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Infrared Tutorial

-- Discovery
In 1800, Sir William Herschel, the royal astronomer to the King of England, conducted an experiment to study the heating effects of sunlight. He used a prism to separate light into the colors of the spectrum and used a thermometer to measure the temperature in each color. As he moved from the violet to the red region, the temperature increased; however, when he placed the thermometer in the region just beyond the color red, the temperature continued to increase, even when no light was visible to the naked eye. William Herschel had just discovered the portion of the electromagnetic spectrum known as "infrared."

-- Properties --

All objects emit infrared radiation. The temperature of an object determines how much radiation is emitted and at what particular wavelength. The higher a body's temperature, the more radiation emitted and the shorter the peak wavelength of the emissions. The graph in Fig. 2 shows the radiation profiles of various objects and conditions. As an object's temperature increases, the location of the "peak" wavelength moves toward shorter wavelengths. The surface of the sun, at 6000 K, has its peak in the yellow region of the visible portion of the spectrum, and therefore, appears yellow in the sky. A fighter aircraft exhaust, at approximately 800 K, isn't hot enough to emit radiation in the visible spectrum. The fighter aircraft exhaust's peak emission occurs at roughly three micrometers (mm) and is located in the infrared region of the spectrum.

Similar to the colors of the rainbow, the infrared spectrum is divided into subregions primarily based on how they are utilized in sensor systems. The boundaries of these regions are not absolute, but normal convention breaks down the infrared region into four basic categories: Short, Medium, Long and Very Long wavelength. Just beyond the color red in the visible spectrum, i.e. with a wavelength --slightly longer than red, is an area known as Short Wavelength Infrared (SWIR). This band generally covers the wavelengths between 1-3 mm and is used by space based sensors to see the bright rocket plumes of boosting missiles. Slightly longer in wavelength and covering from 3- 8 mm is the area known as Medium Wavelength Infrared (MWIR). Space systems use this band to detect and track objects through booster burn out against an Earth background [Below the Horizon (BTH)]. From 8-14 mm, is an area known as Long Wavelength Infrared (LWIR). The long wave band is used by space sensors to see objects Above the Horizon (ATH) against a cold space background. The final region of the infrared, Very Long Wavelength Infrared (VLWIR), is located beyond 14 mm and typically ends around 30 mm. This band is used to track extremely cold targets against a space background.

Because all heated objects emit infrared radiation, the infrared is an excellent spectral region to use for object detection and tracking. Using an infrared detector, an object's emitted radiation can be detected, measured and plotted. Since every object has a unique infrared signature or "fingerprint," a positive object identification can be made based on the received energy.

-- Infrared Detectors --

In order to detect the infrared radiation emitted from heated objects, a material sensitive to infrared radiation is needed. Current space based systems use photon detectors in order to "see" this thermal radiation.

Photon detectors consist of a semiconducting material that is sensitive to infrared radiation. The radiation consists of energy packets called "photons" that interact directly with the material and generate electrical signals. The detector material is divided into small sections called "pixels," and a detector's resolution is determined by the size, spacing and number of these pixels. The name given to a material segregated into pixels is a "Sensor Chip Assembly." Today, most SWIR, MWIR, and LWIR detectors are made of either Mercury-Cadmium-Telluride (HgCdTe) or Indium-Antimonide (InSb); however, Silicon (Si) and Germanium (Ge) are still used for VLWIR detectors. These infrared sensitive materials can be integrated into a larger device called an "infrared sensor system."

-- Infrared Sensor System --

An infrared sensor system is a collection of optical elements and electronic hardware connected to an infrared detector. The optical elements reflect and focus incident radiation from an object onto a focal plane, and electronic hardware attached to the focal plane is used to "read out" the electrical signals generated by each pixel of the focal plane. Signal processors are used to convert these analog voltage signals into digital images that can be used by a computer to determine which infrared signature(s) the detector is receiving. On a space based sensor, each detector collects photons from a particular area on the Earth known as a "footprint." The size of this footprint is determined by the angular field of view of each pixel and the altitude of the sensor (Fig.3). A detector at a high altitude will see a larger area than one at a low altitude; however, a low flying sensor will generally have better resolution. There are two basic types of sensors - "staring" and "scanning" (Fig. 4). In a staring sensor, a square or rectangular Focal Plane Array (FPA) continuously looks at a particular area and watches for changes in the incoming infrared radiation over time. The benefit of this technique is that an area is under constant watch, and depending on how often the electronics read out the incident photon energy on the FPA, it is possible to detect small, quick changes in incident radiation intensities. The drawback is that this kind of focal plane generally needs to be large in order to cover a significant area, and these large arrays are more expensive and difficult to build than smaller arrays. A second technique is to use a smaller array and scan across a region to build a picture of the entire scene. Some common scanning detector methods include the side-to-side toggle scanner, the line scanner or "pushbroom" and the spin scanner or "spinner." The advantage of the scanning sensor is that the FPAs can be manufactured relatively inexpensively compared to large staring sensors while still providing the necessary coverage. The drawback is that as the FPA performs its scanning, it cannot watch an entire scene simultaneously and might miss a change in an event occurring outside its immediate scan area.

The speed at which a scanning sensor returns to a particular spot in the field of view is called "revisit rate." If the revisit rate can be made fast enough, a scanning sensor provides a practical alternative to a staring sensor.

The ultimate decision for which type of sensor to use depends on many factors including satellite configuration, mission, altitude and performance requirements.

Infrared sensors are "passive" devices, which means they do not send out and receive signals as do "active" sensors, such as laser or radar sensors. Instead, they passively wait until infrared energy from an object strikes the detector and is measured. A space based infrared system allows each sensor to view a large area due to its high altitude; however, because satellites are so far away, the infrared radiation needs to travel a great distance in order to reach it, which reduces the amount of radiation received at the detector. In addition, the atmosphere absorbs some infrared radiation at particular wavelengths, thus reducing the amount of radiation reaching the detector even more (Fig. 5). To overcome these factors, space based infrared detectors are designed to be very sensitive. Also, the Earth emits an enormous amount of infrared radiation which adds clutter to the image; however, with the help of software algorithms, the Earth's background radiation can be filtered out of the detector signal (Fig. 6).

-- Summary --

The infrared region of the electromagnetic spectrum is an excellent band for detecting and tracking heated objects. The technology for utilizing the infrared region has advanced considerably, and today, advanced technology infrared detectors are ideal for military space based sensor systems.




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