Eleven years ago I edited Vol. 2, No. 1, of the SPIE Journal (now Optical Engineering), under the title "Infrared Today & Tomorrow." In it I summarized the state-of-the-art of civilian and military infrared technology and ended by stating, "In the future the development of imaging systems which will produce a visible picture of objects sensed only by their infrared energy is anticipated. This will permit the use of photographic instrumentation techniques in connec-tion with systems analyses and data recording activities...," and ". . . the equivalent of television based on IR content." We also (see McFee, op. cit., p. 16) predicted, ". . . the develop ment of techniques to build preamplifiers in juxtaposition to detector elements . . .," and ". . microelectric fabrication techniques . . . to include detector elements in a single package with the electronic circuits."
Abstract Optical image evaluation procedures as specifically applied to the analysis of scanning infrared (IR) optical systems having discrete detectors and viewing isolated point sources in a low-intensity structured background are discussed. Details about the system design requirements determination, the optical performance evaluation, the interaction of the detector focal plane design and evaluation procedure are included. The optical system analysis is performed using the computer program POLYANA, a program developed by The Aerospace Corporation. A complete example utilizing an F/2.5 all reflective Schmidt system is described in detail to illustrate the analysis procedure.
A method for designing and predicting the performance of large aperture sensors is described. The sensor is modeled as an elementary optical system combined with a focal plane consisting of many discrete detectors. The output signals of the detectors are filtered and combined to compute signal-to-noise ratio and to estimate centroiding (target location) error. The dependence of these two performance criteria upon major design parameters is described. The quantum efficiency of the detectors is a critical factor. Optical aperture is also a major factor in design, but lack of a cost function for optical subsystems is a drawback. It is possible to design an optimal focal plane layout for given detector and optics models. The electrical filter is optimized separately. Centroiding error in the crossscan direction depends in an interesting way upon the focal plane layout. Two mathe-matical appendices describe the sensor as a linear, time-invariant system and some of the signal processing.
Infrared detectors are discussed in terms of an equivalent circuit consisting of a photocurrent generator in series with a gain mechanism, both shunted by the detector internal impedance. Detectivity, the usual measure of infrared detector sensitivity, is defined in terms of the parameters of this equivalent circuit. The properties and performance of the most common and best developed infrared sensitive photoconductors and photodiodes are then described in terms of these parameters. Both normal and low background performance are considered, with emphasis on maximum sensitivity in the low-to-moderate frequency region. Short discussions of the current status and recent developments in thermal detectors, high-speed detectors, and infrared detector arrays are included. The importance of preamplification is stressed throughout, and preamplifier principles and current practice for the various types of detector are separately discussed.
The basic processes leading to the absorption, emission, and scattering of infrared radiation in the atmosphere below 70 km are briefly reviewed. The present ability to predict these effects is demonstrated by examples of computed infrared transmittance spectra obtained with low-resolution band model techniques and with high-resolution monochromatic techniques. Several examples are included of the experimental application of these techniques to the derivation of atmospheric composition and meteorological data from infrared observations.
The objectives and general requirements for long-wave infra-red (LWIR) spaceborne sensor calibration are presented and assessed. The development and capabilities of existing ground calibration facilities (calibrators) are summarized. Recent calibrations and spaceborne measurements have identified major deficiencies in the calibrators and sensor response anomalies. These deficiencies, the response anomalies, and the operational conditions that influence the anomalies will be described. Changes in calibration methodology to characterize these effects will be reviewed. A review of the current "effective" method of data presentation and alternate methods will be provided. An assessment of the overall calibration status will be provided along with recommendation for critical calibration technology development.
During turn-around a scanning mirror may be subjected to severe angular acceleration which can cause surface deflection of sufficient magnitude to distort the image significantly. In this paper a solution is developed for a circular mirror for a particular method of application of the direction-reversing forces. Parametric curves are presented which allow computation of the deflection as a function of mirror size and point of load application. An example is computed in which the deflection is a significant fraction of the optical wave length.
Substantial improvements in the thermal imaging state-of-the-art are occurring in forward-looking infrared (FLIR) system technology. These far infrared imaging systems have achieved broadcast television standards for frame rate, resolution, and signal-to-noise ratio, and are moving toward the standardization and volume production necessary for wider dissemination of the technology. Current techniques permit a single-detector system to approach within a factor of two the theoretical limit of thermal sensitivity using a photon detector. FLIR systems now use mechanical scanners and the mini-mum number of detectors necessary to achieve the needed performance, but performance requirements are increasing, and future systems may use mosaics of self scanned detec-tors. Then problems of element-to-element nonuniformities and of background substraction must be solved.
The IR engineer has the choice of many types of cryogenic cooling systems which can be used for cooling components of IR systems. This article is intended to give some guidance in this selection process to the IR engineer, who may not be intimately familiar with various cooler types. The article briefly describes the characteristics of cryogenic coolers which are suitable for cooling components in IR systems to temperatures in the range of 4-120 K. It covers systems which are presently available, or will be available in the foreseeable future. The principles of operation of commonly available cooler types are described, the characteristics of each type are discussed, and the probable areas of application for each type are indicated. The following cooler types are discussed: open cycle systems using expendables (liquid cryogen, solid cryogen, and Joule-Thomson coolers); continuous flow, closed cycle refrigerators (Joule-Thomson and reversed Brayton cycle refrigerators); intermittent flow, closed cycle refrigerators (Stirling cycle; Gifford-McMahon cycle, and Vuilleumier cycle refrigerators, plus split cycle refrigerators based on these three cycles); and passive radiators. A bibliography, containing references to more detailed information on all cooler types, is included.
The Southern Rocky Mountain Region of the United States is not a newcomer to the world of photo-optics. Early work at Flagstaff, Arizona, led to the detection of the outermost planet, Pluto, by Dr. Clyde W. Tombaugh. Rocketry experiments in the 1920s by Dr. Robert Goddard near Roswell, New Mexico, spawned the need for optical tracking instruments to record for later study the performance of extrater-restrial vehicles in flight. The first photographs from space which demonstrated earth curvature were made at White, Sands Proving Ground, New Mexico, in the 1940s. Today, a compilation of significant, discretely separate installations and activities in the geographic area would be a major effort.
Using a very narrow band (0.004-0.013 nm) Universal Bire-fringent Filter, Filter, rapidly turnable between 410 and 700 nm, and the vacuum telescope at the observatory, we have developed photographic and video techniques to observe solar magnetic and velocity fields utilizing the Zeeman and Doppler effects respectively. This is in addition to direct filter-grams which can also be obtained at any wavelength in this wavelength region. The video technique permits virtually real time display. Both techniques are used for time lapse cinematography of solar activity.
A four-pulse multiple cavity laser" (MCL) system was de-veloped for use in recording time sequential holograms of fast events. It was used to record four double exposed holograms of an exploding bridge wire (EBW) on a single 10E75 plate with between 2 and 5 i s framing time. The holograms were written and processed to an optical density of 2.0. The complex change in index of refraction of the plasma created is visible in the fringe structure of the refconstructed image. The shock front was measured to propagate at an average speed of between 0.13 and 0.14 cm/us. plain advantages of the MCL holographic system are: (1) loy cost, (2) greater energy/time interval, and (3) electronic control of the time interval between frames.
The Los Alamos Scientific Laboratory (LASL) maintains an active and broad program of computerized image analysis. Initially begun as an image enhancement adjunct to a flash radiographic facility, the base of experience has now been expanded to include medical and industrial radiography, astronomical observations, and high-speed framing camera images. In addition to classical image enhancement and restoration, there is now activity in precision high-speed data extraction, often as an end in itself without the need for the analyst to view the image once it is digitized and supplied to the computer. Examples of various applications are given. In order to support these activities LASL has developed a library of image analysis subroutines, and is now developing a user-oriented command language. A substantial hardware installation has been developed. An active research program for development of new digital techniques of image restoration is maintained.
An airborne telescope utilizing a scanning interferometer and video recording system has been used by the Los Alamos Scientific Laboratory for coronal-emission-line-profile measurements during the total solar eclipses of 1972 and 1973. A 250 mm diameter f/8.0 achromatic doublet refractor served as the objective lens, and a pressure scanned Fabry-Perot interferometer with narrow band transmission filters provided 0.005 nm spectral resolution of the selected emis-sion lines. A variable-gain channel-plate image intensifier, fiber-optically coupled to an Sb2 S3 vidicon tube, was adapted for use as a high sensitivity image detector. At maximum sensitivity this system was capable of recording a video image with signals as small as 5 x 10-9 J/m2 incident upon the in-tensifier photocathode. The vidicon tube was scanned at a rate of 60 fields per second in a 500-line, non-interlaced television format. The composite video signals were recorded on standard video tapes from which they were digitized for subsequent computer processing.
The paper describes a compact Modulation Transfer Function Analyzer for long focal length, infinite conjugate optical systems testing. It uses a 12-inch aperture collimating off-axis parabola and two beam-folding mirrors. The analyzer scans the line spread function, produced by the optical system under test, with a mask having a sinusoidal transmittance grid which varies as a function of the rotation angle. The transmitted modulated flux generates an AC output rep-resentative of the modulation transfer function, which is plotted on an X-Y recorder. A heavy cross-slide bench as-sembly on a granite base provides the required mechanical rigidity and accuracy.
A nephelometer has been developed for airborne measurement of polar scattering diagrams of atmospheric aerosols. The nephelometer light source is a modulated heliumneon laser; a digital synchronous photon counter is utilized to measure scattered-light intensity. The system has been designed for airborne measurements on a pressurized aircraft using outside air ducted through a 5-cm diameter airflow tube; the sample volume is that which is common to the intersection of the collimated source beam and the detector field of view within the airflow tube. The instrument has been flown on the NASA Convair 990 airborne laboratory to obtain data on the complex index of refraction of atmospheric aerosols. Particle sizing devices were operated simultaneously to determine the aerosol size-number distribution. Calculated values of the angular variation of scattered-light intensity were obtained by applying Mie scattering theory to the observed size distribution function and assuming different values of the complex index of refraction of the particles. The calculated values were then compared with data on the actual variation of the scattered-light intensity obtained with the polar nephelometer. The most probable value of the complex refractive index was taken to be that which provided the best fit between the experimental light-scattering data and the polar scattering diagrams calculated from the observed size distribution function.
Diffraction of light by a plane sound wave in an optically isotropic medium is described for arbitrary optical and acoustic polarizations. The index waves produced by a sound wave propagating along a crystallographic axis of a cubic crystal are determined. A partial wave analysis of the vector wave equation describing the propagation of a light wave is given, and analytic solutions of the equation are obtained in the Raman-Nath and Bragg regions of diffraction. The differences between diffraction of light by sound waves in solids and liquids are pointed out. It is shown that the solutions of the vector and scalar wave equations are significantly different only for large Bragg angles. The direction and phase dependence of a diffracted beam on the acoustic frequency and phase, respectively, is pointed out for real-time wavefront correction.
In the preceding articles, the advantages of Raman analyses have been presented. It is now time to point out a selected number of uses where the Raman approach can uniquely solve environmental analysis problems.