An introductory survey is given of the methods used in modern image assessment. The need for the introduction of the optical transfer function is first considered, together with the validity of regarding image formation as a linear process. The concepts of the contrast transfer function, the phase transfer function, and their complex form the optical transfer function, are introduced by considering the image of a cosine-wave intensity distribution: the relationships these bear to the line spread function (impulse response) of the optical system are established. A tolerancing of production runs, for both contrast and phase transfer functions,is proposed, based on the use of two suitably chosen spatial frequencies. The manner in which the range of spatial frequencies of interest affects the degree of correction of aberration required is also discussed. The final section derives the interrelationships between the point spread function, the line spread function, the edge spread function, and the transfer function.

This paper describes techniques for the calculation of geometrical MTF and diffraction-MTF, using Hewlett-Packard HP-85 and HP-9845 computers.

Most optical workshops and many research workers have the use of simple double-pass interferometers that use laser sources. Many such workers are familiar with the different patterns formed by the interferograms resulting from aberrated optical systems, but few workers seem to appreciate the effects of such wavefront deviations on the final image produced by those systems. This paper uses a novel technique to plot the fringe patterns produced by each of the Sixth Order Aberrations, and then goes on to plot pseudo-isometric views of the resultant Point Spread Functions. A comparison is made with the corresponding geometrically-generated Spot Diagrams, to demonstrate the superiority of the diffraction-based calculation over the old-fashioned geometrical method.

In conventional flight simulators information relating to a terrain model board is relayed to the pilot via closed circuit television. The main disadvantage of this type of system is the limited bandwidth, resulting in either a restriction to the pilot's field of view or low resolution. These limitations are most serious, for instance, in ground attack helicopters where both high resolution and large fields of view are required. To overcome the above limitations Rediffusion Simulation Limited have undertaken a Ministry of Defence supported programme of work to develop a wide angle, high resolution, laser visual system.The work presented in this paper represents part of a contract placed upon Sira Institute by Rediffusion Simulation Limited.

The paper starts with the definitions of the Modulation Transfer Function (MTF) and the Optical Transfer Function (OTF). It contains the MTF of image tube lenses, fiber plates, phosphor screens, mica coupled image, human eye and the component and composit response of Gen I and Gen II image intensifier tubes.

This paper discusses the design, manufacture and testing of a standard 100 mm f/4 triplet lens and its subsequent use for checking the accuracy of OTF measuring equipment. The tolerances on individual lens parameters, established in the design stage, correspond to a maximum uncertainty in MTF of ±0.05 at 20 c/mm (on-axis at full aperture). Precautions necessary during manufacture to achieve this level of agreement between theoretical and measured performance for the assembled lens are described. Interferometric methods were used to assess the accuracy of manufacture, the wavefront aberration so determined being compared with results of calculation from the basic lens design data. The OTF was derived from the measured wavefront data, confirming that performance within the ±0.05 MTF tolerance band had been achieved. The MTF was also measured directly, using the SIRA-BECK Eros IV equipment at RAE Farnborough, and compared with the calculated performance. One result of this comparison was to indicate the presence of an unsuspected source of error in the measurement procedure adopted for this particular test. This demonstrates very clearly the value of having a lens such as this available as a primary standard of reference, and that the lens amply fulfils its original purpose.

Classical and subjective methods, such as resolving power tests, star tests and aberra-tion measurements and the like, have been used in the inspection and evaluation of imaging system performance in China so far for a long time. Beginning from the 1960s,situation in this field is changing. In this paper some types of NSF and OTF measuring equipments developed in China within about 10 years after 1965 are briefly described. Performance of some higher precision OTF measuring equipment is given in some detail. In 1976, comparative MTF measurements for two standard lenses among some labo-ratories using different arrangements were carried out, and results of this study are given. Some discussions are given about problems concerning the assessment methods and the selection of characteristic spatial frequency. The paper also gives a survey about our practice in simplified NSF test conditions and proposed criteria of image quality for photographic lenses.

This paper describes the problems associated with the measurement of the OTF of a wide angle scanned laser visual display system for flight simulation. The application of proprietary OTF equipment to the measurement of the optics under static conditions is described. An outline is given of the development of a device which it is anticipated will permit OTF measurements to be made at the display so as to include the dynamic effects of the electronic and electro-optical components.

This paper presents a method of high-speed production lens testing by modulation transfer function (MTF) measurement. An MTF measuring system is developed for the purpose of routine go-not go testing of photographic lenses. An operator can test one lens in less than ten seconds which includes not only the time of MTF measurement but also the time of operator's manual work for attaching and detaching a lens to/from the system. The photodiode arrays are utilized as scanners of line spread functions. MTFs are measured for on-axis and off-axis position in the image field of 24x35mm film format. There are 14 items which are checked by MTF values, and if all the measured MTF values for each of the items clear the specified values, ao-sign is displayed on the indicator. The system can be used for testing photographic lenses that are in the range of focal length from 35mm to 300mm, and F-stop from 1.2 to 5.6.

An instrument for measuring line spread functions (LSFs) was built. The instrument, which was controlled by a microprocessor, was capable of implementing quality criteria based on the LSF. Because its operation was directed by the software of the microprocessor it was possible to perform varied measurements automatically and to perform analysis of the results.

Limiting resolution is a subjective and therefore unreliable measure of the imaging performance. The modulation transfer factor corresponding to a well chosen spatial frequency is an objective measure that has all the benefits but none of the disadvantages of the limiting resolution. The method of measuring the modulation transfer factor is relatively simple and can be expanded to give also the macroscopic uniformity and the distortion of the imaging device under test. The paper describes an instrument for the measurement of the modulation transfer factor of image intensifiers.

Precision optical systems for the infrared (IR) require optical grade IR materials. Materials mostly used are Ge, ZnSe, ZnS, Chalcogenide glasses. An instrument is des-cribed where dn/dx, the inhomogeneity of the refractive index can be detected and quantitatively interpreted by means of measuring the shift of a fine-line image. The influence of spatial variation of absorption to these results can be eliminated by a double scan at first with the measurement slit in the left flank and secondly in the right flank of the fine-line image. On the other hand, if the slits are removed this instrument enables measurement of local transmission. Results obtained with IR materials are discussed. The same instrument can be used to measure the absolute value of the refractive index by means of the shift of the fine-line image caused by a small wedge. Beside inhomogeneity optical quality of IR materials may be degraded by birefringence. A setup for measurement of birefringence in the IR is shown. Image quality in the IR can be determined by measuring the MTF. An instrument is described with a small slit as test. First the IR fine-line image is measured and the MTF by Fourier Transform. Hereby noise is eliminated with a lock-in amplifier. With a small test slit it is easier to fulfil the theoretical requirements, especially to realize an isoplanatic region and an incoherent illumination. By this means no ground glass is needed. With a precision scanning unit no additional magnification of fine-line image is required. Both remarks are important for IR measurement technology. Furthermore, the influence of the colour temperature of the radiation source onto the accuracy of results is shown. A comparison is made between obtained measured results and the MTF calculations. The possibility to measure the fine-line image is important to describe the image quality of IR systems used in connection with detector arrays.

The reasons why thermal imaging lenses often fail to achieve their design performance specifications are discussed. Examples of lenses having centering errors and figuring errors are shown. The effects of using material with refractive index inhomogeneities are described. Methods for reducing the discrepancies between the design and actual performances in thermal imaging lenses are briefly outlined.

Cathode-ray tube evaluation is becoming increasingly important in connection with the prediction of overall performance for systems incorporating an imaging display. Modulation transfer function has been measured by a method which takes account of the basic non-linearities of the crt and also offers improved accuracy by reducing the effects of phosphor screen noise. Two tests for crt internal veiling glare are discussed. Standard test conditions, which have been successfully used for a wide range of displays are described for both mtf and veiling glare. A contrast index is employed to indicate the extent of nonlinearity in the display transfer characteristic, and the paper also discusses briefly the use of generalised drive characteristics and limiting contrast curves as a supplement or alternative to modulation transfer function for definition of display performance.

This memorandum describes a semi-automatic facility which directly plots the dynamic modulation transfer function of cathode ray tubes. A sine-wave pattern is generated which moves at a chosen rate across the screen. The intensity of the spatial pattern is measured by a stationary optical detector. Some discussion is given of the potential of pulse output signal processing to determine the ultimate performance capability. The high signal to noise ratio obtained is illustrated by some preliminary results.

An equipment designed specifically for testing image intensifiers, including those which incorporate micro-channel plates, is described. The equipment consists of a source unit which projects an image of a narrow slit on the input face of the tube and an analyser unit which effectively does a Fourier analysis of the slit image formed on the output face of the tube. The MTF versus spatial frequency can be plotted on an x-y recorder or read off directly from a digital meter. A facility is provided for rapidly selecting four preset spatial frequencies for use particularly in production testing. The factors which affect the design of an equipment for testing image intensifier tubes are considered in relation to this particular instrument. Examples of measurements made with the equipment are given.

Studies of high resolution phosphor screens suitable for image intensifiers or small cathode ray tubes often require the MTF of the screens to be measured separately from the electron optics. Because electron excitation is essential for realistic measurement a demountable vacuum system is required if a usefully large number of screens are to be measured. These requirements have been met by combining a scanning electron microscope (SEM) with a standard MTF equipment. The screen is placed in the sample compartment of the SEM and a very fine line less than 2 μ;m wide is excited by the focussed beam of the SEM, deflected in the line direction only. The structure of the screen degrades the line and a specially designed optical system is used to relay an image of this degraded line from the SEM to the MTF analyser. Procedures used to ascertain the system response are described together with results from a variety of phosphor screens.

Second generation image intensifiers are generally used with cathode illuminations so low that the system performance is significantly degraded by quantum noise. The noise arises from the inherent quantum nature of the detected radiation and the statistical variation in the gain of the intensifier. A signal to noise measurement made under specified conditions provides a quantifiable assessment of this characteristic for any image intensifier. The conditions that are now generally used in specifications are given and a method of measurement described. Also methods of determining the individual contributions to the overall signal to noise ratio are described.

The importance of the standardization of measuring methods which are employed to derive quality criteria for use in the international exchange of goods is pointed out. As an example, the proposals of standardization of the optical transfer function (OTF) are used to show how to overcome the difficulties and to find a solution of the problems. It should be emphasized that existing national and international standards dealing with the same subject are very helpful to set up a successful ISO Standard.

This paper presents a brief introduction to the concepts of the NATO Standard on the OPTICAL Transfer function of imaging systems.

In the UK a procurement specification for optical germanium has been evolved. The prime requirements for optical grade germanium are stated in terms of conductivity type, resistivity range, refractive index, refractive index temperature coefficient, MTF performance, transmittance, optical absorption and fracture stress. The general aim of the specification is to relate the properties specified as closely as possible to the performance of the material in component form. In the present paper numerical data for each of the above parameters is given together with discussion on the means by which the data has been evolved as a result of materials characterisation where this is appropriate.

To assess the importance of various objectively measured imperfections in visual optical and electro-optical devices it is necessary to appreciate the interactions of the eye/brain system. This system contains not only optical transmission components but also a complex sequence of series and parallel neural processes. Early parts of visual image processing are now sufficiently well understood to be able to develop fairly comprehensive models. Such models need to acknowledge, amongst other things, that diffraction, aberrations and refractive inhomogeneities all play a part in retinal imagery, that the retinal image is rarely in perfect focus due to accommodation errors, that the visual system is inherently noisy, that visual performance varies progressively across the retina and that the eye senses primarily changes in retinal illumination in space and time. The basic stages of early visual processing are briefly described. Models are then discussed for describ-ing the display/observer interface factors which influence retinal image formation, the threshold performance functions for fovea' vision, suprathreshold perception of image fidelity and rudimentary search processes.

The aim of this paper is to identify possible objective test methods for afocal visual instruments which are relevant to user requirements. The problems associated with testing image quality of such instruments are introduced and discussed. A direct concern in this work is providing quality assurance specifications pertaining to sights of a fixed design but constructed in large production runs, not to small numbers of specially assembled sights. The experimental work is divided into two parts. The original investigation examines the levels of correlation obtainable between existing types of MTF-related image quality specifications and subjective resolution tests. It is demonstrated that greater care must be taken, than hitherto, in choosing appropriate test methods. In addition, very low levels of correlation are obtained between subjective resolution tests and objective MTF-related measures. The discussion of possible reasons leads, naturally, to the new investigation. A well-controlled analytical experiment was devised specifically to test the aspect unique to afocal visual instruments: coherent coupling between the sight and the eye. The investigation attempts to mimic, by subjective means, the variables and conditions relevant to (objective) MTF measurements. This investigation has not yet been completed; but the preliminary results do show, however, that expectations of quantitative agreement between subjective and objective test techniques are not borne out. The comparison fails spectacularly, suggesting that coherent coupling effects represent a major source of discrepancy. Some details are given of current measurements aimed at elucidating the precise factors involved in the optical coupling between an afocal sight and the eye.

The application of sine-wave contrast sensitivity as an image assessment tool is examined. The required sinusoidal targets are carefully specified and the construction method is briefly described. Problems associated with the measurements are discussed and an apparatus for the subjective testing of visual instruments is presented. The method is used to investigate the best correction of astigmatism and field curvature in a telescopic system.

This paper provides a summary of results and conclusions of work, to be published in full elsewhere, on visual performance related to system design and assessment. The system transfer functions, MTF and PTF, are widely used, nowadays, for characterising the quality of many classes of imaging system. The aim of the present work is to provide an interpretation of these transfer functions which is relevant to the visual performance of an observer when viewing the output of the systems. Measurements were made of visual contrast discrimination and of phase threshold levels. In both cases, attempts are made to derive general conclusions, since these tend to be more useful for system design and evaluation. From the experiments, a tolerance is derived for MTF measurement and assessment, so enabling MTFs of different designs, or determined by different techniques, to be directly compared. Results presented on the sensitivity of the visual system to changes in phase suggest that the PTF need not be considered for well-designed and constructed systems. Thus, phase threshold levels are small only at low spatial frequencies: the region where phase shifts in imaging systems are usually negligible.

The subjectively assessed quality of photographs taken with a 48 inch lens has been compared with the MTF of the lens measured in the same image planes. The image quality and associated MTF were varied by the through focus technique, using a range of image planes on both sides of the optimum. The photographs have been examined to ascertain the change in MTF, at the various levels, resulting in a just detectable difference in image quality, and whether a 'reduced' MTF could be used which would sensitively indicate the changes. The results will be used to provide MTF production tolerances for lenses used in aerial photo-graphy. They indicate that a measurement at a single selected spatial frequency should be sufficient to enable differences between lenses in a production batch to be monitored reliably.

The user of any imaging system is primarily concerned with securing some desired level of perception performance against real world objects of interest under anticipated typical operating conditions. Developing validated predictive models for this 'field' performance involves a number of complexities and sources of uncertainty which are not encountered when modelling hardware performance from objective laboratory measurements. These problems become apparent in the course of describing the basis of a program for generating the perception probability versus range curve for intensifier sights and a statistical trial which was conducted to validate its predictions.

A series of psychophysical experiments was carried out in order to investigate the effects of scan-line density and pictorial noise upon the recognition of thermal images of military vehicles. The basic test material consisted of a series of 69 thermographs made in the field. The thermographs were displayed indoors using a flying spot scanner system with variable scan line density and variable amounts of noise added electronically in the scanning circuit. The experiments were carried out session-wise. In each session all 69 pictures were presented in random order with the same scan-line density and the same amount of noise. Subjects were asked to identify the vehicles out of 6 alternatives. Big differences are found in recognizability between the vehicles and even bigger differ-ences between different views of the same vehicle. Hot spot front views are most difficult. On the average an identification score of 70% is obtained at about 2 scan lines per meter (measured over the real objects), when no noise is added. The effect of noise is surprisingly small. In additional experiments it was found that the scores at a fixed scan-line density could hardly be affected by MTF-operations, if not pushed to the extreme. We perhaps may conclude that prediction models as well as design approach, should recognize scan-line density as the main parameter, to be considered first.

Transmittance and Veiling Glare Index measurements in conjunction with the Optical Transfer Function can give a set of objective measurements for specifying and providing quantitative data on the performance of components and complete optical systems. The main application of the work described, is to directly viewed telescopic systems. Transmittance, the ratio of transmitted radiance to incident irradiance, of telescopic instruments is usually measured with a small diameter collimated beam, integrating sphere and photo multiplier detector. An alternative method in which the full aperture of the objective is filled by the incident beam will reveal any adverse effects of vignetting or variability of anti reflection coatings and is of importance in subjective/objective comparisons when the eye pupil is greater than the exit pupil of the telescopic system. The MVEE 'audit' and Standard telescopes which were developed for investigation of the application of OTF measurement to visual instruments were found to have markedly different Veiling Glare Index values when measured on a wide glare field apparatus. Subsequently a series of measurements of these telescopes, using several current techniques for the measurement of stray light, indicated the importance of unwanted radiation which came from outside the field of view and the inadequacy of routine VGI measurements with a glare field equal to the field of view of the specimen being tested. For a single measurement test it was concluded that a black spot on a wide field (>±45 degrees) was desirable; this is substantially test condition C2 of British Standard 4995. The Standard telescope, as tested above, had the lowest VGI and the Audit telescope the highest VGI of all systems tested, with a modern 10 x 60 sight having a VGI value close to the former. The final section of the paper discusses possible combined instruments for measuring all three parameters, to give the most effective utilization of apparatus.

Many sophisticated models have been developed for night viewing systems using image intensifiers in order to answer the question "From what distance can a specific detail be detected?" These models are often too complex for everyday design use.

Existing standards on the optical transfer function ( OTF ) can not directly be applied to thermal imaging systems or to similar opto-electronic systems. Imaging characteristics such as a limited linear range, ac-coupling and high frequency boost or jitter, drift and flicker require more sophisticated OTF measurement techniques and a further development of the conventional OTF concept. Sampling and aliasing effects due to the raster structure of the system or to electronic multiplexing force new definitions which allow the expansion of the OTF concept to such systems. The paper gives a review of major system features that affect the OTF measurement and presents the work of an international study group that was directed toward the establishment of an OTF standard for thermal imaging systems.

Johnson's target recognition work has been extended to the concert of minimum resolvable signal (MRS). The detailed methodology of usina MRS to predict field performance is described in terms of the equivalent and apparent scene. Some comments are made on the alternative simulation method. Where possible data sources and computational models are identified.

Minimum Resolvable Temperature Difference (MRTD) is now the most widely used parameter for describing both the temperature sensitivity, and spatial resolution of thermal imaging systems. It can be measured in the laboratory, using fairly simple equipment, it can be calculated from component parameters, and a good correlation has been established between MRTD and the field performance of systems, eg detection and recognition ranges of targets. However, both the strength, and the weakness of MRTD lies in the fact that it is a subjective parameter. It is measured by an observer viewing standard bar targets, and so it combines the spatial resolution and noise characteristics (thermal resolution) in the correct way. It also takes account of the performance degradations due for example to cosmetic defects. On the other hand, being a subjective measurement, there is bound to be some variations in measured values, particularly from one laboratory to another. This causes problems, if only from an administrative point of view, when testing a system against a given specification.

The design and performance of equipment for assessing complete thermal imaging systems is described. The equipment consists of a source/target unit plus control electronics which can be used for measuring the MRTD or MDTD of thermal imager systems. The system can achieve a temperature-difference resolution of better than 0.01°C and response times of the order of 10-15 seconds. Facilities are available for generating linear temperature ramps and for remote changing of targets. A scanning microphotometer using a CCD array has also been developed for analysing the output of the display of a Thermal imager. In conjunction with the source/target unit it can be used for measuring the signal transfer function (SiTF) and MTF of the thermal imageing system. A minicomputer is used for processing the data from the microphotometer. A radiometer is required to calibrate the test equipment and the design of a suitable unit which operates from the same minicomputer as the microphotometer, is outlined briefly.

The test facility is designed to measure the main performances of thermal imaging systems : optical transfer function, minimum resolvable thermal difference, noise equivalent temperature difference and spectral response. The infrared sources are slits, MRTD four bar patterns or the output slit of a monochromator which are placed in the focal plane of two collimators. The response of the system can be measured either on the display using a photometer or in the video signal using a transient recorder. Most of the measurements are controlled by a minicomputer. Typical results are presented.

This survey is principally concerned with the measurement of glare in the visible spectrum for complete optical instruments. It covers the traditional black aperture methods, originated by Goldberg in 1922, and proceeds to the more powerful elementary-source technique which leads to the concept of the glare spread function (GSF). Various national standards for glare are then listed, all of which are based upon the traditional "black-spot" method. It is then shown that the GSF can be directly related to the loss of image contrast and it is suggested that test methods based upon glare spread functions would be more satisfactory than the techniques incorporated in the existing national standards.

This paper presents veiling-glare of current lenses of various types, wide angle, standard, telephoto and zoom lenses; introduces the effect of reflection in camera bodies; proposes criteria for the assessment of veiling-glare. Veiling-glare does not differ significantly with focal lengths of lenses but differs appreciably with manufacturers: less than 1 % to more than 10 %. The increase in veiling-glare due to reflection in a camera body differs with manufacturers: less than 1 % to several per cent. According to the result of comparison of the measured veiling-glare values with actual pictures taken by the lens-and-camera systems under test, the following criteria correspond very well to the actual photographic perfomance: veiling-glare of not more than 1.5 %, very good; between 1.5 % and 3 %, usually good; between 3 % and 6 %, usually poor; more than 6 %, poor.

Experimental measurements of glare in refracting thermal imaging systems are in good agreement with theoretical predictions from ray-tracing, indicating that the primary cause of glare is multiple reflections from the optical surfaces. This is confirmed by measurements of the polar scattering function from blanks of infrared optical materials. Since the publication of this work (1), further measurements on diamond-turned Germanium blanks have shown a scattering level for some samples, which is as low as the best polished Germanium. Also recent measurements on Zinc Selenide, using an almost identical experimental arrangement (2), have provided supporting evidence that the scattering from this material at 10.6 microns includes a component from the volume as well as from the surfaces of the material.

The presence of veiling glare in an image intensifier contributes to the total veiling glare of a complete passive direct view night vision system, it is therefore necessary during the development of an image intensifier to understand the various mechanisms which contribute to the total veiling glare. Second generation intensifiers are designed to operate at relatively low values of screen luminance and have saturation characteristics. Consequently the veiling glare measurement technique requires the accurate measurment of extremely low values of screen luminance (typically 0.01 cd/m2). The paper describes a sensitive scanning system developed to measure the veiling glare, and explains that the veiling glare is only partly optical in origin, there being a contribution from electron backscattering at the input surface of the channel plate electron mutliplier. The veiling glare caused by the back scattered electrons from the channel plate is critically dependent upon the electron optical design of the intensifier and the condition of the surface layer of the input to the channel plate. The measurement technique to be described has been a useful tool in helping to optimise the design of an 18mm low distortion 2nd generation intensifier.