This paper is a review of the various tube types generally included under the classification of image intensifier tubes. They will be categorized by the method of electron optics employed, by the gain mechanism, and by the input and output response characteristics. Use of specific types for specific applications will be discussed. Some of the characteristic measurements conducted on image intensifier tubes will be described, with typical data provided on some tubes.
In the original Daguerreotype photo-graphic process which was described in 1839, the exposure-time requirement was 20 minutes. Since that time, a great deal of progress has been made in reducing the time of exposure for the recording of images. Highlights of the historical record of high-speed photography are shown in Fig. 1.
Magnetic focus image tubes have several properties which make their application in direct contact photograpiv attractive. These properties may be listed as: - High Resolution - Low Distortion - Uniform Response Freedou fro fiber optic windows - Large Apertures with short tube lengths.
Very high luminous sensitivities and quantum efficiencies have been reported for III-V compound photocathodes operated in the reflective mode. Sensitivities as high as 2000 μA/lumen for GaAs have been reported (Ref.l) on a research basis. However, most practical imaging devices require operation of the cathode in a semitransparent mode. When semitransparent III-V photocathodes are synthesized by such techniques as epi layers grown on transparent substrates, much lower sensitivities than reflective-mode cathodes are reported. In addition, the broadband response capabilities of the reflectronic mode are generally compromised to some degree when the latter technique is used. Thus, the substantially increased sensitivity of the reflective mode compared to the transmissive mode is sufficient motivation to evaluate possible methods for using reflective III-V compound reflective photocathodes in imaging devices.
A unique instrumentation technique has been developed by industry and is in regular application at the Night Vision Laboratory to repeatably measure the modulation transfer function of image intensifiers operating at output light levels well below that visible. Two of the most severe problems associated with such measurements, very poor signal-to-noise ratio and difficulty of normalization, have been overcome while maintaining an instrument that may be set up, adjusted, and operated quickly by personnel with a skill level comparable to that needed for routine MTF measurements of lenses at high light levels. These advances were made possible by the inclusion of a digital signal averager, a unique film pattern, and innovative electronic interfacing and analization circuitry. Both the fundamental instrumentation concept and the details of how the practical instrument was realized are discussed in this paper.
Optical system designs specifically tailored for the high apertures required for direct view, passive, image intensifier, night vision systems began in the late fifties with a 14 inch focal length F/1.4 catadioptric objective. Three spherical corrector lenses were used in a flat field design for a 40mm image intensifier system. The principles developed proved quite successful, eventually leading to similar objectives used on most 40mm, 25mm, and 18mm format, first generation, night vision sighting telescopes. The principles are discussed and comparisons are made with several other high aperture catadioptric systems recently discussed in the literature. More compact second generation electrostatic and wafer type tubes opened new opportunities for light-weight system designs. The 18mm microchannel plate wafer tube in particular resulted in a number of new viewing systems, including nairs of 1X wide angle telescopes used as night vision goggles. The principles involved in these systems are described as well as the eyepiece performance and characteristics which are unusually important. For these eyepieces and in fact for all of the newer Magnifier eyepieces, Night Vision Laboratories has developed new methods of testing and criteria for specifications that require considerably better correction that had previously been required. The most recent development in the field of direct view night vision devices has been that of the Biocular eyepiece. The principles and characteristics of this rather successful innovation are discussed.
The observation of objects under low illumination conditions is made possible using photo-electronic image intensification of the available light. The intensifier is normally used in conjunction with a large aperture objective lens so as to gather the maximum number of photons. These photons are then imaged onto the photocathode and after electronic amplification the reconstituted optical image can be viewed at the phosphor by means of an eyepiece. At present the limitation of passive night vision devices is the inadequate spatial frequency response of the image intensifier.
This survey of photocathode research for image intensifiers will begin by defining a photocathode and describing briefly a few general properties and principles of its operation for the benefit of those unfamiliar with the terminology of the field. Basically, a photocathode is a thin film of material inside the faceplate of the image intensifier tube which converts the incoming radiation or photon flux into photoelectrons as illustrated schematically in Figure 1. The photoelectrons thus generated move through the film to the free surface where they are emitted into vacuum for further processing. If the input photon flux contains focussed image information, this image is faithfully reproduced, in principle, as an intensity distribution of photoelectrons emitted from the photocathode surface.
Phosphors, as we all know, are major components of many information display devices, such as cathode ray tubes and image intensifiers. Extensive studies with phosphors are being conducted by the electronic industry to optimize their photoelectronic properties in order to achieve the improved image forming characteristics required by present image display technology. This presentation will re-view state-of-the-art research and design requirements for the development of high resolution phosphor screens for image intensifiers. It would be instructive at this point, therefore, to describe briefly some of the general properties of phosphors, screen structure, and principles of operation.
Fiber optic image couplers are essential components in many types of image intensifier tubes. The paper described the various forms of couplers for such applications and some of the steps leading to their development. The properties and capabilities of image couplers were discussed along with test methods and specifications, also examples of intensifier tubes and systems employing their components.
In the field of direct viewing intensifiers, only one basic principle has been used with any success in approaching the theoretical limits set by photon noise. This is the principle of accelerating photo-electrons to bombard a phosphor screen. Two types of device have developed having different arithmetic.
Gentlemen: I would like to start by thanking you for the opportunity to present this paper to such a distinguished assemblage. Although I doubt that I can significantly add to the body of knowledge pertaining to the design of image intensifiers, I hope that I will be able to impart a broader understanding of their application in the law enforcement community as well as some of their operational shortcomings. Let me also point out that these comments represent my own opinions and do not necessarily reflect those of the IACP or the Police Weapons Center.
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.
Approximately three years ago the commercial closed circuit television marketplace suddenly became inundated with a new category of television cameras, those cameras which were advertised as being Low Light Level Television (LLLTV) cameras. It appeared to most camera manufacturers, both in this country and overseas, that it was incumbent on them to get into this new marketplace. Many manufac-turers did so by taking unbelievable advertising liberties. Thus, in 1970 we saw conventional vidicon cameras being advertised as LLLTV cameras, or advertised with the popular catchphrase of that day, "See In The Dark" television cameras. Silicon Diode Vidicon cameras were advertised and sold as "total darkness" television cameras. The camera marketeers were proud; many cameras were sold. Unfortunately, many of the camera end-users did not feel the same sense of pride in their "LLLTV" systems as the marketeer felt in selling it to them.
The overall thrust for development of Night Vision Systems has been to provide equipment which will satisfy the high performance requirements of the modern Army while keeping both acquisition and maintenance cost at the low level necessary to allow high density utilization, This thrust has been pursued by placing emphasis on modular configuration, which allow common use of major. components such as standard, image. tubes, lens systems, and power supplies. Other papers presented during this seminar have already covered, in great detail, image tube and optical assembly. performance. Therefore, I will cover only field applications, present status and development programs.
Since the introduction of photography as a reconnaissance tool for the military, there has been a desire to collect photographic images on an around-the-clock basis. A large technology has been developed since World War II to provide the necessary illuminants to produce night photography. Night photography was successfully produced by a large variety of flash bombs, cartridges and illumination flares. These pieces of ordinance emitted the vast quantities of light required to expose film in aerial cameras. They also, however, emitted vast quantities of noise and tended to illuminate the reconnaissance aircraft more efficiently than they illuminated the ground.
Three hand-held image intensifiers were studied. Two of these were passive visual aids (Starlight Scope and Uniscope) and one was an active IR viewer (Find-R-Scope). These devices were evaluated in terms of number of targets (trucks, boats, village) recognized on a 1000:1 scale terrain model. Simulated air-to-ground views of 20 observers were provided as they circled the model at a simulated 520 MPH and 8500 ft slant range under a moon-light illumination level. Although all targets were visible through the devices when observers were shown when and where to look, almost no target recognition occurred when any of the aids were used in a search viewing-mode under the conditions of the study.
Over the last ten years, the application of image intensifiers in astronomical observations has become a routine procedure. This year at Kitt Peak National Observatory, 45% of the time on the 2.1-meter telescope is scheduled for image intensifier work. Most of this is work that requires a dark sky for spectroscopic observa-tions of faint stars and galaxies, and so nearly 90% of the available dark time on the telescope is scheduled for observations with some sort of image intensifier being the primary detector. Similar scheduling of intensifiers can be found at many observatories around the world. I would like to review the current application of some of these devices in astronomy and discuss their advantages and limitations.
Electrography has been used for astronomical observations over a period of about 15 years. The method is becoming' increasingly accepted by astronomers due to the possibility of new observational insights, the comparative, ease of opera tion, of the new detectors as developed by Kron McGee, and the improvements in reduction techniques. We choose an electrographic detector detector mainly on the basis of useful field, size for astronomical imagery. Our choice, the Kron detector, still has the largest available field.
This paper summarizes some of the tests of 11 image intensifiers evaluated at a test facility developed at the Optical Sciences Center and the Steward Observatory of the University of Arizona. The present work is mostly a continuation of research reported earlier (Refs. 1 and 2). A more detailed discussion of both the present and earlier work is near completion and should soon be available (Ref. 3).
The requirements and techniques for time resolved (<1 nsec) diagnostics of laser pulses are reviewed. The advantages and limitations of applying gated image converter streak cameras to the problem of laser pulse diagnostics are discussed. The design and operation of a unique 10-psec time resolution streak camera is described. The compact size of the camera results primarily from the use of a micro-channel tube for image intensification. Optically triggered avalanche transistor circuits provide reliable, low jitter electronic operation of the streak tube. The S-1 photocathode response allows direct measurement of neodymium laser pulses. Examples of the uses of streak cameras for laser diagnostics and the data obtained are presented. Comments on additional applications and the development of streak cameras with extended capabilities are included.
The need for application of photographic reconnaissance techniques to nighttime operations is apparent. In many situations, troop movements occur primarily at night and weapons installations and equipment may be camouflaged against daylight detection. The weather during daytime hours in many geographic locations makes daytime photographic reconnaissance difficult because of heavy cloud cover.
The statistical quality of a fluoroscopic examination can be improved without an increase in the dose of radiation delivered to the patient by using an image intensifier to raise the light flux reaching the retina and by further increasing the luminance of the image to the level of photopic vision. All currently used x-ray image intensifiers attempt to accomplish both of these purposes, but, in addition, these devices form the basis of modern television transmission of fluoroscopic images, cineradiography, and small format ( 70-105 mm ) photography of the image intensifier output image.
The homo sapien, when placed in perspetive with other mammals is a diurnal creature; that is, his primary activities are carried out during daylight hours. (Ref. 1) The fact that man is diurnal rather than nocturnal will not rattle the cages of our civilization, but when we look at ourselves relative to other mammals we find the homo sapien making up part of a 15% minority (Ref. 2) of animals. One need not refer to his slide rule to calculate the fact that 85% of all mammals are either nocturnal or functional at night. (Ref. 3)