EO/IR Sensors

EO/IR Sensors

An EO/IR sensor is a device that converts photons in the visible and invisible spectral range into electronic signals that can be processed and manipulated.eoir sensor EO/IR systems are used for surveillance, tracking and targeting of airborne vehicles, ground-based targets and spacecraft. They can be categorized as scanning (with a limited number of detectors that scan) or staring sensors (with large numbers of detectors in rectangular arrays).

Whether a system is imaging, tracking or radar, it must perform two fundamental tasks: finding the target and maintaining its line-of-sight (LOS).eoir sensor The ability to detect a dim target against a complex clutter background is central to EO/IR systems, especially for target detection and tracking systems.

The sensitivity of an imager or radar system is defined by the ratio of the system's noise level to the target signal, and it is determined by the optical system and detector.eoir sensor In the case of EO/IR imaging systems, the Johnson Criteria limits how small the target can be and still be usefully seen.

In the era of digital sensors, the noise limit is joined by a second limit, the ability to transfer and portray contrast differences, which is also called resolution.eoir sensor This is determined by the detector, optics and sampling of the sensor.

Aliasing

A sensor cannot measure the full spectrum of light from a target, so it must sample the light in some fashion.eoir sensor Sampling leads to the problem of aliasing, which introduces spurious frequencies that confuse the task at hand. There is no way around aliasing, but it can be managed. One of the important issues in designing an EO/IR system is the choice of the sampling frequency, which must be at least equal to the Nyquist frequency for good performance.

The choice of the spectral band in which a system operates is heavily influenced by the target phenomenology and physics.eoir sensor Missile launch detection, for example, is dominated by the very hot exhaust from the missile, which produces a signature in the IR band.

Unlike most radar systems, which emit their own illumination, EO/IR cameras depend on the illumination of a target by sunlight, moonlight or the target's own emissions. They also must cope with the hostile environment, which can refract, absorb or scatter the photons that reach the sensor. All of these factors can limit system performance. To improve performance, EO/IR systems can be grouped into point target and fully imaged systems. The former involves locating a target amongst a clutter background without ever forming an image (such as the original Sidewinder air-to-air missile seeker); the latter requires an image of the target against a sky background.

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