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Noise comparators and associated thermal standards have been developed to provide accurate noise source calibration facilities at S and X Bands. The comparators are based on a null-balance form of the Dicke radiometer, but eliminate problems of RF switching by using an IF switch. Matching facilities are incorporated. The thermal standards include both waveguide and coaxial forms and operate at 400°C. The most successful type is described in detail. Gas discharge noise sources have been developed as highly reproducible transfer standards, with outputs stable over many thousands of hours to within a few hundredths of a decibel.

This paper outlines the work currently being carried out at the Royal Radar Establishment (RRE) in the RF and Microwave National Standards Division of the British Calibration Service, on both hot and cold noise standards and comparator systems. The design of three cryogenic noise sources for use in WG15 are described in detail along with the theoretical calculation of their output noise power spectral density. A noise-adding radiometer is analyzed, and finally the sources of uncertainty involved in a recent calibration are discussed.

An easily constructed differential radiometer is described which can be calibrated analytically from its geometric and physical parameters. An estimated 3% accuracy is achieved using common handbook values for the physical parameters, and the accuracy of the calibration is confirmed by experiment. The instrument measures net thermal radiation exchanged between surfaces near normal room temperature. It responds quickly to changes in incident flux and reaches equilibrium within about 1 min. It is sufficiently stable and rugged to be considered for student experiments and for demonstration lecture work. A number of elementary experiments are described which use the radiometer to demonstrate the behavior of long wavelength infrared radiation and to determine its interaction with materials.

Radiometric technique for absolute temperature measurement of microwave noise sources under field conditions

The thermistor radiometer consists of a Wheatstone bridge formed of four thermistors, two of which are exposed to incident radiation and two of which are hidden from it and act as thermal compensators. The output from the bridge is suitably amplified and fed into a microammeter on which deflections are read in microamperes and related to radiant temperatures by means of calibration charts. Calibrations are made against standard blackbody radiators and at present have been carried out for use of the instrument in ambient temperatures between −50°C and +30°C for measuring radiant temperatures between −185°C and +60°C. The instrument has been used successfully in (1) the measurement of skin temperatures indoors at normal room temperature and outdoors in ambient temperatures as low as −45°C; and (2) the measurement of the radiant temperature of the sky and ground in temperate and in arctic climates.

Of modern British work seeking to establish a scale of radiation intensity the most important is that of the late Professor H. L. Callendar (1910), who developed a very accurate instrument, the Callendar Radio-balance, for measuring radiation. Subsequently Callendar used the radio-balance to calibrate various types of radiometer produced by British instrument makers. After his death, arrangements were made for this work to be continued at the National Physical Laboratory. The instrument used to establish a scale of radiation intensity at this Laboratory was a radio-balance made to an improved design of Callendar’s shortly before his death. It was made in the Physics workshops of the Imperial College of Science and calibrated at the National Physical Laboratory as described in the second of these papers. Other scales, notably those known as the Smithsonian Scale of 1913 and the Ångström Scale, have also been employed in this and other countries, particularly for the calibration of meteorological instruments; and it has been recognized for some time that these scales are not in agreement. The discrepancies indicated the importance of obtaining independent evidence of the accuracy of the N. P. L. scale rather than continuing to rely on the properties of a single instrument. Accordingly, the first steps taken in a long investigation, which ultimately led to complete justification of the faith reposed in the radio-balance, concerned the development of absolute radio-meters of an entirely different type. Descriptions of these new instruments, and of the method of using them, form the subject of this first paper.

Investigations into the use of a silicon solar cell to measure solar radiation intensity are described. The effect of optical path length ratio and atmospheric constituents are discussed. A survey is made of other photovoltaic devices. It is concluded that, on presently available data, the silicon cell is the most suitable for the purpose. Comparisons are made between the signals obtained from a silicon-solar-cell radiometer and a Kipp thermopile type solarimeter used as a standard. One set of comparisons is made for 30-minute intervals on clear days and is analysed so that known effects due to angle of incidence are allowed for and variations in the signals caused by spectral quality of the radiation can be separated. This set of comparisons shows an extreme spread of ±13 percent on a reference constant determined for midday clear sky. This extreme spread includes variations of +6.5 to −3 percent at midday together with variations during the course of any one day. A second set of comparisons made during whole days or long periods, during which conditions were stable, gave rise to two calibration constants, one for clear-sky and one for overcast-sky conditions. If an over-all calibration constant is desired for all conditions, then a value of 15.79 mA cm−2 min/Langley is obtained with a probable error of −312to +812 percent. The skew effect here is because more readings were taken for clear-sky conditions, thus favoring the lower constant for these conditions.

The response of a net radiometer to a sudden irradiation is shown to give generally a marked overshoot which during calibration is mostly overlooked. This overshoot also simulates a high speed of response. An improved design of net radiometer minimizes this effect and also gives a high sensitivity.

A radiometer is described which is calibrated on an absolute basis from its geometric and physical parameters. An estimated 3% accuracy is achieved using common handbook values for the physical parameters. Experiment confirms the calibration, including predicted dependence of sensitivity on temperature. The instrument gives integrated response to diffuse thermal radiation exchanged between surfaces at normal room temperature. When used with a commercial microvoltmeter, the present device measures net heat fluxes ranging from 0.1 to 100 Btu/h‐ft2. It responds quickly to changes in incident flux and reaches equilibrium within about 1 min. It is sufficiently rugged and stable for practical field work and is suggested as a tool for research and teaching.

A description is given of the scale of total irradiance maintained at the National Research Council. This scale, in contrast with other national scales, is derived from a low temperature (50 to 150 °C) blackbody, rather than from a high temperature (> 1000 °C) blackbody or an absolute radiometer. The irradiance from three tungsten-filament lamps has been measured on this scale as part of an international programme to intercompare irradiance scales. The results of several tests for systematic errors in these measurements are discussed.

Long-wave calibration procedures which expose a shielded net radiometer to different air temperature on the two sides or which expose an effective-pyranometer to an air temperature different from the instrument temperature cause a major increase in apparent sensitivity due to (1) an unwanted sensible heat flux through the instrument and (2) differential radiation coming from the shielding if the latter is not very transparent. Calibration procedures using cavities directly placed over the shielding should therefore be avoided.

Laboratory calibration of Beckman-Whitley's long wave radiometer was carried out by using two radiators of the Dines type, as shown in Fig, 1. Of these radiators, the surface temperature of one is near the room temperature, about 10°C, the other about 20-45°C. Let the temperature of the surface be T1 and T2, the electric motiv force V and V′, then have(1-e)⋅κ⋅a⋅σ(T14-T24)=k⋅(V-V′), where e is an absorptivity of water vapour and CO2 in the wooden box, a is blackness of radiator, κ is a constant depending upon the solid angle of the radiator seen from the element. k is the calibration constant, and obtained from Fig. 2.These value of k is significant for the column radiation from the upward. However, a value for the total radiation from all parts on the hemisphere is desired in the measurement of atmospheric radiation. Assuming the relative value of absorptivity of the receiver surface depended on the direction of incident radiation and intensity of the atmospheric radiation from each direction, we can obtain a value required, that is larger than the value obtained from Fig. 2.

To allow the absolute calibration of field and laboratory radiometers in the range 0.1 to 1.5 cal/cm(2) sec, a continuous flow radiation calorimeter has been constructed, based on a design originally developed by the British Joint Fire Research Organization. The temperature difference established between a water outlet and an inlet is sensed by a thermistor bridge and the radiation intensity measurement is made in terms of the level of the electrical power, applied to a built-in immersion heater, which is necessary to reproduce the same temperature difference., Reprinted from: Canadian Journal of Physics, v. 39, no. 11, November 1961, p. 1583-1594. ISSN: 0008-4204

A simple technique for the long-wave calibration of net and one-way radiation probes is described. The materials required for the operation are inexpensive and easily assembled. An error analysis indicates that calibrations accurate to ± 3% may be routinely achieved. Calibrations of several Fritschen-type net radiometers obtained by this technique and by a chamber technique agreed to within better than ± 3%.

The suitability of the solar furnace for the generation of high heat fluxes of known, stable intensity has been investigated. Absolute flux measurements have been performed using an absorbing cavity of new design, which traps a larger portion of the incident flux than the sphere. The results indicate that there is a well-defined correlation between normal-incidence solar radiation and heat flux in the focal zone. This correlation makes it possible to calculate the heat flux when it can-not be directly measured during experiments. On the basis of this correlation the solar furnace can be used as a radiation standard up to 250 cal/cm 2 -sec intensity. A new coating has been adapted to the Gardon radiometer, which makes it possible to use this instrument to the upper flux limits of the solar furnace. A method has been developed for the calibration of the radiometer in the solar furnace.

A balloon-borne instrument is described which measures the downward flux from atmospheric water vapour in the 20–60 μm region. The instrument uses a thermistor bolometer as the infra-red sensor. A special infra-red filter composed of beryllium oxide embedded in polyethylene provides the desired optical characteristics for the instrument. In-flight energy calibration is carried out with two reference black bodies. The radiometer-sonde is flown in conjunction with a conventional radiosonde, which provides atmospheric temperature, and pressure data for the ascent. Results have been obtained from 3 ascents. In the stratosphere the observed flux is consistent with the flux calculated for a water vapour mixing ratio of 2 × 10−6, and using published spectral data. The divergence of the flux corresponds to cooling at a rate of 0.4°C/day.

The accuracies of conventional microwave radiometers are influenced by instabilities mainly in their RF components A radiometer is described that uses two reference temperatnres. The signal processing eliminates the influences of all RF parameter changes on the radiometer indication. Thus the overall stability depends only on the reference temperatures and the low-frequency and dc components. These can be made extremely stable by careful design. The paper presents a detailed analysis of the radiometer performance. The minimum detectable signal is greater than that of a comparable Dicke radiometer, but approaches this, if the reference temperatures are appropriately chosen according to the particular measurement problem. The great advantage of the radiometer is that no measurement time is lost for frequent recalibration. Practical results of an X-band radiometer for airborne use are given at the end of the discussion. They are in good agreement with the theory and demonstrate the feasibility and superiority of the concept.

A three-instrument infrared spectral emittance experiment, comprising a rapid-scan spectrometer (6.7-13.3 ?m), radiometer (10.375-12.1 ?m), and boresight camera, has been flight tested over selected geological terrain in central and southern California and Nevada. Preand post-flight calibrations of the infrared spectrometer were performed both by using polished samples of "standard" rocks (quartz diorite and gabbro) as well as the more familiar blackbody radiance standards. From these latter spectra the instrument transfer function (A??) was derived. In-flight calibrations of wavelength were achieved by the rapid insertion and removal of a polystyrene film in the optical train of the spectrometer, as polystyrene is a material whose transmission spectrum is constant and well known. By flying over a body of water (?6.7-13.3?m = 0.98) and recording the radiance spectrum of that target one can determine the transmission spectrum of the atmospheric path between the aircraft and the water (at least to a first approximation) as both the spectral emittance of lake water and the optical transfer functions of the instrument are known or can be calculated. So far, flights have been made only at low altitudes (2000 ft above the lake), with the lake surface at 2000 ft (near Pisgah Crater, S. Calif.) or 6000 ft (Mono Lake, E. Calif.) above sea level. The lake should be in the area to be studied geologically.

The construction and calibration of two portable radiometers, suitable for micrometeorological studies, is described. One instrument measures the hemispherical short-wave radiation and the other measures directly the total radiative flux through a plane Attachments to the latter instrument permit total hemispherical radiation and psychometric measurements to be made. Small dry batteries, a tiny motor, a simple Wheatstone's bridge and thermistors are used.

Many commercially available net radiometers using polyethylene shields to protect their sensing surfaces have 5% of these surfaces covered with a highly reflective white paint. The purpose of this paint is to reduce the sensitivities of these instruments to short wave radiation and make them equal to a supposedly lower long wave radiation sensitivity. Theoretical considerations, however, yield no conclusive evidence for such a differential transmission of short and long wave radiation by thin films of polyethylene. Furthermore, rederivations of the net long wave radiation equations for the calibration chambers of J. P. Funk [J. Sci. Instrum. 36, 267 (1959); J. Geophys. Res. 67, 2753 (1962)] and L. J. Fritschen [J. Appl. Meteor. 2, 165 (1963); 4, 528 (1965)], who pioneered in the construction and evaluation of these instruments, indicate that certain approximations made in the original derivations completely account for the original 5% discrepancy. New experiments verify these findings and demonstrate the ...

Impulse generators are not conventional instruments and present their own calibration problems. A new method for calibrating the output spectral density level of impulse generators is explained. A Dicke-type radiometer is used for measuring the equivalent noise temperature of the output power spectral density which can easily be converted to the spectral density (amplitude). The switching sequence of the radiometer should be synchronized with the impulse generator in order to have all the impulses delivered to the receiver. This method eliminates the need for computing impulse bandwidth and the need for measuring bandpass characteristics of the receiver. It also does not require a sophisticated radiometer but a simple Dicke radiometer. An impulse generator with 50 pulses per second (pps) rate is calibrated with a Dicke radiometer with a square wave switching sequence of 400 Hz. There is reasonable agreement between the calculated and measured values.

A series of measurements of the infra red emission of the atmosphere has been made at Kew Observatory with Dines and Linke-Fuessner radiometers. Precautions necessary during the calibration and use of these instruments are discussed, the results obtained with them compared, and the accuracy of the measurements assessed. Computation of the atmospheric emission by Elsasser's method is discussed, and compared with the experimental results. It is suggested that the divergence found is due to over-estimation of the emissivity of water vapour in the presence of carbon dioxide by Elsasser, together with the presence of a variable additional component of radiation of from 0 to 8 per cent of the total. Some comments are made on the possible origin of this radiation. A modification of the method of F. A. Brooks is used to derive values of water vapour emissivity from the measurements, and by combining these with the laboratory values of Elsasser and Brooks an emissivity curve is obtained which differs from that now generally accepted. This is used to compute the water vapour radiation, and the construction of an empirical radiation chart is described. Finally it is shown that the outward radiation of the ground may be obtained from the reading of an ordinary thermometer, and that by combining this with an estimate of atmospheric emission based on routine upper air data, the net infra red radiation of the surface may be estimated with an accuracy better than 15 per cent.

Atmospheric absorption in the 600-Gc/s region was measured throughout the actual atmosphere by means of a DICKE-type superheterodyne radiometer receiver using second harmonic mixing. The average measured value of horizontal attenuation was approximately 34 dB/km/g/m3. The variation of water vapor absorption with respect to water vapor density was also indicated in the measured results. The minimum detectable temperature difference (ΔT) MIN was obtained by calculating the rms value of output deflection and the use of the calibration curve for the radiometer. The best value achieved was 5.2°K. From this result, the radiometer receiver noise figure and the mixer conversion loss were able to be determined indirectly. The results were 33.2 dB and 26.4 dB, respectively. A thermal calibrator was used to adjust and calibrate the radiometer, and as a source of radiation for the measurement program.

This paper describes in some detail a radiometer for measurement of ultraviolet radiation from sun and sky. As designed, the instrument is selectively responsive to a narrow band of wavelengths and is capable of detecting and measuring low‐energy levels. The radiometer consists of a photoelectric sensing element, an ultraviolet transmitting window, and a dry‐cell battery mounted in a small housing. Calibration of the instrument shows satisfactory agreement with Lambert's cosine law and a linear relationship between energy flux and output of the phototube. For recording the signal from the radiometer the output is fed to a direct current amplifier which in turn drives a recorder. A typical recording for a cloudy day is presented.

A radiometer is described which was designed to measure the radiation flux near external walls of buildings. The instrument is simple in construction and requires no artificially induced air-blast to compensate for convection as does the standard meteorological instrument. A simple theoretical treatment is given; the calibration of the radiometer is described and the results of comparative observations with an air-blast radiometer are discussed.

A shielded, gold-disk radiometer, specially designed for field use, is described. It measures thermal radiation in the range 0.01 to 1.2 cal cm-2 s-1. The calibration is not greatly affected by ambient temperature or by prolonged exposure and the error introduced by winds of up to 25 mile/h is less than 8%.

During operation, space-borne sensor system calibrations may drift away from preflight measured values. Present sensor internal radiometric calibration systems are also subject to error and drift. Natural targets can be used to determine temporal radiometric response changes. In addition, use of common targets having uniform reflectance and thermal emittance permits cross-checking the responses of different sensors.© (1974) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Publisher Summary This chapter describes some basic aspects of net radiometers and techniques for using them. It discusses calibration techniques, basic considerations of data acquisition, and methods of modifying net radiometers for a variety of different applications. The primary component of a net radiometer is a thermal transducer or thermopile. Two different approaches are used to minimize or equalize convection effects from its upper and lower surfaces; these are to ventilate both surfaces equally or to shield them with transparent domes. The chapter describes the most common method for calibrating net radiometers—that is, the occulting technique or shading technique . Bet radiometers are calibrated for sensitivity to short-wave radiation by simple shading techniques employing standard solarimeters for comparison. Long-wave calibrations are obtained either from special calibration chambers or from a simple flat-plate technique. The chapter indicates that popular polyethylene-shielded net radiometers have comparable sensitivities for both short- and long-wave radiation. Height placement of net radiometers is dependent upon the homogeneity of the underlying surface and its temperature relative to that of ambient air.

A ground-based radiometer for measuring the cosmic background radiation at a wavelength of 2. 1mm is described. The instrument, including black body radiators for calibration, is self-contained and operable in the automatic mode. Output data is punched on paper tape for later computer analysis.

Vacuum optical bench /VOB/ spacecraft testing facility for calibration of telescopes, radiometers, spectrographs and monochromators and measurement of intensity distribution, radiation source and contamination

Tungsten strip lamps spectral radiance calibration, discussing accuracy requirements for radiation constants, wavelength and black body temperature

Volume I begins with a description of the design of a program to measure the radiation from weapon muzzle flash in the spectral region from 0.8 to 5 micro meters. Instruments designed to measure the spectral radiant intensity and spectral radiance of these phenomena in narrow spectral bands are described. A discussion of the calibration techniques used to quantify accurately and precisely the results from the instruments is also included.

Measurement of the radiation produced by a hypersonic body during re-entry into the earth's atmosphere is' receiving attention under several experimental programs. Imaging radiometers are applicable to this requirement. This type of instrument provides spacial resolution as necessary for individual measurement of closely spaced multiple point sources and for measurement of line source intensity, gradients. Evaluation of S-10 and S-20 image orthicon tubes, in the radiometric application, has been performed. Quantitative results of test's defining radiometric response, resolution, and calibration stability are described, along with a comparison of S-10 and S-20 capabilities.

Description of the optics, thermal compensation measures, and calibration procedures for a scanning imaging spectroradiometer to be used in earth resources sensing applications. The spectral range covered extends from 4000 to 8500 A. Two possible modes of operation include (1) a high-spatial-resolution terrestrial survey mode with moderate spectral resolution, and (2) a high-spectral-resolution ocean survey mode with decreased spatial resolution. Two high-aperture f/1.4 objective lenses (providing fields of view of 10 and 25 deg) are interchangeable to give added flexibility in planning experiments with this instrument. A diffraction grating disperses the spectrum which is then scanned by the image sensor.

The quarts gauge apparatus for measuring the vapor pressure of postum was completed. The first sample of postum oxidized in the course of preparation, while in a presumably sealed and evacuated quartz tube. an error of 0.4 per cent appears to be present in the vacuum weighing of postum. a diffusion pump is being installed to reduce the pressure, in the hope of minimizing the radiometer effect. Repetition of the effusion experiment with a rebuilt crucible yielded four points below those reported last time by a factor of ten in pressure. A leak appeared in the crucible, requiring construction of a new one. X-ray patterns were again obtained for the Q metal sample, {number_sign}33, and oxide sample, {number_sign}42. Work on the calibration of the two new powder cameras is proceeding satisfactorily. Two samples of cadmium were made for the purpose of developing a technique for producing single crystals of metals. A sample of tellurium nitrate yielded the same pattern as that reported in the literature. Data on the characteristics of a newly designed tub type moderator as compared with the Hanson cylinder are presented. Some notes on the calculation of probable error show the minimum error to depend upon the ratio of source strength to background.

Equipment for the initial phases of the template spectroscopy work has been completely assembled and is being refined. A number of templates have been made using both the sun's spectrum and an iron arc source. Best results to-date have been obtained by matching the iron arc template against the iron lines in the sun's spectrum. Very good discrimination was obtained at the match point with a dc detection system, but changes in light level were a problem. The addition of a wobbulator to impart vibrating motion to the slit of the spectrograph now permits the use of ac detection. Results with the latter system have been very encouraging. A detection sensitivity of 10-15 m/sec has been achieved initially. For the optical heterodyning phase, the radiometer has been placed in operation and checked out satisfactorily. Mounts and motor drives for the optical components have been completed and a lamp holder cavity for the Hg source has been designed. Development of the TWT mixer has been delayed pending an investigation of the use of a cw laser as local oscillator. Analysis shows that the use of a laser would considerably ease the broad band requirement and made possible the use of standard photomultipliers in the mixing stage. (Author)

A cryogenically cooled, filtered infrared radiometer built on a previous contract has been calibrated, installed on a high altitude balloon platform, flown three times and refurbished between flights. This radiometer has several unique features which include the options for the remote operation of two filter wheels each containing six filters at helium temperature, and a rotary chopper which may be run at 200 cycles per second or positioned open. It contains a 10 in. diameter Cassegrain telescope with nine field stops in its focal plane. Each of these field stops is reimaged on a separate detector for stray radiation rejection. The fields of view of the detectors are small and varied in size and shape. The telescope is mounted on a two axis alt azimuth/ artillery type/servo operated gimbal which makes sky scans under the control of an on-board programmer. The cryogenically cooled telescope is fitted with an antifrost device which protects the system at altitudes above 35,000 ft. The system is capable of absolute spectral radiometry in bands from 3 to 25 microns.

A radiometer described was developed in the Department of Medical Physics at Newcastle General Hospital. Its main purpose was to calibrate a high power monochromator for dermatological irradiations. Other uses include the measurement of total light intensity, energy measurements in spectroscopy, the calibration of thermojunctions and thermopiles, and total heat absorption measurements.

A scanning radiometer, using eight interference filters, has been developed at the Smithsonian Radiation Biology Laboratory for monitoring the spectral distribution of daylight. The unit has been designed to perform in the natural environment with a minimum of maintenance. The range of intensities it can cover is three to four decades, depending on the filtering and detectors.

The radiance of the solar disk was measured as a function of wavelength from 4.0 μ to 5.0 μ with a spectral radiometer system which was calibrated in terms of absolute radiance. The measurements were taken at an altitude of 31 km and, therefore, are relatively unaffected by the terrestrial atmosphere. The radiance values are presented at 0.1-μ intervals over the 1.0-μ range, and it is shown that the effective radiation temperature of the solar disk decreased from 5626°K ± 100°K at 4.0 μ to 5270°K ± 150°K at 5.0 μ. This result represents the only measurements which have been made of solar radiance vs wavelength beyond 2.5 μ so no direct comparison is available; however, the result fits with a single measurement which has been made at longer wavelengths.

THERMAL methods for the measurement of power at centimetre wave-lengths normally involve a substitution process in which the high-frequency power is assumed to be identical in magnitude with the d.c. power causing the same rise in temperature or change in resistance. While in some techniques this assumption results in no appreciable error, it is a general limitation of thermal methods that the high-frequency and d.c. powers are not dissipated in precisely the same manner. This feature of existing methods is particularly evident at low power-levels, of the order of 1 milliwatt, at wave-lengths of 3 cm. and less. Although a calibration of centimetre-wave milliwattmeters can be made against standard equipment operating at higher power-levels, this operation requires much auxiliary apparatus and attenuation measurements of high precision. There is a need, therefore, for further convenient power-measuring techniques at the milliwatt-level in which the limitations inherent in existing methods are avoided. It seems probable that a form of film radiometer or bolometer, in which the ideal of accurate substitution of d.c. power for high-frequency power is approached very closely, will fulfil these requirements; the results of some preliminary experiments in the development of such an instrument are summarized in this communication.

Two cavity-type radiometers have been developed, based on first principles, having the capability of measuring an irradiance with an indicated error 0.3%. The prerequisites for this accuracy are a measuredaperture area, a measurement of electric voltages, and an effective absorptance of its blackbody cavity from 0.998 to 0.999 throughout the uv, visible, and ir spectral ranges. The first cavity-type radiometer is designed to operate only in a vacuum of10(-5) Torr and to measure intensities from 10 mW cm(-1) to 300 mW cm(-2). By using this radiometer in an evacuated cold chamber, an experimental determination of the Stefan-Boltzmann constant is obtained at a value that differs from the theoretical value by 0.3%, which indicates the degree of confidence that can be expected in measurements made with blackbody cavity radiometers. The second type of radiometer is designed to operate in either air or vacuum. Although its aperture opening is windowless, it is unaffected by wind. The range of intensities accurately measurable is from about 10 mW cm(-2) to 800 mW cm(-2); the indicated accuracy is also 0.3%.

Klumb-Schwarz type radiometer gauge (Knudsen gauge) is an absolute gauge for high vacuum measurements, but since its theoretical sensitivity is so far unknown, in practice it needs calibration against another absolute gauge (e. g. McLeod). In this paper a simple theory to determine the sensitivity of a Klumb-Schwarz gauge is described. The optimum angle of the vane is decided as 24° by theoretical considerations. It is further shown that in this gauge the force due to electromagnetic radiations limits the lower range and is something in analogue with the soft X-ray effect in conventional hot-cathode ionization gauges.

An f/2.5 spectral radiometer, consisting of telescope and monochromator mounted on a movable head, is described. By comparison with a blackbody standard and appropriate calibration, spectral radiances can be obtained in watts/cm2-micron-steradian. Observation on many targets indicates a nearly identical radiance for all. Space scans at a given wavelength do give target distinguishability however, and a discussion of possible cause is given. Rapid variations in radiance have been observed, and are still being investigated.

A calibration method is described which makes it possible to measure the brightness or steradiancy of an extended source with a photometer or radiometer which may have nonuniform directional response. A uniform extended source of limited size is successively observed, first as a point source and then as an infinite extended source, with the same radiometer. A circular aperture, slightly larger than the radiometer lens, is illuminated through diffusing sheets of opal glass. The brightness of this source is computed from the total intensity measured with the radiometer at a distance large enough to permit use of a “point-source” calibration scale. Then the radiometer is brought up to the source, where both aperture and field are completely filled, to obtain the response to an “infinite” extended source of known brightness.

Radiometric measurements of the solar corona have been made in the spectral region 1.15–5.1 μ during the total solar eclipse of 20 July 1963. A 78.7-cm diam, f/2 radiometer equipped with a thermistor bolometer detector was employed. Four wide band interference filters were used for spectral isolation, namely, 1.15–2.60 μ, 2.9–4.2 μ, 4.5–5.1 μ, and 7.6–13.4 μ. Coronal spectral intensities were measured as a function of radial position. The radial dependence of these intensities was calculated and compared with the van de Hulst model of the visible corona. Radiometric measurements were made during and after totality.

With the increasing number of commercial radiometers employing rather varied and sophisticated optical designs, the determination of the spectral responsivity of these radiometers has accordingly become more involved. A joint National Bureau of Standards-U. S. Army (Missile Electronics Warfare Technical Area, White Sands Missile Range, New Mexico) calibration of an ir radiometer was conducted at the White Sands Missile Range facility. The radiometer chosen for the measurements employed an uncooled PbS detector with a 10-cm Cassegrainian optical system. The selection of this particular type of instrument permitted two methods of measurement (each employing different standards and techniques) to be investigated. In order to simulate infinitely distant point sources, a 30-cm off-axis collimator was used in both methods of calibration. The methods of calibration and the uncertainties associated with each method are described and the application of the responsivity values to field measurements is discussed. The agreement of the spectral responsivity values as determined by each method is within the estimated uncertainty computed for each procedure.

The paper describes high-speed infrared radiometers utilizing thermistor infrared detectors. Instruments with 4- and 8-in. diam Cassegrain mirror optics are described. The method of operation and the calibration of these radiometers is discussed.The radiometers are uniformly sensitive from 0.7 to 35 μ. They have a response time of 16 msec and a sensitivity of the order of 10−10 w/cm−2 corresponding to a blackbody temperature differential of 0.01°C.The radiometers are used for quantitative measurements of infrared radiation and remote temperature determinations.