THERMAL IMAGERS?

Thermal Imagers are utilized in a number of day-night applications and provide positive visual target identification. Thermal imagers operating in either the long-wave (8-12 micron) or in the mid-wave (3-5 micron) spectral region are available. Mid-wave imagers using the 3 to 5 micron spectral region provide some significant advantages to long-wave cameras. The technology for mid-wave focal array fabrication is more advanced than that for long-wave focal planes allowing production of high-resolution staring focal plane arrays at modest cost.

Thermal imagers are typically across-track scanners that detect emitted radiation in only the thermal portion of the spectrum. Thermal sensors employ one or more internal temperature references for comparison with the detected radiation, so they can be related to absolute radiant temperature. The data are generally recorded on film and/or magnetic tape and the temperature resolution of current sensors can reach 0.1 °C. For analysis, an image of relative radiant temperatures (a thermogram) is depicted in grey levels, with warmer temperatures shown in light tones, and cooler temperatures in dark tones. Imagery which portrays relative temperature differences in their relative spatial locations are sufficient for most applications. Absolute temperature measurements may be calculated but require accurate calibration and measurement of the temperature references and detailed knowledge of the thermal properties of the target, geometric distortions, and radiometric effects.

Because of the relatively long wavelength of thermal radiation (compared to visible radiation), atmospheric scattering is minimal. However, absorption by atmospheric gases normally restricts thermal sensing to two specific regions - 3 to 5 μm and 8 to 14 μm. Because energy decreases as the wavelength increases, thermal sensors generally have large IFOVs to ensure that enough energy reaches the detector in order to make a reliable measurement. Therefore the spatial resolution of thermal sensors is usually fairly coarse, relative to the spatial resolution possible in the visible and reflected infrared. Thermal imagery can be acquired during the day or night (because the radiation is emitted not reflected) and is used for a variety of applications such as military reconnaissance, disaster management (forest fire mapping), and heat loss monitoring.

Thermal imaging instruments measure radiated infrared energy and converts the data to corresponding maps of temperatures. A true thermal image is a gray scale image with hot items shown in white and cold items in black. Temperatures between the two extremes are shown as gradients of gray. Some thermal imagers have the ability to add color, which is artificially generated by the camera's video enhancement electronics, based upon the thermal attributes seen by the camera. Some instruments provide temperature data at each image pixel. Cursors can be positioned to each point with the corresponding temperature read out on the screen or display. Images may be digitized, stored, manipulated, processed and printed out. Industry-standard image formats, such as the tagged image file format (TIFF), permit files to work with a wide array of commercially available software packages.

Images are produced either by scanning a detector or group of detectors, or by using with focal plane array. A scanning system in its simplest form, could involve a single element detector scanning along each line in the frame (serial scanning). In practice, this would require very high scan speeds so a series of elements are commonly scanned as a block, along each line. This cuts down the scan speed from having just a single detector but the scan speed and channel bandwidth requirements are still high. It does, however, give a good degree of uniformity. The frame movement can be provided by frame scanning optics (using mirrors) or in the case of linescan type imagers, by the movement of the imager itself. Another method is to use a number of elements scanning in parallel (parallel scanning). These have one element per line but scan several lines simultaneously. Frame scan speeds are lower but this method can give rise to poor uniformity.

Another way thermal images are produced is with a focal plane arrays (FPA) (or staring array). A focal plane array is a group of sensor elements organized into a rectangular grid. A high magnification image of a portion of a micro bolometer focal plane array is shown to the right. The entire scene is focused on the array; each element cell then provides an output dependent upon the infrared radiation falling upon it. The spatial resolution of the image is determined by the number of pixels of the detector array. Common formats for commercial infrared detectors are 320x240 pixels (320 columns, 240 rows), and 640x480. The latter format is nearly the resolution obtained by standard TV. Spatial resolution, the ability to measure temperatures on small areas, can be as fine as 15 microns. Temperature resolution, the ability to measure small temperature differences, can be as fine as 0.1° C. Temperature sensitivity and measurement range cover broad ranges.

The advantage of FPAs is that no moving mechanical parts are needed and that the detector sensitivity and speed can both be slower. The drawback is that the detector array is more complicated to fabricate and manufacturing costs are higher. However, improvements in semiconductor fabrication practices are driving the cost down and the general trend is that infrared camera systems will be based on FPAs, except for special applications. A micro bolometer is the latest type of thermal imaging FPA, which consists of materials that measure heat by changing resistance at each pixel. The most common micro bolometer material is vanadium oxide.