GB1478761A - Thermal imaging system with redundant object space scanning - Google Patents

Abstract

1478761 Infra-red imaging system HUGHES AIRCRAFT CO 15 April 1975 15429/75 Heading H4F Thermal imaging system comprises optics 30-38 directing an infra-red scene image on to an azimuth-scan rotating mirror drum 40 and then via an elevation oscillating mirror 44 to an array of detector elements 48, the electrical outputs of which are connected via individual channels to an identical array of light emitting elements 60 whose light outputs are directed via an elevation oscillatory mirror 64 (mounted on the other end of the pivotally yoke 43 on which mirror 44 is mounted), azimuth-scan mirror drum 40 and optics 66-68 to a viewer or vidicon camera. With this arrangement each detector element views every point in the field of view during each scanning frame period. The detector array 48, Fig. 3, (which is identical to the LED array 60) specifically comprises 18 detectors arranged in six groups each of three detectors aligned in the azimuthal scan direction so that a complete field scan is effected as shown in Fig. 4. At the end of each field scan the elevation scan structure is indexed by 1¢ effective scan lines (fractional relationship between azimuth and elevation cycles) so that the scan lines of the fields are interlaced, and as shown in Fig. 4 four field scans comprise a complete frame scan. Thus during one frame period a number (i.e. 18) of registered pictures each of which is indicative of the thermal energy distribution within the field of view is produced, this number corresponding to the number of detector element and LED pairs. The detector array has a 50% "over scan" since the lower one-third portion of the three detectors of each group is scanned by the same segment of the field of view as the upper onethird portion of the three detectors of the adjacent group (this overlap is not indicated in Fig. 4). The input optics include an objective 30, a focusing lens 32, a boresight prism 34, a field switch lens 36 and a relay lens 38. The output optics comprise lenses 66 and 68, and a reticle pattern 67. The system is such that channel to channel non-uniformity does not cause line to line or region to region non-uniformity, and in addition the resulting image is insensitive to the presence of dead channels. The system is suitable for use with an afocal magnifying system.