Continuing developments in photoelectronic detector technology in the last few years have given users the benefit of greatly enhanced performance characteristics. Major improvements have been made in the following key areas: photocathodes, proximity focusing, microchannel plates, phosphor screens, electron bombarded Si diodes and CCDs, image tube intensified SSA cameras, fiberoptic windows and tapers, and photon-counting imaging. Highlights of the improvements in these areas are reviewed, and trends for the future are discussed.

Nighttime, nap-of-the-earth pilotage requires information from several sensors including thermal and image intensified sensors. Traditionally, the thermal imagery is displayed on a CRT; the image intensified imagery is displayed with a night vision goggle (NVG), a direct- view device worn immediately in front of the pilot''s eyes. If electronic output data from the image intensifier could be displayed on a CRT, the pilot''s safety and mission effectiveness would be greatly enhanced. Conventional approaches to using charge coupled devices fiberoptically coupled to image intensifier tubes have failed to provide the resolution, contrast, and sensitivity that pilots are accustomed to with night vision goggles. To produce image intensified sensors with performance comparable to an NVG, an intensified sensor that is optimized for coupling to solid state sensors and eliminates all fiberoptic-to-fiberoptic interfaces was fabricated. The Integrated Taper Assembly (ITA) sensor has a fiberoptic taper built into the vacuum of the image tube. The fiberoptic taper minifies the 18 or 25 millimeter (mm) output of the image intensifier tube to the 11 mm diagonal of the high resolution CCD. This requires one optical coupling -- at the CCD surface. By offering high resolution, high sensitivity, and a simplified optical path, the ITA image intensifier overcomes the shortcomings that normally limit the performance of intensified CCD cameras.

An efficient method for calculating electron-optical characteristics of electrostatic electron- optical systems (EOS) for focusing and deflection (gating) in image intensifiers is proposed. Dwelling on this method based on perturbation theory (which in electron optics is usually called the aberration theory) the paper presents EOS characteristics estimated using software support for the relevant computer-aided design (CAD) system. Some examples of CAD of different EOS constructive implementations are provided.

An ultraviolet imaging device has been developed for use in nuclear safeguards. The handheld device detects and amplifies a restricted portion (250 - 350 nm) of the faint Cerenkov glow produced in water around irradiated nuclear fuel stored in used-fuel pools. A 105 mm quartz objective lens, a UV-pass filter and a microchannel-plate image intensifier tube form the real- time visible-light image, which can be photographed or viewed directly through an eyepiece. Normal artificial lighting of the fuel storage bay does not interfere with the Cerenkov-light image. Anomalous fuel assemblies can be detected in the presence of normal assemblies by differences in the Cerenkov-light image. The latest version of this Cerenkov Viewing Device, the Mark IV CVD, is being used routinely by inspectors from the International Atomic Energy Agency to verify declared inventories of irradiated light-water power-reactor fuel. Its design and operation are illustrated and described in this paper, together with plans for further enhancements of the instrumentation.

Among various devices intended for radiant flux imaging, those involving direct conversion are most space-saving and cheap. The major difficulty in their implementation is selection of the recording medium and the physical principle of data read-out from this medium. We have developed and tested a wafer tube for direct conversion of thermal images into visible ones using ferroelectric LiTaO3 as the recording medium and employing for read-out purposes the effect of exoelectron emission that accompanies alterations of a ferroelectric polarization. The design and operation principle of a vacuum tube are as follows. Thermal radiation passing through the tube''s germanium input window is focused onto an absorbing coating and transformed into a thermal field which in turn is transformed pyroelectrically into polarized charges distributed over a LiTaO3 target surface facing a MCP. Owing to a vacuum gap between the target and the MCP, the distributed pyroelectric charges are transformed into an electric field that sets over the target surface. Under the action of exoelectron emission the resulting pyroelectric field turns into an electron flux directed toward the MCP input surface. The emission flux amplified by a chevron-type MCP produces an image on a phosphor screen. The earliest manufactured tube enabled resolution of up to 3 p.l./mm for (lambda) equals 10 micrometers .

A variety of applications require coupling of vacuum tube intensifiers to CCD cameras, in a variety of configurations. Commercial and laboratory systems now being developed continue to expand the performance limits of such electro-optic hybrid systems. We review the state of the art in both CCD and intensifier technology, and present some test data on the most common practical configuration''s resolution and dynamic range. The merits of Gen I intensifiers coupled to cooled CCDs are discussed, and comparisons are made between the two most likely hybrid system configurations. Significant future performance gains appear possible in developmental systems.

In diagnostic imaging, primarily because of the low optical coupling efficiency and the limited sensitivity of video cameras, the use of an x-ray intensifying screen coupled to a camera by a lens is considered suboptimal. Modern electronic x ray acquisition employs image intensification in order to achieve an adequate signal for the camera at the output end of the tube. The high gain of modern intensifiers enables the acquisition of images with noise characteristics limited by the number of detected x-ray photons. However, the advent of ultra- sensitive low-noise CCDs have prompted a re-examination of non-intensified optical coupling approaches for limited applications in medical x-ray imaging where image intensifiers are not optimal. The measured characteristics of a cooled CCD system, with 2048 X 2048 pixels are presented as they apply to the requirements for some x-ray imaging applications. Measurements with a lens coupling suggest that the x-ray quantum limit may be reached in such a system after careful optimization of the components, but much better results may be attained with fiberoptic coupling. The potential merits of intensified versus non-intensified approaches are also discussed.

A number of time analyzing image converter tubes integrated with thinned, backside- illuminated, electron-bombarded (EB) CCDs were developed. Among them are PV001, PV003, and PIF01 streak tubes, differing from each other by their internal electron optics and external (metal-glass or metal-glass-ceramic) geometry. All tested EB CCD/streak tubes reveal relatively good behavior due to the long-term stability of their input S1 photocathode and reproducibility of the matrix parameters. These tubes differ advantageously from their regular type phosphor screen analogs by their rather high spatial resolution (40 lp/mm at 10 MTF for PV type tubes and 25 lp/mm at 10 MTF for PIF type tubes), reasonably good threshold sensitivity of 10-10 J/cm2 in standard, 10 MHz CCD scanning mode at 850 nm input radiation, and picosecond temporal resolution (better than 5 ps) at a relatively acceptable value of the light transfer function (approximately 102).

The design, implementation, and evaluation of a high-resolution vidicon-based reconfigurable imaging system for integration into a photon-counting streak camera that can be readily coupled to a standard interface and computer have been achieved. Experimental results are reported which demonstrate that the design goals are met, providing the capability to measure differential time to better than 3 picosecond accuracy. Augmented by real-time calibration, the accuracy, linearity, noise levels, and stability of the system are adequate to support dual wavelength laser ranging.

In choosing an image tube''s phosphor type, the system designer is confronted with several unknowns, depending on the tube''s gating time, repetition rate, etc. This paper reports on some measurements made toward a better understanding of the factors involved -- temporal and spectral issues, and efficiency of coupling to a silicon detector -- for some of the common phosphors, such as P-20, P-43, P-46, P-47, and a red rare-earth phosphor.

Advances in four key areas of microchannel plate (MCP) technology are discussed. The development of centrifugal shearing techniques promises to reduce significantly or eliminate the characteristic distortion introduced into curved MPCs during the channel bending process. The Advanced Technology Microchannel Plate, now under development, will allow microfabrication of high performance MPCs for electronic imaging and space science applications. The Ultra-Small Pore Microchannel Plate with channel diameters as small as 5 microns has been successfully fabricated. These devices, when incorporated into image intensifier tubes, raise the limiting spatial resolution from 30-40 lp/mm to 60-80 lp/mm. Clean Plate Technology can now be applied to high aspect ratio straight-channel microchannel plates, yielding electrical performance rivaling curved MCPs.

Life performance of image intensifiers containing Litton High Performance microchannel plates is examined. Luminance gain, microchannel plate electron gain, and photocathode response decay data are presented and explained in terms of a simple model in which a source species population is reduced by combination with a poisoning species population.

The application of fractal tiling theory has been shown to produce fiber array devices with a very high degree of order [1 Previous work has concentrated on optical waveguide arrays (i. e. fiberoptic faceplates). In this study we review the construction and performance of microchannel plates (MCPs) made using fractal tiling methods. MCPs with 40 mm active areas having nearperfect channel ordering were produced. These plates demonstrated electrical performance characteristics equivalent to conventionally constructed MCPs. To the best of our knowledge these are the first MCPs which have a sufficiently high degree of order to permit single channel addressability. Potential applications for these devices and the prospects for further development will be discussed.

We have fabricated 18mm format vacuum photodiodes incorporating GaAsP/A1GaAsP photocathodes (Eg 8eV) with P-20 phosphor screens. The photocathode response peaks at approximately 550nm. The quantum efficiency at 550nm is in excess of 55 (electrons per incident photon). The photocathode dark current for these tubes is less than i014 Amps/cm2 at room temperature. We have compared this cathode with the GaAs/AlGaAs photocathode.

ITT has an ongoing research program to develop a new generation of image intensifiers that uses a charge-coupled device (CCD) operated in the electron-bombarded mode in order to detect electrons directly from a GaAs photocathode. This type of image intensifier combines signal-to-noise levels typical of microchannel plate (MCP) image tubes with the modulation transfer function (MTF) performance of a diode tube design and generates electronic output compatible with standard video displays. The first successful electron-bombarded CCD (EB- CCD) image intensifier with a GaAs photocathode was demonstrated in August 1991. This photocathode was a standard ITT negative-electron-affinity (NEA) GaAs photocathode. Imagery was obtained from this device. The intensifier incorporated an RCA back-illuminated frame-transfer CCD, designed for electron detection and mounted in a vacuum compatible package. The cathode had a photoresponse of 965 (mu) A/lm at 6 kV -- equivalent to 1,166 (mu) A/lm at an electric field typical of proximity-focused image intensifiers (80 V/mil). A full-well signal was obtained with 2.5 X 10-5 fc photocathode illumination and 6 kV cathode-to-CCD bias. A photocathode life in excess of 280 hours at 1 X 10-4 fc illumination was demonstrated.

Coulomb interaction in electron beam is one of the main factor limiting maximum time resolution of subpicosecond image converter tubes. Among the investigations devoted to theoretical analysis of this problem one can distinguish two different approaches. In one of them1 the simplified model concept for the beam motion equations is treated in order to estimate the typical system parameters within the accuracy of one order of magnitude .Suchapproach can not give the detailed characterization of the beam structure required for formation of space—time modulation transfer function (MTF). Another approach2 is related to the analysis of closed system of equations describing the initial boundary nonstationary selfcoordinated problem for electron beam. It is well known that in general case the correct numerical solution of a such kind problem may be obtained only by application of rather complicated and massive numerical procedure which realization is difficult even for modern powerful computers. The important condition describing the behaviour of Coulomb interaction in dynamic emissionimaging system incorporates the necessity of space charge calculation at the level of "optical" accuracy when initial photoelectron angular and energy distribution have to be taken into account. It is also highly important that in the mentioned above systems the inherent Coulomb field of the beam proves to be as a rule several orders of magnitude integrally weaker than the applied external field and therefore Coulomb field may be considered as a perturbation3' Two indicated circumstances, namely, the necessity of high accuracy attainment and the possibility of application the perturbation theory are put into consideration in this work.

Experimental results are reported which measure the variation of SNR performance with input image size for selected Generation 2 microchannel plate intensified CCDs (ICCDs). The evidence shows that SNR performance can begin to decrease when the image size drops below 300 microns. A generalized theoretical model for ICCD SNR performance is also presented.

Many space applications of photon counting detectors (PCDs) are particularly challenging due to the requirements for high local and global count rates and for sensitivity down to the soft x ray region. The count rates per pixel required may be as high as several hundred counts/second and overall count rates as high as 1,000,000 counts/second. At the Rutherford Appleton Laboratory (RAL) we have been developing a range of modular detector systems in the visible and UV which are built specifically with solar UV observing in mind. The high resolution/high count rate imaging performed pushes the technology to the limit and shows up, in some detail, problems in intensifier and microchannel plate (MCP) manufacture. Close cooperation with the manufacturers of the intensifiers used, Photok Ltd., has led to improvements in the intensifier design. In the following paper the performance of these detectors is discussed in detail along with plans for their future development.

A modified PV006S image-converter tube is suggested for use as a primary receiver component for laser ranging systems. The design features of the tube itself, its two- dimensional 2 GHz response, and the sensitive deflection system are reviewed. Some experimental results are also presented which indicate that a 500 MHz circular-scan camera prototype has an experimental temporal response function of less than 5 ps in cw mode of operation.