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Unformatted text preview: A means for calculating the optics MTF of an under-sampled IR imaging system Stephen Burks, Jeff Olson, James Stevens US Army RDECOM CERDEC Night Vision and Electronic Sensors Directorate 10221 Burbeck Road, Fort Belvoir, VA USA 22060-5806 ABSTRACT Using measured quantities, it is possible to arrive at a reasonable approximation for the optics MTF of a longwave undersampled imaging system. Certain reasonable assumptions concerning the format of the data from the imaging system should be made in order to make sure that there is no image processing artifacts. For systems that contain imaging artifacts, such as an analog output, there are too many secondary effects that will degrade the predicted Optics MTF beyond a reasonable approximation. Keywords: Modulation Transfer Function, Longwave Imagers, Undersampled Imaging Systems, Optics MTF 1. INTRODUCTION The pre-sample modulation transfer function of an under-sampled imaging system is one of the two most impor- tant measurements needed for determining an electro-optical system’s performance. Along with the 3D Noise measurement, this measured quantity can be used to compute the overall system performance (typically the pre- sample MTF is the limiting MTF for longwave undersampled imaging systems). 1, 2 The pre-sample modulation transfer function includes the effect of the objective lens, the detector array, the readout electronics, and any additional signal processing that may occur before the signal is able to be captured by a digitizer. For most under-sampled imaging systems to be well-designed, the limiting Modulation Transfer Function (MTF) of the above four MTF components should be the objective optic MTF. 3 It is also commonly the case that the objective MTF for most under-sampled imaging systems is not close to approximating the diffraction limit for an optic. Instead, distortions and aberrations cause the MTF to be lower than an ideal diffraction- limited MTF. For a typical comparison of a measured optical MTF versus an ideal optics MTF, consider Figure 1. As a note, an under-sampled imaging system is defined to be any system whereby the pre-sample MTF is greater than zero for any frequency past the Nyquist frequency. Most electro-optical systems that are tested do not have removable optics. Therefore, without invasive and sometimes destructive methods, the MTF of the objective lens is not directly measurable. This paper presents a simple technique for approximating the optics MTF from the system MTF if a certain number of assumptions are met. One of the main applications for this technique is for systems that have a complex sets of optics that are not easily measured as a singular system. When the optics that are used to focus onto the detector plane are more than a single objective lens, it sometimes can be challenging to determine the effect of the optics system as a whole. Even though multiple optic cases were not verified from this study, the same principles used for single lens systems should apply to situations like those.lens systems should apply to situations like those....
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This note was uploaded on 02/04/2012 for the course ECON 101 taught by Professor Gulipektunc during the Spring '11 term at Middle East Technical University.
- Spring '11