Imagers based on focal plane arrays (FPAs) risk introducing in-band and out-of-band spurious responses, or aliasing, due to undersampling. IR systems can use microscan (or dither) to reduce aliasing. We describe a generic microscan technique and the benefits of microscanning, including an analysis of and experiments on four-point microscan employed in IR imagers, in which the image is mechanically shifted by 1/2 pixel between fields, in each dimension. Our purpose is to describe the benefits of microscanning for IR systems employing sensitive detectors. Through analysis and experiments on production systems, we show that microscanning is an effective way to improve the resolution of imaging systems. In addition, we present experimental data that shows that this increased resolution results in lower minimum resolvable temperatures (MRTs) than an equivalent nonmicroscanned system; and that this improvement in MRT is accompanied by an increase in detection, recognition, and identification (DRI) range performance in a real-world system. The microscan hardware can also be used to null out residual gimbal jitter in a stabilized imaging system, resulting in a jitter reduction of 35 to 50%. We show that this technique, known as microscan stabilization (MSS), is complementary to microscan, and further increases the imaging system performance.
Imagers based on focal plane arrays (FPA) risk introducing in-band and out-of-band spurious response, or aliasing, due to undersampling. This can make high-level discrimination tasks such as recognition and identification much more difficult. To overcome this problem, three-chip color charge coupled device (CCD) cameras typically offset one CCD by 1/2 pixel with respect to the other two. Analogously, monochrome imagers including infrared can use microscan (or dither) to reduce aliasing. This paper describes a generic microscan technique and benefits of microscanning. Covered are analysis and experiments on four-point microscan employed in infrared imagers, in which the image is mechanically shifted by 1/2 pixel between fields, in each dimension. Four of these offset fields are then combined to form one frame of high-resolution video. We show that microscan reduces aliasing, which results in higher resolution and improved image quality resulting in improved performance.
Experiments and analysis were used to determine the number of resolvable cycles across an alphanumeric character required for readability. This has serious implications for the resolution needed for a surveillance camera to present a “readable” image to a human. Fourier analysis was used to predict the number of cycles required for readability. Using two-dimensional Fourier transforms, the set of 26 English letters and 10 Arabic numerals was analyzed and classified. This theory is supported by empirical data based on user identification of random English letters and Arabic numerals. The results strongly indicate that accurate readability (defined as 90% correctness or better) can be accomplished with approximately 2.8 cycles across a block letter. This appears to suggest a lower resolution requirement than that generally accepted for unknown target identification. The reason is the limited data set of only 36 alphanumeric characters, of which the observer possesses a priori knowledge. Moreover, the ability to read an alphanumeric is a steep function of the resolution between 2 and 3 cycles per character height. The probability of correct “Reading” can be expressed similarly to that of Detection, Recognition and Identification by using a postscript such as “Read90”.