Proc. SPIE. 5076, Infrared Imaging Systems: Design, Analysis, Modeling, and Testing XIV
KEYWORDS: Thermography, Signal to noise ratio, Imaging systems, Spatial frequencies, Image processing, Modulation transfer functions, Minimum resolvable temperature difference, Thermal modeling, Systems modeling, Visibility
Minimum Resolvable Temperature Difference (MRTD) has long been used to describe the performance of thermal imaging systems. The Visibility Model II developed for second generation thermal imaging systems includes sampling and aliasing issues without assumptions about the observer. As with the earlier reported Visibility Model, applicable for first generation imagers, both objective and subjective measurement schemes can be accommodated. The visibility concept has been demonstrated to be applicable in predicting the objective MRTD. The laboratory measurement of objective MRTD provided the data to evaluate the performance of Objective VISMODII model. Although the measurements are limited in scope, they do demonstrate that the VISMODII predictive model can also be applied to objective MRTD measurements.
We present a new model for predicting the minimum resolvable temperature difference (MRTD) curve of thermal imaging systems. The analysis for the new model concentrates on contrast reduction due to spatial frequency limiting factors of subsystem components. Curves have been generated for this model for a system with typical component values. These results are compared with curves generated from the NVL's static performance model. The proposed visibility model leads to a relatively simpler development for a MRTD predictor which can readily account for artifacts due to a nonzero system phase transfer function. In addition the visibility model makes no assumptions regarding the recognition process and therefore is adaptable to the goal of modeling an objective MRTD measurement. The visibility model agrees with the static performance model except at very low and very high spatial frequencies where the proposed model appears to be in better agreement with observed trends in measured MRTDs.
Two commonly used optical correlation techniques, matched spatial filtering and joint-Fourier transform correlation, are briefly reviewed. A recently proposed real-time joint-Fourier transform correlation is then discussed and demonstrated by computer simulations.
Optical beam distortion generated by an acousto-optic sound cell can be characterized in terms of a linear system methodology. Comparison of various first order diffraction profiles numerically obtained for a Gaussian input field demonstrates an inherent frequency-dependent asymmetry.
Current methods for real-time optical correlation employing a jointFourier transform approach use a spatial light modulator (SLM) in the focal plane to store the interference intensity which is subsequently read out by coherent light. Because the resolution required for accurate representation of the image transform is extremely high the spatial light modulators which meet the specifications are prohibitively expensive. We propose a novel approach to jointFourier transform correlation (JTC). The method obviates the use of the spatial light modulator in the focal plane and still preserves the real time aspects of the measurements. The approach incorporates the use of acousto-optic technology for optical mixing SLMs for image representation and electronically tuned detection of the heterodyne optical signal.
An exact solution for the crosscorrelation between two different apertures of circular symmetry under defocused conditions in a two-pupil system is developed. Application to some nonstandard pupil topologies is described. Previous experimental and theoretical work demonstrates that two optical fields with different temporal frequencies when mixed through an optical heterodyne action at a photodetector can if properly preconditioned produce an electronic signal related directly to the crosscorrelation of the two corresponding pupil functions . Applying this concept several aperture topologies are proposed which lead to OTFs with bandpass and bandreject properties. The approach presented herein is to calculate without approximation the crosscorrelation of two different circular apertures in a two-pupil system. This result will serve as the essential function through which OTFs for various more complicated apertures are expressible. This approach is based on an extension of a previous analysis carried out for the autocorrelation of a circular aperture with defocus .