Joseph C. Richmond

Data on total and spectral solar irradiance.

This paper presents a brief survey of the data available on solar constant and extraterrestrial solar spectral irradiance. The spectral distribution of solar radiation at ground surface, computed from extraterrestrial solar spectral irradiance for several air mass values and for four levels of atmospheric pollution, is also presented. The total irradiance at ground level is obtained by integration of the area under the spectral irradiance curves. It is significant that, as air mass increases or as turbidity increases, the amount of energy in the infrared relative to the total increases and that the energy in the UV and visible decreases.

Effect of Surface Roughness on Emittance of Nonmetals

It has been observed experimentally, both at NBS and elsewhere, that the emittance of polished metals can be markedly increased by roughening the surface, by as much as a factor of 2 or 3. For nonmetals and particularly white ceramic materials, on the other hand, the emittance appears to be essentially independent of surface roughness, at least for wavelengths below 7 or 8 μ. This apparant anomaly is explained on the basis of the differences in the optical properties of the two types of materials.

Reflectance errors in infrared thermography: errata.

Laser Raman spectroscopy is highly suitable for concentration and temperature measurements in gases. As well as other factors, the detection limit of the Raman probes is dependent on the sensitivity of the photomultiplier tubes (PMTs) used. Highly sensitive PMTs (for example, RCA type C31034A), which are often used for cw laser Raman spectroscopy and which are necessary to detect low gas concentrations, may easily be overcharged at high concentration values when giant pulse lasers are used with pulse widths of the order of nanoseconds. This can happen because the electric charge of the photoelectrons produced within the PMT—being a measure of the molar concentration of the gas component under investigation—now strikes the PMT anode within a time interval reduced by orders of magnitude compared with that interval when the same charge is produced by cw lasers of identical energy output. This means, however, that the anode current of the PMT, a very critical quantity, is increased by the same order of magnitude as the time interval is reduced. The use of other PMTs with higher anode currents is not advantageous for low gas concentration measurements, as this is usually connected with loss of PMT sensitivity. The ex-

Reflectance errors in infrared thermography

Infrared thermography produces a monochromatic image in which the brightness of an area in the cathode-ray tube (CRT) image is related to the apparent radiance of the corresponding area in the scene viewed. These apparent radiances are frequently interpreted in terms of the radiance temperature of the area in the scene or, more commonly, in terms of the temperature differences between different areas in the scene, which can be converted into true temperatures, or temperature differences, if the emittances of the materials viewed are known. Such interpretation is subject to errors, which are frequently overlooked. The principal errors are due to atmospheric absorptance and the reflected radiance of the areas viewed. Only the latter error is discussed in this letter. The problem is easier to deal with when the measurements are made in an enclosed area with walls at a reasonably uniform temperature. These conditions apply both to IR thermography in an enclosed area and optical pyrometer temperature measurements inside a furnace, but this discussion is confined to IR thermography in a room. First, we define a blackbody radiator and discuss its properties. A blackbody is a surface having the properties of (1) absorbing all incident radiant energy and (2) emitting the maximum possible spectral concentration of spectral exitance (or radiance) for any body at its temperature at all wavelengths. The basic physical laws of radiation were derived for blackbody radiators. They are expressed as the Planck equation, which relates the spectral exitance (or radiance) of a blackbody to its temperature and the wavelength of the emitted radiation; the Stefan-Boltzmann equation, which relates the spectrally total exitance (or radiance) of a blackbody to its temperature; Lamberts law, which in essence states that the radiance of a blackbody radiator is independent of direction; and Kirchhoffs law, which states that, for a body in thermal equilibrium, heated and cooled by radiation only, the radiant energy absorbed is equal to that emitted. No true blackbody surface exists in nature, but a completely enclosed cavity with opaque walls at a uniform temperature is a true blackbody. If a small aperture is made in the wall of the cavity, the blackbody radiation can be observed with only a very slight degradation of the quality of the blackbody. Since the opaque walls of the cavity are at a uniform constant temperature, they are in thermal equilibrium and reemit all incident energy they absorb, and since the walls are opaque, all incident radiant energy not absorbed is reflected. Hence, the emittance and reflectance of the walls sum to one, and the exitance of any area on the wall is equal to the irradiance on that area regardless of the emittance at that point. Let us first assume that we have a room 7 m long, 5 m wide, and 3 m high with walls that are Lambertian (isotropically diffuse) reflectors with an emittance of 0.80. The room has a 1.0 × 1.5-m window, centered on one end. Let us first consider what the apparent emittance of the wall opposite the window will be if the window is open, and no radiant flux enters the window. In any real case there will always be some radiant flux entering the window, but a worst case situation has been chosen deliberately. The basic equation, derived by Gouffe is

Errors In Passive Infrared Imaging Systems Due To Reflected Ambient Flux

Passive infrared imaging systems produce a signal in which the amplitude at a particular spot is related to the radiance of the corresponding spot in the scene viewed. The differences in signal levels in different areas of the image is usually interpreted in terms of radiance temperature differences in the scene viewed, and may be converted to true radiance temperatures if the scene includes an object whose radiance temperature is known. The radiance temperatures are usually converted to true temperatures by correcting for the emittance of objects in the scene. This would be correct in the absence of reflected ambient flux. However, for scenes at ambient temperatures, ambient flux is always present in significant amounts. Temperature errors due to reflected ambient flux are discussed from a theoretical standpoint, and a procedure for experimentally evaluating the ambient flux is suggested.

Three-Dimensional Imaging In Law Enforcement

We survey the variety of three-dimensional imaging methods now available including stereo photography, integral photography, holography, laser stereometry, and moire photography. We then survey the applications of those methods in law enforcement offering a guide as to which methods are best for which application.

A Proposed Standard For Monochrome Television Cameras For Courtroom Use

This paper briefly describes a Proposed Standard for Monochrome Television Cameras for Courtroom Use. In this brief description the physical parameters (size, weight, marking, user information, etc.) are ignored, and the operational parapeters (format, sync signal, power requirements, connections, etc.) are mentioned only briefly. The performance parameters, 1) relative spectral response, 2) total response, 3) signal-to-noise ratio, 4) limiting resolution, 5) contrast transfer function, 6) shading and 7) geometric distortion, are described in some detail. Minimum acceptance levels and methods of evaluation are given for each of the performance parameters.

A Standard for Passive Night Vision Devices for Law Enforcement Use

A draft Standard for Passive, Hand-Held Night Vision devices has been developed for the Law Enforcement Standards Laboratory of the National Bureau of Standards. This Standard is now being circulated for comment prior to adoption as a Standard of the National Institute of Law Enforcement and Criminal Justice of the Law Enforcement Assistance Administration of the Department of Justice. The paper mentions some of the philosophy behind the standard, lists the performance requirements and describes briefly the test procedures for (A) focus adjustment, curvature of field and distortion of the eyepiece lens, (B) optical gain, optical gain stability, light equivalent background, light induced background, luminance of output screen, luminance uniformity, cathode and screen quality, contrast transfer function, distortion and flare of a night vision device complete with objective lens, but with the eyepiece removed, and (C) for resistance to vibration, high and low temperature storage, operation and thermal shock and humidity of night vision devices complete with both objective and eyepiece lenses, and (D) boresight adjustment, click movement and resistance to mechanical shock of night vision devices intended for use as rifle sights.

A Standard For Night Vision Devices For Law Enforcement

The purpose of this paper is to describe the Standard for Passive, Hand-Held Night Vision Devices that has been developed at the National Bureau of Standards under a project sponsored by the Law Enforcement Standards Laboratory of NBS, which is financed by the National Institute of Law Enforcement and Criminal Justice of the Law Enforcement Assistance Administration of the Department of Justice. The Standard is intended for use by law enforcement agencies in procurement of night vision equipment. The Standard is now undergoing review prior to promulgation. Because of the size of the complete standard, it will not be possible to describe it in detail. What is intended is to mention the philosophy behind the standard, describe briefly the test procedures, and mention the performance requirements.