Radio waves and light are two examples of electromagnetic radiation, a physical phenomenon generated by a moving electric charge. The charge can be in a wire or on the surface of any object. It can also be made to move by almost any external power source, from an electric voltage difference to the motion of molecules caused by heat. The moving charge generates a field with two transverse components that are perpendicular to each other. These two components are in phase producing this result – an electromagnetic field propagating away from the generator, as seen below.
The EM waves have two properties that can vary – the amplitude and the wavelength. The amplitude represents the intensity of the wave, perceived for instance, as the brightness of a light source, while the wavelength indicates the ‘colour’ of the wave – colors like red, green, and blue in the case of visible light but colours with other sorts of names in different ranges of wavelengths.
PHOTONS
Electromagnetic radiation can also be thought of as little packets of energy emitted by the source that travel along the direction of propagation. In that interpretation, the amplitude represents the number of photons and the wavelength tells how much energy each photon has. Note the distinction introduced by the photon interpretation. While the wave could allow any amplitude to exist, the photons are packets carrying energy in discrete amounts. Whether EM radiation behaves as a wave or as photons depends on the situation. Conveniently, the wave properties show up in situations where the energy packets cannot be detected and the photons materialize where wave behavior is unimportant. In situations where both wave and particle behavior provide explanations for observed behavior, both representations can appear simultaneously.
DETECTING VISIBLE AND INFRARED
Both the wave and particle properties of EM radiation affect detector design and performance. The wavelength relates primarily to the size of the detector elements while the photon energy relates to the materials used. In the simplest, most universal case, the detector is a material that responds in some way when its temperature changes. Thermocouples, for instance, generate a voltage that changes with temperature and many materials change resistance with temperature. To convert arriving radiation into a temperature change requires only the use of an absorber that takes in radiated energy and converts it to heat. When you stand in the sun, you can feel your skin do this.
The absorber heats the detector material and then circuitry, with appropriate calibrations, reports the temperature change. An array of these elements fitted with a lens to make an image will take a picture of the emitted or reflected energy variations in a scene. If the absorber has a uniform spectral absorption, all of the energy that can pass through the lens contributes to the picture. Devices of this type are used for room occupancy monitoring, for instance. In this type of detector, the presence of photons has little significance; what is detected is simply absorbed energy.
However, detectors that do recognize photons as individual packets have some advantages related to noise performance and are generally amenable to fabrication in smaller structures. As a result, these types are preferred in all but a few situations. To see why, consider the energy contained in the photon’s corresponding wavelengths. Typically, photon energy is measured in electron volts (eV), the amount of energy an electron takes on when accelerated through a potential difference of one Volt. For reference, a photon with one eV of energy corresponds to a wave with a wavelength of 1.2µm in the near infrared (NIR).
The problem with photon detectors arises because they convert incoming photons into free charge that can be collected to form the output signal. In the visible spectrum, this can be done with a silicon detector which requires less than one eV to knock a charge free with low extraneous signals at room temperature. Materials that can generate free charge with only a tenth of an eV are much too noisy at room temperature to be useful as detectors and must be cooled, to a temperature of 77K for reasonable performance or even to 4K (using liquid helium) for best results. This makes equipment using these detectors expensive and bulky. Other commonly used coolers present additional problems. Mechanical coolers such as Stirling engines are large and have moving parts with resultant lifetime limits. Thermoelectric coolers require a lot of power and substantial heat sinks and still generally do not deliver temperatures low enough to support operation of these photon detectors.
DETECTION DEVICES AND OPTICS
To sidestep the size and weight limitations of photon detectors, better and smaller thermal energy detectors have been developed. The technology chosen for this purpose is the microbolometer. The bolometer is a thermal detector made of a material that changes electrical resistance with changes in temperature.
Honeywell developed an implementation of the bolometer small enough to be fabricated in arrays suitable for imaging. These “microbolometers” now use either vanadium oxides or amorphous silicon as the thermally-variable conductor. Cryogenic cooling is not necessary for these devices to produce usable images of room temperature objects. As a result, they have been widely applied to the manufacture of small lower-cost cameras with resolution, speed, and sensitivity suitable for real-time video imaging.
The 8 to 14µm band used in the Owl AI Thermal Ranger system most often associated with room-temperature thermal imaging. LWIR can see through fog and smoke better than shorter wavelengths because the particles in these obscurants are smaller than the LWIR wavelengths.
LWIR is absorbed by common glass, so other materials are needed to fabricate lenses. Some special glasses can transmit LWIR but these are difficult to work and must generally be moulded rather than ground and polished. The quality of moulded optics is not sufficient to produce images at the diffraction limit so other materials are used. The most common is germanium, which is transparent beyond 1.5µm. Germanium has two characteristics that must be accommodated. First, it has a very high index of refraction, so about 40% of the arriving radiation will be reflected. To mitigate this, high-performance anti-reflection coatings must be applied to all surfaces. Second, the thermal conductivity of germanium drops substantially as its temperature increases, leading to thermal runaway in the presence of high-power illumination sources. For passive thermal imaging, runaway is not a danger, but germanium lenses (and germanium photodetectors) must be shielded from potential bright infrared sources including focused sunlight.