I have no novelties nor new results to bring to you since my recent work is in the region, λλ < 20. Accordingly, I am constrained to give a summing up of my long-wavelength experience--aimed to be appropriate now when this spectral region is burgeoning.
The paper deals with the optical design for the infrared region. The problems of different materials for the various IR regions are discussed, taking into consideration the refraction, dispersion and absorption losses, as well as the material deficiencies. Ways of measurements and the varying testing methods are analyzed. In addition to the basic optics, the Zoomar orbiter and two Zoomar IR lenses are described.
In order to test the effectiveness of spatial diversity averaging as a method of reducing laser image speckle, an experimental 10.6-micron active imaging breadboard was constructed and used for obtaining day and night distant images at ranges exceeding one kilometer. The system was unusual in its use of employing dual acoustic-optic units for both generation of optical local oscillator signal and for beam combining of object reflected power and local oscillator power in an optical heterodyne detection system.
The optical transfer function (OTF) of an IR imaging system using coherent (laser) illumination was compared to its performance when imaging with incoherent (thermal) radiation. The imaging system under consideration is a basically passive framing infrared system in which the IR detector is scanned throughout an 8°x10° field of view (FOV) to develop an IR image. The system can be operated in the laser imaging (active) mode by synchronously scanning a CO2 laser beam with the detector. The linespread function method was used to derive the respective OTFs. From the measured lineshape response, the OTF was computed using Fast Fourier Transform (FFT) techniques. These measurements and analyses have shown that (1) the OTF is a valid and reproducible measure of the system's frequency response, even when coherent illumination is used, and (2) the active mode tended to give a reproducibly higher MTF than the passive mode for all wire targets used.
Imaging experiments in the far infrared are described. Transmission data are presented for several common media. Far infrared pictures of metallic objects are presented. The practicality of a far infrared imaging system is discussed.
Diffusion and photoetching technology enable one to diffuse a well defined layer having a high temperature coefficient of resistance on the surface of a silicon chip. The resistance and the temperature coefficient can be tailored to optimize the device for use as a bolometer at any operating temperature between 1K and 4.2K and possibly much lower. A new type of device is described in which a thin film heater has been evaporated onto the insulating surface of the bolometer. This enables one to directly calibrate the bolometer's response to absorbed power. The heater is useful in situations where the background power changes from one measurement to the next and one wishes to calibrate the bolometer in situ. The performance characteristics as a function of background power can easily be measured with the heater. A description of the device and performance data are presented.
The far infrared (FIR) optically pumped waveguide laser is a compact, practical, coherent source. Operation of this laser within the spectral region of 40 μm < X< 2 mm with various gases is described. A theoretical description is presented and compared with the experimental performance.
Photoconductive detectors in the 10-1000μm range have been developed and flown in rocket borne liquid helium cooled telescopes for sky survey and auroral emission studies. Measurements under background limited conditions have been made in the spectral range of 60-300μm using the 30cm telescope aboard the NASA Lear Jet. We present the results of the detector sensitivities used under these two environments.
The transmittance of most optical materials used in long wavelength infrared systems is strongly temperature dependent. When these materials are used at cryogenic temperatures, the room temperature data on optical properties is frequently unreliable for system design. This paper presents low temperature transmittance data on several materials which are commonly used in the long wavelength infrared.
An electro-chemical process has been developed which, when applied to aluminum and most of its alloys, produces a surface which is highly absorbent in most of the wide spectral range from the near ultraviolet (0.27 micrometers) to the far infrared (125 micro-meters). Its infrared characteristics in the band from 8 to 125 micrometers are discussed in this paper, where its absorptivity and emissivity remain above 0.985. The surface is space qualified and has flown in the Skylab missions. It also withstands exposure to hydrazine (a rocket fuel), commonly used cleaning reagents, and many other chemicals. This surface is one of the most absorbing yet developed over an unusually wide range of radiation.
This talk consists of two parts: 1.) a brief account of some of the most important astronomical results obtained through infrared observations, and 2.) a look at future developments in this rapidly changing field, with emphasis on new instrumentation. A recent review of experimental techniques has been made by Low and Rieke.(1)
Far infrared atmospheric transmission spectra using the sun as a source have been obtained with a Fourier spectrometer from mountain top and from an aircraft in the low stratosphere. The spectra cover the range 15-60 cm-1 with a resolution of 0.065 cm-1. Some of the mountain-top spectra were made on an extra-ordinarily dry day when the quantity of precipitable water overhead was less than 250 ppt μm. Absorption features due to oxygen, water vapor, ozone, and nitrous oxide are present. The known rotational parameters of these molecules allow computation of the expected atmospheric transmission for a given distribution of absorbers. The methods of computation will be described and the comparison of observed and calculated transmittances discussed. The insensitivity of the absorption to the details of distribution of absorber limits the information which may be obtained from below about the distribution of absorber at higher altitudes.
The problem of spectral filtration and beam splitting in the far infrared portion of the spectrum is, at present, a more difficult problem than in the shorter wavelength region of the spectrum multilayer dielectric stacks are used almost exclusively in the short wavelength region. The theory for the design of this type of filter is well known and the behavior of a filter can be predicted to a high degree of accuracy. Even though interference dielectric stacks offer better characteristics than other filtering methods presently used in the far IR, the use of this form of filter has not been given a lot of consideration above 30 to 40 μm. This is due, in part, to several problems arising from material characteristics and the inherent difficulty in vacuum deposition of very thick layers. This paper reports on a preliminary experimental study to examine the feasibility of constructing multilayer dielectric interference filters for the 2 to 1000 μm region of the spectrum. Test results are given for two filters which were constructed to operate at 100 μm.
The 2.4 meter aperture Large Space Telescope to be flown by the National Aeronautics and Space Administration in 1982 will include a far IR spectrophotometer which will operate from 2 to 1000 microns. The sensitivity of the instrument will be limited by the statistical fluctuations of the photons readiated by the primary, secondary, and asso-ciated relay optics over a large portion of the spectrum. To maintain the required sensitivity the detectors within the instrument must be cooled to 2 deg K as are the optical filters and some of the relay optics. At the long wavelengths diffraction effects within the relay optics must be included in the analysis because the optics dimensions are comparable to the Airy disc dimensions. The radiometer has been designed to minimize modulation noise resulting from a symmetrical imaging and non uniform background flux distribution.
Coherence in radiometry, or the radiometry of radiation in any state of coherence has had a short and checkered history. So short, in fact, that the very subject needs definition, elucidation, and bounds. In this article I would like to outline several specific areas in which coherence theory of some sort and radiometry come into contact. The twain meet and in several different ways. The first has to do with the noise equivalent power (NEP) of thermal and photon detectors. The second has to do with the theory of partial coherence and extended definitions of radiance, radiant emittance, radiant intensity, etc. The third deals with the conjunction of radar theory and the measurement of "coherent" returns from specific objects.
Meaningful prelaunch calibration of it space sensors requires low-background test facilities for the controlled generation and absolute calibration of radiant flux over wide dynamic ranges. The problems inherent in such measurements are multiplied by the extremely low radiant energy levels at which they have to be performed and by the high selectiveness of most detectors. A procedure is described for the in situ determination of the spectral content of the generated radiant incidance. An analysis of all perti-nent calibration errors is performed. The attainable accuracy is ultimately limited by the inherent uncertainty in our knowledge of the radiant exitance of blackbody-type radiation sources.
When an LWIR sensor is calibrated, the question generally arises as to what the precision, accuracy, and repeatability are for the calibration. The measurement precision is readily determined; however, general agreement does not exist in the LWIR community as to the formulation and interpretation of the terms "accuracy" and "repeatability". A derivation is presented in this paper in which a logical formulation of these terms is obtained. Forms for both short-term and long-term uncertainties are given, In addition, the use of confidence intervals in connection with sensor calibration is discussed, along with an appropriate example. A compilation of uncertainity terms and the definitions of these terms are proposed for adoption by the LWIR community,