Accuracy

Accuracy describes the difference between a target’s real-world position and where the camera points when directed to that target. Accuracy does not, however, measure repeatability. Imagine a surveillance system that aims six feet to a target’s left half the time and six feet to a target’s right half the time. Such a system is considered “accurate” because the aiming points are grouped around the target’s actual location.

Superior surveillance systems require precision and resolution as well as accuracy. The design and engineering of a system’s motion control mechanism, or positioner, determine the speed and reliability of target acquisition.

Cooled vs. Uncooled

Thermal imaging systems are available with either uncooled or cooled infrared sensors. Cooled sensors have a integrated refrigeration system that reduces sensor temperature to cryogenic levels (approximately -195°C or -320°F). Such low temperatures reduce thermally induced noise in the sensor material. As a result, systems with cooled sensors have significantly better sensitivity, (ability to distinguish small differences in temperature). This is important, for example, when trying to find a warm human in the middle of a warm desert. Better sensitivity means more detailed images at longer distances.

Uncooled sensors have the advantage of being mechanically less complex and much less expensive than cooled sensors. They can be a good choice for shorter-range LWIR applications.

Field of View

Field of view (FOV) is a key capability for surveillance systems. On one hand, a large, panoramic FOV is critical to detecting an intrusion throughout a 360° perimeter. On the other hand, the narrow FOV of a telephoto lens is necessary to distinguish friend from foe at a safe distance. A superior surveillance system has both capabilities.

Image Fusion

Image fusion combines outputs from a visible light camera and an infrared camera aimed at the same target. Visible light images provide spatial resolution and texture details in a way that is familiar to the human visual system. Infrared images differentiate targets from backgrounds based on their different thermal signatures. Fusing both signals into a single display provides the advantages of both systems.

Image Intensifier

An optoelectronic system, for use under low-light conditions, that increases the intensity of available visible light. It works by converting photons to electrons, which are then amplified and converted back to photons using a fluorescent screen. Image intensifiers are often found in night-vision goggles and similar devices.

Infrared Imaging

Infrared imaging, also known as thermal imaging, is the collection and display of energy patterns emitted from or reflected by an object. It is similar to visual imaging, except that infrared light has longer wavelengths than visible light.

Passive infrared imagers detect infrared light emitted by any potential targets, such as heat from a human body or automobile engine.

Active infrared imagers require a source of infrared light, which targets then reflect back to the imager. Active IR imagers are easily detectable by potential threats, and their detection range is limited by the infrared emitter.

LWIR

Long-wave infrared (LWIR) is a subset of infrared frequencies, with wavelengths from 8 µm to 14 µm. Home inspectors often use LWIR to spot poor insulation or water damage. LWIR penetrates smoke or aerosols better than MWIR, making it the preferred infrared band for firefighting and certain military applications. However, MWIR is less affected by humidity and has better atmospheric transmission than LWIR. MWIR therefore is preferred for longer-range applications (>2 km). Uncooled LWIR sensors tend to be relatively inexpensive; however, they show less detail and thermal range than cooled sensors. Cooled LWIR sensors exist but tend to be very expensive. See also SWIR and NIR.

MWIR

Middle-wave infrared (MWIR) spans wavelengths from 3 µm to 5 µm. (The gap between LWIR and MWIR is because Earth’s atmosphere absorbs wavelengths from 5 µm and 8 µm.) MWIR sensors exhibit excellent detection of any thermal contrast between a target and its background. MWIR systems are relatively unaffected by humidity, making them ideal for surveillance in coastal or tropical areas. MWIR can detect gas leaks invisible to the unaided eye, as well as exhaust plumes from aircraft or missiles. However, MWIR sensors must be cooled, making them more expensive than uncooled LWIR sensors. See also SWIR and NIR.

Night Vision

Night vision systems use Image Intensifiers to amplify any visible light provided by, for example, the moon or stars. Because they require visible light to be present, “night vision” systems are completely different from infrared or thermal imaging.

NIR

Near-infrared (NIR) is non-visible light with wavelengths from 0.75 µm to 1.4 µm. NIR can improve surveillance images under low-light conditions. NIR also is used in the medical profession to differentiate chemical compounds. See also LWIR, MWIR, and SWIR.

Payload

Payload refers to a package of sensors or effectors that is mounted on and aimed by a positioner. The ease or difficulty of payload installation and replacement gives an early indication of a surveillance system’s maintainability and configurability.

Positioner

A computer-controlled, motorized system for aiming sensors, effectors, or any other payload. Positioners are distinguished by their precision, accuracy, and resolution. These, in turn, are a function of the positioner’s mechanical design, component quality, and control system. The specifications for any single component – a rotary encoder, for example – are NOT a reliable indicator of overall system performance.

Precision

Precision describes the difference between successive attempts to aim a camera at the same point. In other words, it measures repeatability. To use a target sports analogy, a tight shot grouping has good precision, while a loose shot grouping has poor precision. However, a system can have excellent precision (repeatability) and still miss the intended target every time.

The best surveillance systems provide accuracy and resolution as well as precision. Design and engineering of a system’s motion control mechanism, or positioner, determine the speed and reliability of target acquisition.

Resolution

Resolution, for surveillance systems, most often refers to the rotary encoders used to control a camera’s pan and tilt angles. Resolution can be described either as the number of steps per single 360° revolution, or as the number of bits output by the encoder. For example, an “8-bit” encoder has 2⁸ or 256 steps per revolution, so it can move a camera in 1.4° increments. A 12-bit encoder has 4096 steps per revolution, or one step every 0.88°. But, at a distance of 2 kilometers (1.2 miles), 0.88° means that the camera will pan left or right in increments of 10 feet (3.1 meters)!

Resolution and accuracy are independent. Two encoders with the same resolution can have different accuracies, and vice versa. Also, other components and design decisions can, and do, affect the precision and accuracy of the complete system.

SWIR

Short-wave infrared (SWIR) is non-visible light with wavelengths from 1.4 µm to 3 µm. SWIR is particularly useful for sensing high temperatures (>150° C), differentiating substances (e.g., food vs. contaminants), and atmospheric imaging. Water droplets and dust particles in the air scatter visible light, but not SWIR. This makes SWIR ideal for hazy or foggy conditions. SWIR sensors tend to be more expensive and less sensitive than either MWIR or LWIR. See also NIR.

Thermal Imaging

Thermal imaging is synonymous with infrared imaging. Thermal imaging operates in the infrared spectrum. It is different from “night vision“, which operates in the visible light spectrum.