A computer model that takes into account the effect of aerodynamic and solar heating, sky irradiation, and radiative cooling on infrared emission from missile noses is presented. The heat transfer equation was solved with numerical techniques both in the steady-state (constant speed and altitude flight of cruise missiles) and in the nonstationary regime
(quickly variable speed and altitude of short to medium range ballistic missiles) to give the temperature distribution on the skin surface. The
corresponding head-on absolute infrared emission in the 3 to 5 pm and 8 to 12 jim spectral bands was computed as a function of time of flight and missile altitude. Results show a strong dependence of temperature and radiant intensity distributions on thermal and geometrical properties of the skin material, on the character of the aerodynamic flow (laminar or turbulent boundary layer), and on the physical characteristics of the atmosphere. By varying these parameters into reasonable ranges, infrared emissions spanning over more than an order of magnitude were obtained. The comparison with data corresponding to a skin uniformly kept at the boundary layer effective temperature showed that huge overestimates of the infrared emission can be found when the target is flying at high speed and high altitude. By computing the ratio of the in-band total radiant intensity in the 3 to 5 p.m and 8 to 12 p.m spectral regions, the relative merits of these regions for the detection of a given target are given as a function of missile altitude.
Twelve-bit digitized images taken with PtSi Schottky barrier detector arrays have been processed on Sun work stations. Two techniques for 8-bit global display are compared: the standard method of histogram equalization and a newly devised technique of histogram projection. The latter assigns equal dynamic range to each occupied level, while the former
does so according to the density of the occupied levels. The projection technique generally gives distinctly superior results based on an extensive
set of indoor, outdoor, day, and night imagery. For cases in which the two algorithms have complementary advantages, the techniques can be combined in effect by a weighting of their distribution functions, which often gives the desirable features of each. The new projection algorithm also can be used as a powerful and robust local contrast enhancement technique.
An alternative method of contrast enhancement, a global algorithm based on modular (sawtooth) displays, affords a comparable degree of enhancement at less computational cost.
We have developed a technique that enables us to acquire images of high speed (>60 Hz), repetitive thermal phenomena. A prototype system has been built and used successfully to image events occurring at frequencies as high as 1 kHz. The theoretical upper frequency limit for the system is 4 kHz. The time required to acquire images with this technique
is proportional to the frequency of the event of interest. The prototype system consists of an off-the-shelf imaging radiometer (Inframetrics lR 600), a microcomputer, and specially developed hardware and software. In this paper, we describe the theory and operation of the system.
A psychovisual experiment with 80 observers determined the amount of additive noise an observer could tolerate in resolving 36 different bar targets. The results show that the detection threshold noise can be represented by a simple analytical expression with contrast and spatial frequency as variables. The expression that closely matched the data is Vnoise (6.4C + 0.68)/v' , where Vnoise IS the amount of noise that can be tolerated, C is the contrast, and is the spatial frequency in cycles! radian. The numerical values are a function of the monitor used and are readily obtained by a simple measurement.
The need for high performance analog to digital (AID) converters, the advantages of optics, and current approaches to AID conversion are reviewed. A novel method, for which a U.S. patent was recently awarded, is proposed that uses comparators, optical logic, and a table look-up to provide optical digital output from an optical analog input. The advantage of the new method is the ability to produce any digital code, optical input! output, and longer word lengths at high speed. Implementations of the optical logic are proposed that use 1-D spatial light modulator devices under development, including self-electro-optic effect devices (SEEDs), double heterostructure optoelectronic switch (DOES) devices, and spatial
light rebroadcasters (SLRs). Optical AID converters could make digital multiplication by analog convolution competitive with electronic systems.
Opt. Eng. 29(2), (1 February 1990) doi:10.1117/12.55580
TOPICS: Filtering (signal processing), Forward looking infrared, Electronic filtering, Signal to noise ratio, Sensors, Optical sensors, Systems modeling, Error analysis, Monte Carlo methods, Image sensors
We present a methodology for tracking the centroid of small extended objects using measurements from a forward looking infrared imaging sensor. The statistical characterization of the centroid of a frame as a noisy linear measurement of the centroid of the target is obtained. The offset measurement noise is shown to be autocorrelated. State vanable
models and the corresponding filters for tracking the target centroid with these measurements are presented. Their performances are compared, and it is verified with simulations that the filter that models the autocorrelated measurement noise provides the best performance.
The Naval Ocean Systems Center airborne platform was utilized during research project FIRE (First International Satellite Cloud Climatology Project Regional Experiment) to investigate the low level horizontal variability ofthe marine boundary layer structure. Flights were made near San Nicolas Island to determine how well island-based measurements represent upwind open ocean conditions. The meteorological profiles revealed differences between the open ocean conditions and those taken near the island. The observed differences were not uniquely tied to a specific island effect since the upwind fluctuations bounded the island measurements. A higher concentration of surface-based aerosols existed
around the island, and if the resulting extinction parameter were used to model open ocean ship-to-ship electro-optical propagation conditions, errors
would result. This is not so for vertical ship-to-satellite propagation since the total integrated vertical optical depths as measured at the island
are not significantly affected by the shallow surface-based aerosol layer. Sea surface and cloud top temperatures taken near the island are not always representative of those observed upwind.
Replacement of a large continuous aperture by a set of small apertures offers a number of advantages if the subapertures can be placed within the array so as to achieve minimal spatial frequency redundancy. Using an approach based on combinatorics and integer programming, we have found solutions for a linear array for N (number of subapertures) up to eight. In my opinion, too much emphasis has been placed on the subaperture positioning problem and not enough on the resultant diffraction imagery. To this end, we have calculated the diffraction images of extended
incoherent objects for these optimum arrays. We then proceed to discuss inversion techniques whereby we determine the object intensity from the diffraction images.
Performance degradations due to polarization mismatch between subapertures of an optical array (Young system) are explored. Two tools are utilized: The first is a mathematical model for a linear array of N subapertures. The second makes use of a wavefront propagation code. The former aids our understanding of polarization mismatch on the interference
pattern, and the latter ties in the diffraction envelope along with 2-D array behavior.
An RF optical modulation technique for multiplexing and selfreferencing a number of fiber optic intensity sensors is described. The optical transducers are incorporated into recirculating optical fiber loops connected in parallel between transmit and receive optical fibers. A linear RF ramped optical signal is coupled into the system and the detected optical signal is electronically mixed with the input. Beat signals are produced in the frequency domain in the form of pulse trains that characterize the output of each sensor module. The relative magnitudes of the frequency components are insensitive to varying optical loss characteristics between the sensor modules and the signal processing location. The theoretical basis of the technique is presented and experimental results are given.
Photoelasticity has been used recently by the authors to conduct many tests related to the determination of stresses in the field of tall beams and to the optimization of those beams. Numerical methods (in particular, finite elements and solutions of Laplace's equation) have also been used to supplement or verify the results obtained photoelastically. This paper deals with selected aspects of some of the results obtained and evaluates the advantages and disadvantages of the methods used in the solution of
the particular problems addressed.