This special issue on Infrared Optics is the first of what we expect will be an annual issue and an Optical Engineering tradition. Included in this issue are papers on the design and testing of infrared optical systems and components, including lenses, mirrors, coatings, and filters.

Some of the familiar kinds of optical designs for the visual region of the spectrum take on new meaning when applied to infrared problems. The availability of high indexes of refraction, for example, accentuates byways of optical design in the visual into quite unusual results in the infrared. As a first example, the ROsch combination of catadioptric system derived from the Schmidt telescope leads to a kind of wide-field system of low aperture ratio and low obscuration, although at a sacrifice of compactness. In another such instance, it is feasible to design an optical system in the IR that can illuminate some appropriate form of detector from a major fraction of a 4-pi solid angle over a field of view of appreciable size. Similarly, it is possible to design a form of retroreflector for the IR that has a 4-pi field of view, as for the Luneberg sphere. Further examples of special design for the infrared are discussed, including all-refractive, and all-reflective and catadioptric combinations. The author reviews some of these useful facets of optical design where appropriate infrared materials can be employed, and suggests how some of these forms may be applied to infrared problems.

Optical systems are normally usable over a restricted spectral waveband. A refractive optical system will only transmit radiation over a limited range of wavelengths, and it may produce well color-corrected images over only part of this transmission band. This paper discusses the design of lens systems for use over the extended waveband from about 0.4 gm to 12µm, which encompasses the visible, the 3 to 5µm mid-infrared, and the 8 to 12 um thermal wavebands. Discussion is given to the available optical materials, including glasses formed by chemical vapor deposition and crystalline materials. The relationships between the refractive and dispersive properties required for wideband color cor-rection are formulated, and several designs are described that use two or three optical materials. Some discussion is given to the coatings required for such optics, viz., ultrawideband antireflection, mirror, and beam splitting coatings. The potential use of this type of optical system is in multisensor applications such as dual visual/thermal observation systems, perhaps employing staring array technology and/or CO2 laser incorporation.

For any application where the mutually exclusive demands of moderately wide field of view and high resolution occur, lenses having a variable image scale have an enormous advantage over fixed lenses. In the field of thermal infrared optics, where diffraction limits ultimate resolution, there is therefore an increasing requirement for continuous zoom telescopes. Until recently, however, such telescopes have been precluded due to their excessive length. A continuous zoom telescope has been designed that combines wide maximum field of view with high maximum angular resolution inside an overall size less than that of many conventional fixed-magnification infrared telescopes. This compact size has had to be achieved commensurate with reason-able manufacturing tolerances. The manufactured unit has justified the expectations of high performance.

The Infrared Astronomical Satellite (IRAS)* has completed an unbiased all-sky survey at wavelengths from 10 to 100 Am. The design and performance of the focal plane array is described with emphasis on in-orbit measurements of the sensitivity and stability. In the four broad spectral bands centered at 12, 25, 60, and 100 Am, the system noise equivalent flux density (NEFD) values are, in Jy/Hz1/2, 0.03, 0.025, 0.046, and 0.21, respectively (Jansky = 10-26 W/m2/Hz). For point sources, a single scan at the survey rate of 3.8 arcmin/s yields limiting flux densities at the 3-sigma confidence level of 0.36, 0.30, 0.39, and 1.2 Jy. The dc stability of the junction field effect transistor (JFET) amplifiers and the excellent off-axis rejection of the tele-scope permit total flux measurements of extended infrared emission at levels below 106 Jy/sr. Response of the extrinsic silicon and germanium photo-detectors to ionizing radiation is described.

The Infrared Astronomical Satellite (IRAS)* was successfully launched on January 25, 1983. This paper presents results based on analysis of early scientific data returned from IRAS. Among the early results of IRAS are the discovery of comet IRAS-Araki-Alcock, evidence for a shell of large particles around the nearby bright star Vega, detection of stars in the process of formation, and detection of many infrared bright galaxies. These early results demonstrate that the IRAS data will be a treasure chest for astrono-mers for years to come.

This paper presents the design of the balloon atmospheric mosaic measurement (BAMM) IIA optical subsystem. The BAMM IIA system is a cryogenically cooled infrared mosaic sensor built for a high altitude balloon measurement program. This paper discusses the optics subsystem designed to operate in the temperature range of 78 K to 85 K. The optics subsystems consist of three infrared telescopes mounted on a turret to permit in-flight selection, 11 dielectric filters for band selection in the spectral region from 2.6 to 5.0 um, a cold chopper, and two reference radiometric sources. This paper presents some aspects of this design and evaluation of the components making up the optics subsystem.

The influence of polishing time on the surface structure of optical elements is shown for germanium lenses. The surface structure is measured with a profilometer and expressed in the form of center-line average (CLA) values. Veiling glare of single lenses manufactured under different conditions is measured in the infrared spectral region (IR) between 3 and 10 Am. An instrument to measure the relative veiling glare ratio in the IR is shown. The results are discussed. The image quality of these lenses is shown as fine-line images and modulation transfer func tion (MTF) measurements. An imperfect surface structure affects more greatly the shorter spectral region. The results are interpreted. The roughness of single-point diamond-turned lenses is compared with that of optically polished lenses.

The development and operation of a 300 mm aperture, high precision scanning interferometer with associated software for the semiautomatic testing of far-infrared lenses and system are described. The instrument is capable of analyzing and evaluating systems in terms of the optical transfer function (OTF), point spread function (PSF), wavefront aberration, ray intersection patterns, polynomial fitting, aberration contouring, and refractive index variation mapping and of providing a large range of representatives of data. The speed of the analysis is commensurate with established techniques with an improvement in the accuracy and flexibility of use of the instrument over alternative measuring systems. The introduction of this instrument has allowed the complete diagnosis of system defects, along with an accurate measure of any error and its effect on system performance in terms of wavefront error, modulation transfer function (MTF), etc. Results on a variety of systems which exhibit errors are presented, and their root cause and subsequent cure are discussed. The value of such an instrument for both prototype building and production runs is described.

The Martin Black process has been used to coat baffles on a wide variety of visible and ultraviolet range instruments. Its infrared applications include baffles for the Infrared Astronomy Satellite and the Spacelab 2 Infrared Telescope. Because of the increased emphasis on stray light suppression in the infrared, the Martin Black process was modified with the objective of creating a better black surface for use in the near-, mid-, and far-infrared regions. These modifications resulted in the creation of an infrared absorbing surface, called Infrablack, which retains the excellent visible absorption properties of its predecessor and has increased infrared absorption. Hemispherical and specular reflectivity and bidirectional reflectance distribution function (BRDF) measurements were made on the Infra-black surface. The specular reflectance at 17° incidence ranges from less than 1% at 60 i.im to less than 15% at 450 /um. BRDF measurements made at the University of Arizona at 10.6 pm, at incidence angles of 10°, 30°, and 60°, indicate that the surface behaves in a nearly Lambertian manner at this wavelength. At 10 incidence the BRDF for Infrablack is about one-seventh that of Martin Black, and the curve is noticeably flatter.

Amorphous hydrogenated hard carbon (a-C:H) is a promising new optical coating material for passive infrared materials. It offers the rare combination of extreme hardness, chemical inertness, and optical transparency over a wide spectral range. We give an overview of the optical properties of rf-plasma deposited a-C:H coatings and compare them with vacuum-evaporated infrared coatings. For many applications, a-C:H solves the problem of a mechanically and chemically resistant 8 to 12 Am coating despite its moderate absorption in the 6 to 20 Am range. The tunability of the refractive index between 1.8 and 2.2 allows single layer coatings on Si and Ge with zero reflection. State-of-the-art applications, possible future developments, as well as remaining technological problems of a-C:H are discussed.

Perkin-Elmer fabricated two 9.5 in. (24.1 cm) diameter beryllium mirror blanks using an advanced powder metallurgy process to produce mono-lithic, lightweight (2.16 lb = 0.98 kg) structures with honeycomb interiors. The new beryllium material exhibits improved isotropy as measured by x-ray diffrac-tion techniques. The mechanical properties meet or exceed the published values for commercially available vacuum hot pressed beryllium block. In addition, the material is readily polished to good figure and surface quality and has demonstrated potential for lower scatter than standard material. The monolithic sandwich construction demonstrated in the program is exceptionally stiff, permitting large lightweight mirrors to be constructed. A scaled-up 36 in. (91.4 cm) diameter mirror would weigh only 31 lb (14.1 kg) using the technology demonstrated in the project. It is estimated that with minor modification in the process this weight could be halved.

Two unconventional Fabry-Perot interferometer cavities are used in series to permit transmission of both wide and narrow spectral bands in the infrared. The use of two cavities, in series, blocks unwanted harmonics and effectively extends the free spectral range of the device. The regime of operation covers the middle- and far-infrared spectra, as well as portions of the near- and extreme-infrared. Very small cavity spacings are used to produce a very wide-band transmission window when the device is operated in the zero-order mode. Tuning the device with a closed-loop capacitive control system produces vari-able cavity spacings in higher-order modes, resulting in variable transmission bandwidths over a large spectral range. The design and fabrication issues for two-cavity Fabry-Perot interferometers are discussed, and theoretical perfor-mance and test results are presented for two infrared devices.

A number of applications require the precise tracking or position estimation of an object unresolved in the system optics. This paper evaluates several one-dimensional interpolation algorithms (odd N-point centroids, N = 3, 5, 7, 9, and three-point and five-point quadratic curve fits) designed to make these estimates to subpixel accuracy. Analytic, Monte Carlo, and experimental results are presented. The tracking sensor examined was a scanning linear array of infrared detectors assumed to be background-limited. The detector size and physical spacing were varied parametrically, with realistic fabrication constraints, to determine the relative performance and to obtain the optimum configuration. The optics blur spot was assumed Gaussian. The sources of error considered to affect the algorithm performance were the systematic algorithm bias, the random noise, and the postcalibration residual detector responsivity nonuniformities. Track accuracy improves with signal-to-noise ratio (SNR), until limited by algorithm inaccuracies or focal-plane nonuniformity. Among the algorithms tested, the three-point centroid performs best, provided that systematic algorithm bias is corrected. An experimental infrared tracking focal plane, used in a tracker simulation, closely confirmed the analysis. With the three-point algorithms, an experimental accuracy to smaller than 1/100 a detector (<1/250 a blur spot) was obtained at high signal-to-noise ratios.

By utilizing the unique x-ray test facility at NASA's Marshall Space Flight Center, high resolution data have been obtained for the scattering of x rays from smooth optical flats. Using x rays with energies in the range from 2.99 keV to 8.06 keV, the resulting reflected images from five state-of-the-art polished mirror flats with various substrate and coating materials are presented and analyzed. Evidence for large angle scattering is shown to exist for some energies and is discussed in terms of theoretical models for surface defects.

The theoretical performances of several f/1.5 triplet lenses suitable for use in the 3 to 5um spectral region are compared. Examples are given in which all the materials used have a finite visible transmittance. As centering errors can thus be considerably reduced, it is expected that such a lens will in practice approach more closely its theoretical performance. These lenses are also considerably less sensitive to manufacturing tolerances as a result of lower index materials, and the use of low dn/dT materials offers other advantages over the more usual silicon-germanium combination.

We discuss alternative techniques for microholography of biological specimens including requirements and constraints on the optical elements and recording media. We derive spatial and temporal coherence requirements for four representative holographic techniques and relate coherence and recording-medium resolution to maximum specimen volume. We give estimates of coherence length necessary to image a variety of specimens under a realistic set of assumptions. We discuss matters of wavelength selection with emphasis on the problem of obtaining high contrasts for nitrogen-bearing biological constituents. We use the eikonal approximation to derive equations for diffractive holographic imaging with particular attention to specimens of low contrast, and use these to illustrate the benefits of using 27r9 resonances to image molecular structures.

Wolf's formulation for inverse scattering in the first Born approximation is reviewed and discussed from an experimental point of view. Some of the experimental problems associated with obtaining enough data to satisfy the requirements of this theory are described, and a method for using symmetry to reduce the data requirements for a class of scatterers is explained. A successful experiment that employed this theory to reconstruct structure in an object with rectangular structure is reviewed. It appears that this theory is extremely difficult to apply to a more general scatterer with no simplifying symmetries.

We have previously reported on the conceptual design and predicted performance of a new type of dedicated astronomical telescope to be used for a deep photometric survey for galactic and extragalactic variability and polarization.' Data derived from this survey will be useful for a wide variety of astronomical investigations including the definition of a complete sample of quasi-stellar objects (QSOs), based on their variability and nonstellar colors, the detection of supernovae on the rising branch of their light curves, and the determination of the supernova production rate as a function of galaxy color, morphology, and red shift. The telescope we are producing to accomplish this survey is a transit instrument. It will incorporate a 1.8 m primary mirror of very high quality, fabricated as part of the program to develop the Space Telescope mirror technology, and, as its detectors, two RCA CCDs (512 X320, 30 /Am pixels) used in the time-delay and integration (TDI) mode. By means of a dichroic beam splitter the two CCDs view the same region of the sky, subtending about 8.2 arcmin in declination. The effective integration time on the sky using this technique is about one minute, resulting in a faint limiting magnitude of about 22 per night. The data from the digitized strip of sky are recorded and analyzed in real time for specific events such as supernovae. The CCD/transit technique has been demonstrated using the Steward Observatory (SO) 2.3 m telescope with the drive switched off. The results of this demonstration are shown and discussed. We report on progress in all aspects of the CCD/transit instrument (CTI) development, including the design of the transit telescope, which is optimized for wide-field, seeinglimited imaging. In particular, we describe a two-mirror field corrector system which realizes the potential image quality of the high-precision primary mirror over a wide field and over the wide spectral bandpass of the CCDs.

Optical waveguide operation in the mid-infrared waveband requires the use of alternative materials to those conventionally based upon silica, this being a necessary consequence of fundamental absorption mechanisms. In selecting such materials the anionic component is the primary consideration. For high performance waveguides it is necessary to have as nearly coincident zero material dispersion and minimum loss wavelengths as possible in order to utilize the IR low-loss potential and also to maximize bandwidths for monomode operation. Short distance power or image transfer applications are dependent solely upon the material minimum loss wavelength.

The ray matrix formalism is extended to include resonators with annular optic axes and is applied to the half-symmetric unstable resonator with an internal axicon (HSURIA). The formalism is used to study the effect of defocus in the reflaxicon on the parameters for an annular resonator. It is shown that, for a large range of defocus, geometric resonator parameters are not significantly affected and are approximately linear functions of the defocus.

The resolution characteristics of a digital subtraction angiography (DSA) system have been evaluated under two different magnification techniques, namely, the geometrical magnification technique and the electro-optical magnification technique. It has been found that the DSA system resolution is less dependent on the focal spot size [or modulation transfer function (MTF)] than that of routine radiographic angiographic studies. The evaluation results have been utilized to reestablish the DSA examination procedure.

In common usage the words "child's play" often indicate something extremely simple. Your Editor has been impressed by his recent readings of the pioneer child psychologist Piaget Piaget suggests that child's play ceases abruptly when the task is no longer challenging. Our usual connotation therefore slanders the child. Piaget further suggests that what interests the child is the idea of a problem known to be challenging yet suspected of being solvable. Great joy and elation come from the satisfactory solution of such a problem. Clearly, science is child's play.