The image quality of modern infrared systems has almost to be diffraction limited. Therefore high demands in lens design, choice of materials and the manufacture are required. Typical advances in lens design using full advantage of aspherics are shown. For the materials used homogeneity and birefringence have to be in certain limits and kept under control. Optical surfaces for the IR may be manufactured applying the classical polishing technique or by single point diamond turning, whereby diamond turning is used especially for the manufacture of aspherics. Imperfect surface structure will cause veiling glare and thus reduce image quality. Typical examples are shown.
This paper will examine how optical designers can use diamond machining techniques to design, test, and construct multimirror optical systems. An unusual two-mirror telescope will be used to demonstrate the advantages of these techniques. A review of the use of non-traditional surface descriptions, a diamond machining approach to aspheric surface metrology, and techniques for system construction and assembly will be described.
An important requirement for many optical systems, is stability of performance over a relatively large temperature range. The method by which this is best achieved varies from case to case. Generally, the techniques involved are referred to as athermalisation. In this paper, as an example of an athermalisation problem, the approach adopted for the stabilisation of an infrared zoom objective will be indicated. The mechanism devised for introducing the adjustments will also be described. The objective was designed for use in the 8.0 micron to 13.0 micron band, with an operating temperature range in excess of 130 degrees C. It employs Germanium and Zinc selenide refracting elements.
A cryogenically cooled (~70K) grating spectrometer has been designed for use at 1 to 5 μm at the focus of the UKIRT 3.8 m infrared telescope. An image of a slit (90 x 20 arcseconds) is formed on to the detectors (2-D arrays of up to 2 mm x 10 mm). Image quality is better than 35 itm (typical size of detector) and wavelength resolutions of between 100 to 50,000 are achievable. Aluminium Diamond turned optics allow the alignment to be insensitive to temperature. Spot diagrams are presented for various wavelengths from 1 to 5 μm using a real simulation of the diffraction produced by the gratings or the Echelle. The chromatic aberration effects, due to the ZnSe camera lens, on image quality and defocus is evaluated. Rotation of the slit in the focal plane is plotted as a function of the grating angle. Vignetting effects on the transmission of the system for various detectors in the 2D-array is studied and found below 20%. The effects of temperature changes on the optical performance is presented. A complete tolerance analysis including manufacturing errors of the diamond turning aspheric components and mounting errors has been performed. Effects of these errors on spot size, RMS wavefront aberrations, change in focus and lateral image displacement are presented.
A scanning servo-stabilized Fabry-Perot filter system has been developed for use in the infrared. Compact devices with 5mm clear aperture have been developed for use as wavelength selectors in fibre networks. A range of special purpose three channel controllers and etalons with apertures up to 150mm have been developed for ground-based and space applications.
A blazed zone plate has been made in germanium by ion etching, into the surface, a circular fringe pattern recorded in photoresist. An efficiency of 64% at 10.6 μm was measured which is consistent with independent measurements of the groove profile.
The purpose of this paper is to review the application of infra-red technology to airborne military tactical reconnaissance and to outline some of the problems and goals which have to be addressed to provide equipments which meet the tactical reconnaissance needs of the future.
An imaging sensor has been designed and built to enable the performance of two dimensional staring arrays of pyroelectric detectors to be measured. The paper describes the sensor design and presents recent results of performance measurements.
This paper describes the design, implementation and use of an infrared imaging radiometer, operating in the 8 to 14 micrometre waveband, which employs an uncooled linear array of pyroelectric detectors. The radiometer system consists of a sensor head, which incorporates all the necessary electronics for image capture and data and video output, and a power supply and display unit, which incorporates a display monitor, system power supplies and drive electronics for the panning unit. The system can be interfaced to a microcomputer for data calibration and analysis.
The DART system is a thermography system based on the UK Class II Thermal Imaging Common Modules (TICM II). It uses a turret containing 4 filters as the means for selecting 4 different target temperature ranges. The physics of such a system is discussed paying particular attention to the CMT SPRITE detector. In particular, the effect of the total photon flux incident on the SPRITE regarding the detector responsivity is discussed,together with the spectral responsivity. Finally, the, consequences, of this detector behaviour are shown in terms of (i) the relative NETD between a DART' system and an alternative 'switched-gain head-amplifier' based system, and (ii) a discussion of the sensitivities of these two systems.
Optical and electronic modifications have been made to a TICM II thermal imager by NPL to allow its use in near-focus radiometric measurements. A GEMS image processing system has customised enhancements to the existing GEMMA sofware permitting pixel-by-pixel restoration and radiometric calibration of images with user-defined algorithms. To allow emissivity measurements to be made at near-ambient temperatures, a non-reflecting cryogenic sample chamber is necessary to remove the reflected component of sample radiance. The design and construction of such a sample chamber is discussed in detail in relation to the NPL facility nearing completion for measuring the emissivity of non-uniform materials or objects. Particular features are the avoidance of vacuum systems for purging or insulation, and the geometrical and thermal design to give ease of handling and a long operating period from a single filling with liquid nitrogen.
This paper reviews the present position and highlights some deficiences, particularly in MRTD measurement. It then discusses some suggested methods of measuring MRTD objectively. Some new problems are then examined, briefly, which are caused by the undersampled nature of many of the new staring array thermal imagers and some approaches are mentioned which may possibly lead to resolution of these problems. The importance of measurement methods suitable for production and in-service testing is emphasised.
The minimum resolvable temperature difference (MRTD) and to a lesser extent the minimum detectable temperature difference (MDTD) are two of the primary criteria used in evaluating the performance of thermal imaging systems. They are used both at the design stage, where they are calculated from known parameters of the system, as well as in acceptance testing of production systems. The measurement of MRTD requires the operator to judge the point at which the temperature difference of a 4-bar target is sufficient for the target to be just resolved. It therefore contains a subjective element which results in measurements done by different observers frequently yielding significantly different values. There is considerable pressure from purshasers, manufacturers and users of thermal imagers to find an objective alternative to this subjective measurement. This paper describes two techniques for obtaining an objective measurement of MRTD which are currently being developed by the authors and discusses some of the difficulties associated with producing an acceptable approach to this problem.
The 3-5 and 8-12 micron wavelength bands are the two main atmospheric transmission windows used for thermal imaging. For military and other applications of these imagers it is important to have data about the emissivity and reflectivity of different surfaces and surface finishes and how these vary with the angle of incidence and the angle of reflection. In general the appearance of a surface in a thermal image will depend not only on its temperature but on the angle at which it is viewed, the radiation incident on it from surrounding sources (eg the sun, sky or clouds) and the manner in which these are affected by the emissivity and reflectance characteristics of the surface and their angular dependence. The equipment described in this paper is designed to measure these characteristics of a surface in either of the two thermal wavelength bands or for any limited wavelength range in these bands. The bi-directional reflectivity is the primary parameter measured, since in principle the emissivity can be calculated from this. However the equipment can also be configured to measure emissivity directly.
A system is described which enables the spectral sensitivity of thermal images to be measured. The instrument covers the spectral range from 2.4 to 24 microns, and so it is useful for analysing the response of both 3.5 microns and 8-14 micron imager systems. It offers the facility to evaluate the linearity as well as the spectral performance of the imaging system.
The current status of in-house characterization and testing of IR materials and components is described. A range of techniques are described, which were developed largely to evaluate the ZnS produced at BAe, but are equally applicable to other materials intended for use in the 8-14μm waveband. Measurement techniques are described for determining the Transmission, Refractive Index, dn/dT, Refractive Index Homogeneity, and the Modulation Transfer Function (M.T.F). These are based around a Twyman-Green interferometer using a CO2 laser as the radiation source with the exception of the M.T.F measurement, which is done using Lateral Shearing Interferometry at the single wavelength of 10.6Am. This method is preferred to the derivation of the M.T.F from the Twyman-Green fringe pattern because of its superior resolution which allows the effects of high frequency surface patterns (e.g. tool marks on diamond turned optics) to be included in the calculation. An in-house test facility has been constructed which allows all of these measurements to be carried out on a system based around the Twyman-Green arrangement with add-on sections to accommodate the test and component under examination. Using this facility, domes as large as 200mm in diameter can be measured, as well as flat plates.
Numerically controlled single point diamond turning is an important alternative manufacturing technology to conventional polishing for the manufacture of infra-red optics. The technology is capable of creating high quality aspheric surfaces on materials such as germanium, silicon zinc sulphide, and zinc selenide. It is essential that in-process optical test methods are applied consistent with a logical sequence of manufacturing steps which can be tested for appropriate accuracy at each stage. Certain machines are suited to machine based optical test using laser interferometric methods. Test methods are described which allow aspheric lenses to be created. Results are presented for a 50 mm F/2 germanium lens which outline the in-process tests employed and are supported by IR performance data for phasefront aberration and M.T.F. The optical performance agrees well with the design and represents current state of art diamond machining.
Manufacturers and Users of the Thermal Imagers have spent very much time upon the definition and measurement of the generally accepted performance curve: MRTD (Minimum Resolvable Temperature Difference). The need for a cheap and fast, objective measurement method has considerably increased since the large scale introduction of thermal imagers. This paper contains a contribution to such a method, based upon simple targets, a CCD-camera and an IBM-PC with frame-grabber unit and fast computation algorithms.
The RSRE Dual Waveband Imaging Radiometer has been used to collect simultaneous infrared signatures in both of the 3-5 micron and the 8-12 micron atmospheric transmission wavebands. The use of internal temperature references enables these results to be scaled to apparent temperature, that is the target temperature as perceived by the imager after passage through the atmosphere. The derived temperature profiles for a target in the two prime thermal wavebands can therefore be compared with ease. Results will be presented showing both the similarities and the differences between the signatures in the two wavebands.
The ability to accurately predict the infra-red signatures of air targets in defined regions of the infra-red spectrum and in known weather conditions is improtant for military applications. The evaluation of the detection and recognition performance for Infra-Red Search and Track systems, surveillance systems, and missile guidance systems relies upon good target emission data. esigners of airframes also require this information in order that signature reduction and stealth techniques can be applied to the design. Making measurements would appear to be the optimum method of obtaining data on targets. However, measurements are notoriously expensive and difficult to make in representative scenarios. The actual intended targets may also be impossible to measure since cooperation is required for meaningful trials to be conducted. BAe Dynamics Division (Hatfield) have been involved in signature measurement and model prediction for many years. This has been primarily in relation to missile guidance applications. For simple tracking and missile systems empirical models are usually regarded as sufficient for basic performance calculation. The extra complexity of future missile systems means that more complex and accurate signature prediction models are required. This includes models which calculate the detailed chemical and thermodynamic properties of the plume for example. Models like these have been implemented and used for performance modelling at Hatfield for some time. However, these models often suffered from the lack of true validation sand were not fully integrated with other model components and did not consider all of the influencing factors in signature prediction (e.g. airframe shielding and reflection). It was of some interest to BAe to discover that a fully integrated and complete model was being developed by NATO Research Study Group AC243 (Panel IV/RSG 6). BAe obtained a working version of the model through the UK panel representative in early 1987 sand were funded by the UK Ministry of Defence (Contract A52b1333) to implement and run the program on a VAX 11/785 computer. Extra routines were written by ourselves to ease data input and output. The model is called NIRATAM (Nato Infra-Red Air TArget Model) and we are currently using Version 1.2.
Under contract to the Admiralty Research Establishment (ARE), Smith Associates have produced a computer model for predicting the infrared signature of ships placed in specified environmental conditions.