. In addition, the effect of the gaseous transmittance on the retrieved optical depths of the short- wave IR bands are presented. The data are obtained by finding the band-averaged transmittance for each filter under several atmospheric and view conditions using the output from MODTRAN3.
ASTER will be calibrated in the laboratory by reference to sources traceable to NRLM and NIST standards and through the use of transfer radiometers. Partial aperture on-board calibration systems will be used in the solar-reflective range and an on-board blackbody source will be used in the infrared. An important independent source of calibration data will be provided through the in-flight radiometric calibration of ASTER by reference to well- characterized scenes. The latter is the subject of this paper. Methods that make use of ground reflectance and radiance measurements made simultaneously with atmospheric measurements at selected sites and used as input to radiative transfer codes are described. The results of error analyses are presented indicating that, depending on the method used, the predicted uncertainties fall between +/- 2.8% and +/- 4.9%, for the solar-reflective range. In the thermal infrared, the goal is an uncertainty of less than 1 K. A method that provides in-flight cross calibrations with other sensors also is described.
It is shown that the far-infrared scatter from smooth mirrors can be dominated by the scatter from just a few very small particles or defects. This emphasizes the necessity for good cleaning techniques and good clean-room procedures. Several effects of this finding are discussed, as well as several other related topics: the cleanliness required for the scatter to be dominated by a mirror's surface microroughness; a proposed specification for low-scatter infrared mirrors; incident angle invariance of clean and contaminated mirrors; the shape ofthe bidirectional reflectance distribution function (BRDF) curves; and the relation between surface cleanliness level, clean-room cleanliness class, and BRDF.
The scattered light from dust-contaminated mirrors was measured at λ = 10.6 μm, and results are compared to that predicted by a modified Mie theory. The theory is in good agreement with experiment. The far-infrared measurements of the "clean" mirrors were, for angles beyond about 8 deg from specular, limited by the scatter from particulates and not the surface microroughness. Similar limitations can be experienced by other irregularities such as scratches, digs, and pinholes in coatings. It is shown that contaminant scatter dominance in the far-infrared requires only a few very small particles, so the necessity for good cleaning techniques and good clean room practice is quite evident.
The scattered light from dust-contaminated mirrors was measured at λ = 0.6328 μm, and the results are compared to those predicted by a modified Mie theory. Comparisons between theory and experiment indicate that the theory accurately predicts the forward-scattered and backward-scattered radiation.
The scattered light resulting from polystyrene spheres residing on mirrors was measured at λ = 0.6328 μm, and results are compared to that predicted by a modified Mie theory. The method for cleaning the samples, counting and measuring particles, the measurement procedure, and the theoretical model employed to predict the scatter from contaminants on mirrors are discussed. The comparisons between theory and experiment indicate that the theory predicts the forward scatter, but the backscatter predictions are not as successful. The indication is that the developed model can accurately predict the scatter from dust on mirrors.
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