The subject of this book is uncooled thermal imaging focal plane arrays and systems. In this context, âuncooledâ refers to not employing artificial means of reducing the temperature of the infrared array, such as by means of cryogenic solids or liquids, mechanical refrigerators, thermoelectric coolers, or Joule-Thomson coolers. The infrared array operates at the ambient temperature, whatever that might be. If unstated, the temperature is assumed to be âroom temperature,â generally considered to be 295 K or 300 K. Some detection mechanisms require the use of temperature stabilizers upon which are mounted the thermal imaging arrays. Although their construction is similar to that of thermoelectric coolers, they maintain the array at or very near room temperature; thus they are not considered to be coolers.
The term âthermal imagingâ refers to the ability of the array in its system to image room temperature scenes. Here again, âroom temperatureâ implies 295 K or 300 K. A thermal image of a scene refers to an image of that scene made entirely by detecting the thermal (infrared) radiation emitted by everything in the scene. Thus there is no use of artificial (lamps, lasers) or natural (sunlight, moonlight, starlight, airglow) illumination of the scene. Because âroom temperatureâ radiation has its spectral peak emittance at about 10-Î¼m wavelength, this implies that the spectral response of the thermal imaging array extends beyond 3-Î¼m wavelength; such arrays are usually designed to operate in the 3â5-Î¼m or 8â14-Î¼m atmospheric windows.
The term âfocal plane arrayâ as used herein includes not only linear arrays of pixels but also two-dimensional arrays. In the former, the pixels are arranged in a single row or column. In the latter, the pixels are arranged in a matrix of columns and rows. Although some publications reserve the term âfocal plane arrayâ to matrix arrays only, that is not true herein.
In general, a focal plane array consists of two parts, the infrared sensitive material and the read-out integrated circuit (ROIC), sometimes referred to as the âsubstrate.â The ROIC includes the means to electrically address the pixels sequentially.