Camera Technology

Modern IR cameras work very much like digital consumer video cameras (VIS cameras). They are based on a 2-dimensional detector chip which generates electrical signals when exposed to light. The detector chip is exposed for a certain time (exposure time or integration time) and the read out pixel per pixel. The analog signals from the individual pixels are converted to digital signals immediately after read-out. After read-out of all pixels the image is complete in a digital form. The digitization of IR cameras is done with a relatively high dynamics of 14 bits (VIS cameras: typ. 8 bits). Thus, quite good sensitivities down to NETD 10 millikelvin are reached in a wide temperature range of some ten Kelvin. Two types of infrared detectors are distinguished by their functional principle: thermal detectors (microbolometers) and quantum detectors.

The less sensitive microbolometers are based on exchange of thermal radiation between detector and measurement object. This causes a temperature change which is measured as the detector signal. The microbolometer detectors do not have to be cooled but only thermally stabilized. Thus, they are often referred to as "uncooled" detectors.

In quantum detectors charge carriers are excited by the absorption of IR photons. The detector must be made of a special semiconductor material with low band gap energy, such that the low energy of IR photons is sufficiently high to generate an electrical signal. The operation of a quantum detector requires cooling with a Stirling cooler down to temperatures between 60 and 80 Kelvin. Quantum detectors are also referred to as "cooled" detectors. Quantum detectors have a high sensitivity and permit very fast, low-noise measurements. For quantum detectors, different operation modes exist: the "snapshot" mode is used for very sensitive detector types. The integration of all pixels occurs simultaneously. With two capacitors per pixel, simultaneous integrating and read-out is possible ("integrate-while-read"). The "rolling frame" integration mode is used for detectors with less sensitivity. The integration is interrupted by the line-by-line read-out only.

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