Glossary
Accuracy
Accuracy generally quantifies how close a measurement is to its real value. In the practical context of pointing an imaging system, it describes how close to the intended viewing angle the imager actually achieves. Accuracy is generally discussed in connection with precision. Both quantities can best be described in terms of a series of measurements, since there can be variations from one occurrence to the next. Read More
Bit Depth
Electronic imaging systems must digitize the analog intensity signals that are generated by each pixel. The number of discrete levels that the signal range of that pixel is broken into is called the Bit Depth. Read More
Cooled vs. Uncooled Detectors
Some thermal imagers have what are called cooled detectors, while other have uncooled detectors. This has to do with whether the detector chip needs to be cooled to cryogenic temperatures to allow the imager to work properly. As a general rule (though there are some limited exceptions), uncooled detectors operate in the Long Wave Infrared (LWIR) band, which is generally from about 7.5µm to 14µm (there can be small variations in spectral range from different manufacturers). Cooled detectors generally operate in the Mid Wave Infrared (MWIR) range, which is generally from about 3.0µm to 5.0µm. Read More
Critical Dimension
The Critical Dimension refers to the smaller of the two principle dimensions (width or height) in an image. This is important to correctly interpret the Johnson Criteria, which is used to evaluate (based on number of pixels covering a target in an image) how well an average observer can perform the imaging tasks, using the given imagery. In some cases, the square root of the target area (width x height) is used as the target critical dimension. Read More
Detection
Detection (along with Identification and Recognition) is one of three terms commonly used to describe an imaging system task. In this context, detection means that an observer can tell that something is in the field of view, but cannot distinguish any features about the target. Read More
Field of View
Field of View (FoV) is the observable area you can see through an optical device. In an imaging system, FoV is a function of focal length, the size of the detector elements (called pixels), and also the number of pixels (array size).
Identification
Identification (along with Detection and Recognition) is one of three terms commonly used to describe a task of an imaging system. In this context, Identification means that the observer can differentiate small differences between targets of the same class. For example, identification is able to determine whether a human target (the class of target) has a rifle or a shovel in their hand (the small difference between targets within the same class). Read More
Image Fusion
Image Fusion combines the output of multiple images (usually from different spectral sources) into a single image. Often, thermal images are fused with either visible imagery or image intensified imagery, to provide the best details of both in a single image. Read More
Image Intensifier
Image Intensifiers (I2) amplify existing visual light, to allow users to see details that normal human vision cannot discern. I2 imagers operate on a spectral range from about 400nm to about 1100nm, which corresponds to visual light through the beginnings of the near-infrared regions. Modern I2 systems can amplify visible light by a factor of about 30,000, meaning scenes that look nearly imperceptible to human vision can be clearly imaged. However, some small amount of ambient light must be available to reflect off the target. This is different than thermal imaging, where the energy being imaged is usually generated by the temperature of the target (or background) itself. In closed, very dark rooms, I2 systems must use an IR illuminator to form an image, due to the need for at least a small amount of ambient light. Read More
Infrared Spectrum
The term infrared means “longer than red.” This is a very broad term that means it covers the portion of the spectrum with longer wavelengths than red light, which includes from about 750nm (which is 0.75µm) on out to about 20µm. There are four defined imaging bands within the infrared spectrum; these include near infrared (NIR) from about 0.75µm to 1.5µm, short wave infrared (SWIR) from about 1.0µm to 2.5µm, mid wave infrared (MWIR) from about 3.0µm to 5.0µm, and long wave infrared (LWIR) from about 7.5µm to about 14µm. Read More
Infrared vs. Thermal Imagers
The terms infrared and thermal are not interchangeable. Infrared imagers are defined as any imager that operates in the spectral region from about 0.75µm, all the way out to about 14µm. However, the shorter wavelengths in that spectral range carry very little thermal energy (at least, at the temperature ranges commonly occurring in surveillance applications). Therefore, thermal imagers are those imagers in the MWIR and LWIR regions, where sufficient thermal energy is radiated (at typical temperatures) to produce thermal imagery. This means that detectors that operate in the NIR and SWIR spectral region really are infrared imagers, but are not thermal imagers. Read More
Line Replaceable Unit (LRU)
A Line Replaceable Unit (LRU) is a self-contained module that is component within an imaging system. LRUs can be replaced by soldiers or operators in the field, with little to no training and few (or no) tools. Using an LRU-type approach has many benefits, as the system becomes modular. This allows the system to be installed in a piecemeal fashion, which is quite convenient when carrying a heavy system up a mounting tower. It also allows the system elements to be replaced individually; if one LRU fails, it can be replaced without replacing the entire system. It also provides a simplified upgrade path, where future advancements in technology can be easily incorporate into existing systems, simply by replacing LRUs. Read More
Long Wave Infrared (LWIR)
Long Wave Infrared (LWIR) is the part of the infrared spectrum with wavelengths from about 7.5µm to 14µm. Generally (although not always), thermal imagers operating in this wave band are uncooled imagers. LWIR imagery works best in dry or moderately humid environments, but tends to work less effectively in maritime or especially humid environments. Read More
Mid Wave Infrared (MWIR)
Mid Wave Infrared (MWIR) is the part of the infrared spectrum with wavelengths from about 3.0µm to 5.0µm. All thermal imagers operating in this wave band are cooled imagers. MWIR imagers can work well in any environment, but these cooled imagers tend to be more expensive than LWIR systems, especially for shorter range applications. Read More
Near Infrared (NIR)
The Near Infrared (NIR) region of the electromagnetic spectrum, spanning approximately 0.75µm to 1.5µm, offers a unique range of applications and imaging capabilities. Although NIR falls within the infrared spectrum, it is important to distinguish it from thermal imaging technology. NIR imaging devices are commonly known as night vision or Image Intensifier (I2) systems, and they serve distinct purposes in various fields. Read More
Night Vision
Night Vision refers to image intensified cameras, which operate in the NIR portion of the spectrum. Imagery from this spectral region is often false-colored green, as green is the area of the visible spectrum that the human eye is most sensitive to. The advantage of this type of system is that it sees visible details that may look different in the thermal imaging spectrum, so the images can be relatively easy to interpret.
Payload
The term Payload (in the context of an imaging system) refers to sensors or emitters in an imaging system. These sensors are attached to a positioner (sometimes called a “pan and tilt” or a “gimbal”) which aims the sensors in the desired direction. Payloads are generally Line Replaceable Units (LRUs).
Pixel
The term Pixel refers to a small energy sensing element, many of which make up an imaging array. Each individual pixel is a light (or energy) sensitive detector, which generally can detect different levels of incoming light or energy. An imaging array is composed of many pixels; when viewed together, the output of an array of pixels forms an image.
Positioner
A Positioner (sometimes also called a “pan & tilt” or a “gimbal”) is a device that sensors (LRUs) are mounted to. The positioner then aims the sensors in the desired direction. The positioner itself is considered an LRU.
Precision
Precision generally quantifies how closely grouped a series of measurements are to each other. In the practical context of pointing an imaging system, it describes the smallest move that the positioner can controllably make. Precision is generally discussed in connection with accuracy.
Recognition
Recognition (along with Detection and Identification) is one of three terms commonly used to describe a task of an imaging system. In this context, Recognition means that the observer can differentiate between targets of different classes; for example, between a human target and a vehicle. Read More
Resolution
In the context of imaging systems, the term Resolution refers to the level of detail that an image presents. An image with lots of detail is said to have good resolution. Often, resolution is expressed in terms of the array size (how many pixels are present in the imaging array). While this is not strictly accurate (pixel count is only one factor; others include pixel size, the spectral sensitivity of the pixel, and the bit depth of the system), the number of pixels in the array conveys a general feel for how resolved the image is.
Short Wave Infrared (SWIR)
Short Wave Infrared (SWIR) is the part of the infrared spectrum with wavelengths from about 1.0µm to about 2.5µm. Imagers in this portion of the spectrum are referred to as SWIR imagers. While this is a portion of the infrared spectrum, devices in this range are not thermal imagers. SWIR imagers excel at seeing through fog, to a much greater degree than any other portion of the spectrum. They also can detect certain commonly used lasers, though they have no real night vision capability. Read More