Spatial resolution is defined as the number of line pairs a camera can resolve per millimetre. Among the several methods available for measuring the resolution of an optical system, modulation transfer function (MTF) is in common use.
Dynamic range of the ICCD is governed by the CCD section and varies inversely with gain of the ICCD. A higher dynamic range CCD used in the ICCD will result in a higher dynamic range ICCD camera. As gain increases, smaller signals that can be accommodated are compensated by the lower read noise to keep the dynamic range constant.
When the read noise drops below a single photon level, dynamic range of the ICCD starts dropping as gain increases further. Frame rates of an ICCD are governed by CCD specifications, especially the number of pixels and pixel readout rate.
CCD and CMOS sensors have a spectral response sensitive to the visible region of the electromagnetic spectrum, with the trailing part extending slightly to near IR region.
A thermal imaging sensor, on the other hand, uses focal plane arrays comprising IR sensing elements that respond to mid (3 to 5 microns) and long IR (8 to 14 microns) regions. IR energy radiated by the object to be imaged is incident on the thermal imaging sensor, which uses a series of complex mathematical algorithms to construct the image visible to the viewer. As the thermal imaging sensor does not need visible light for operation, it can see in total darkness.
Thermal imaging sensors are far more expensive than their visible spectrum counterparts. In view of their military applications, high-end devices are often export-restricted.
A thermal imaging sensor makes use of thermal radiation emitted by the target or scene of interest to generate its image. Essentially, it comprises front-end optical system, two-dimensional array of IR detector elements and image processing circuitry to produce output in the desired format. The front-end optical system focuses IR radiation emitted by all objects in view on a two-dimensional array of IR detector elements, which create a detailed temperature pattern of it, called a thermogram.
The thermogram is generated from several thousand points in the field-of-view of the detector array. The thermal imager measures very small relative temperature differences and converts otherwise invisible heat patterns into clear, visible images that can be seen through either a viewfinder or a monitor.
Most thermal imaging sensors scan at a rate of 30 times a second. These can sense temperatures in the range of –20°C to +2000°C, and can sense temperature changes as small as 0.1°C. In the next step, temperature pattern is translated into electronic impulses. The signal processing unit converts these electronic impulses into data for the display. Fig. 6 illustrates the concept of thermal imaging.
Types of thermal imaging sensors
There are two distinctive detector technologies: direct detection (or photon counting) and thermal detection. In direct detection, the detector element translates the photons directly into electrons. The charge accumulated, the current flow or the change in conductivity is proportional to the radiance of objects in the scene. Detectors in this category include lead selenide (PbSe), mercury cadmium telluride (HgCdTe), indium antimonide (InSb), platinum silicide (PtSi) and more.
All thermal imaging sensors based on direct detection technology, except those working in short-wave IR (SWIR), use detectors cooled to cryogenic temperatures close to –200°C. Newer photon-type IR sensors operating at elevated temperatures are now available. This has allowed solid-state thermal electric coolers and sterling coolers to be used.
Cooled thermal imagers are highly susceptible to damage from rugged use, have a long cooling time of typically a few minutes, limited MTBF of a few thousand hours, high cost, large size and weight, and high electrical power consumption leading to short battery life. The biggest advantage of these detectors is excellent spatial resolution and sensitivity that results from detector cooling.
Thermal detection, on the other hand, uses uncooled detectors. These make use of secondary effects such as relation between conductivity, capacitance and expansion, and detector temperature. Detectors in this category include bolometers, thermocouples, thermopiles and pyroelectric detectors. These sensors operate at room temperature and are lightweight. This feature, for example, allows microbolometer thermal imagers to be mounted on helmets.
Different generations of thermal imaging sensors
There are different generations of thermal imaging sensors. Each successive generation has incorporated not only a major change in the type of detector but also a major change in optical systems used to image the target onto the detector.
Four distinct generations of thermal imagers have been designed, based on IR detector technologies developed during the last 35 years, and classified according to the number of elements contained in each group. First-generation thermal imagers contain single element detectors, or detectors with only a few elements (1×3). A two-dimensional mechanical scanner was usually used to generate a two-dimensional image.