IR Imaging Explained: A Brief Introduction to What You Can’t See

Infrared (IR) Radiation

In everyday life we encounter electromagnetic radiation in many different forms. Visible light, ultraviolet light, radio waves, and X-rays are all examples of electromagnetic radiation, differing only in wavelength. Infrared radiation occupies that region of the electromagnetic spectrum between visible light and microwaves. The figure below shows the major divisions of the electromagnetic spectrum, along with some specific features within the spectrum.
All objects constantly emit IR radiation as a function of their temperature. As an object gets hotter, it gives off more intense infrared radiation, and it radiates at shorter wavelengths. At moderate temperatures (above 200°F), the intensity of the radiation gets high enough so that the human body can detect that radiation as heat. At high enough temperatures (above 1200°F), the intensity gets high enough and the wavelength gets short enough so that the radiation crosses over the threshold to the red end of the visible light spectrum. This is why hot steel glows red. As an object gets even hotter (for example, the tungsten filament of a light bulb at 5000°F), the intensity gets even higher (that is, it glows brighter) and the wavelength gets even shorter (more white). But even at low temperatures, below the threshold of visible light emission, everything glows with this longer wavelength infrared light, and objects at different temperatures glow with different wavelengths and intensities. The radiation from these objects creates an infrared scene, similar in nature to a visible light scene.

IR Imaging Systems

The human eye cannot detect infrared light. But infrared energy can be detected electronically. Sophisticated electronic instruments exist which can scan a scene and convert the infrared light to an electrical signal which can be displayed on a video monitor, analysed or recorded. Electronically the output of these instruments is very similar to the output of a conventional video camera.

Unlike the human eye or a conventional video camera, which rely on reflected visible light to illuminate the objects in a scene, these infrared cameras are detecting IR energy which is being emitted by the objects in the scene. So an infrared camera works as well in total darkness as it does in normal daylight.

IR imaging systems are designed to satisfy different performance parameters, depending on their intended use. Military applications, such as missile guidance, require the highest level of accuracy and reliability. Many commercial applications require low cost, ease of use, and ease of maintenance.IR imaging applications are virtually limitless. They allow you to see at night or through haze and smoke. They allow you to measure the temperature profiles of objects at great distances with high accuracy. Military applications include target acquisition, missile and weapon guidance, navigation, reconnaissance, surveillance, and terrain analysis. Commercial applications exist in many fields: industrial (plant maintenance, quality control, non-destructive testing), environmental and scientific (earth and solar sciences, pollution control, energy conservation, resource development), medical (mammography, detection of arterial constriction, evaluation of soft tissue injury), and civil (law enforcement, fire fighting, surveillance, border patrol), to name just a few.

Historically the majority of sales activity has been in military applications; the high cost of infrared imaging systems has limited its use in commercial applications. However, the last few years have yielded significant breakthroughs in detector technology and associated support electronics, resulting in major reductions in the cost of imaging infrared systems. The price of these new systems now opens up vast new markets for IR equipment.
Content extracted from "IR For Dummies 1999" written by Santa Barbara Infrared, Inc. a HEICO company

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