The infrared spectrum has provided a rich, available technology for the accomplishment of many important and practical tasks. This tutorial text provides a brief introduction to the language, processes, and some of the instrument design techniques that are available to the engineer today. It arose from lectures given at about ten different tutorial sessions at meetings of SPIE, the International Society for Optical Engineering. The material was adapted from a full, one-semester, three-hour course, given at The University of Arizona for twenty five years.
The infrared spectrum is part of the electromagnetic spectrum, with wavelengths ranging from about 1 Î¼m to about 1000 Î¼m (1 mm). It lies just above the visible spectrum that spans the region from 0.3 Î¼m to 0.8 Î¼m (300 nm to 800 nm) and below the millimeter region. There are various names for the various parts of the infrared spectrum, and they are used differently by different people. Most people would consider the far infrared as ranging from about 25 Î¼m to 1000 Î¼m. It is used chiefly by astronomers and solid state physicists. That part of the region will not be covered in this text. The remaining part of the spectrum, ranging from 1 Î¼m to 25 Î¼m, is divided into the short-wave infrared (SWIR), from 1 to 3 Î¼m, the midwave infrared (MWIR), from 3 to 5 Î¼m, and the long-wave infrared (LWIR), from 8 to 12 Î¼m. The remaining region, from 12 to 25 Î¼m, is not used a great deal, but has been called the very long wave infrared (VLWIR). These regions are defined by the transmission of the atmosphere and the spectral sensitivity of various detectors. The last-named region is used only above the atmosphere.
The general plan of this text is to discuss first some of the existing applications of infrared as a sort of appetizer and incentive to learn the techniques discussed in the later chapters. Then a review of optical fundamentals, the requisite radiometry, and of detector types and properties is given. The main body begins with a rather mathematical treatment of the equations of sensitivity. These are done in two different ways. The first is in terms of the summary figure of merit, specific detectivity, that has been used and misused for almost a half century. It is useful in many applications; it does have limitations that must be recognized. The second method is the counting of photoelectrons by charge-collection devices. Idealized equations for signal-to-noise ratio and noise-equivalent temperature difference are developed and discussed. A review is then given of the nature of the infrared scene. The infrared system designer must keep in mind that the world around him is rich in infrared radiation, and the infrared characteristics of all these objects can be vastly different than their visible counterparts. Snow is black in the infrared! This information can then be incorporated appropriately in the sensitivity equations. The radiation from the scene is transmitted, absorbed, and scattered by the atmosphere as it travels to the sensor. Techniques for making these calculations are then discussed. The information and noise bandwidths are also important in sensitivity calculations. They are covered next, along with a variety of scan patterns and techniques, but not scanner realizations.