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IR (Infrared) Technology Parameters and Tradeoffs
Category: Infrared Technology | 27/04/2010 - 15:29:38
There are many technical parameters used to describe the performance of infrared detectors and camera systems.
They are ways to help quantify image quality and predict range performance. Among others, NETD measures image quality. Responsivity and thermal time constant are parameters of the detector and help understand the quality of the detector. Finally, MTF and MRTD measure bar targets of varying size and distance apart. The performance at these differing spatial frequencies can simulate range performance.
Noise Equivalent Temperature Difference (NETD)
NETD is the most commonly discussed IR technical parameter. It is a measure of noise in an IR image. It therefore directly relates to the overall quality of an image. In the audio world, before an audio signal is converted to sound by speakers, there could be noise on top of the desired audio signal. Once it is converted to sound, this noise can be measured in terms of sound, or decibels. The same is true for noise on top of an infrared signal. Once the infrared signal is converted to an image representing temperatures, the noise can be measured in terms of temperature, or degrees. This measurement is NETD, or the equivalent temperature difference of the noise. Typically NETD is expressed in units of Kelvin (K). Cooled infrared camera systems typically have low noise levels, in the range of 10 - 30mK. Uncooled infrared cameras systems are typically a little noisier, in the range of 30 - 120mK. Noise in an image can be spatial or temporal. Spatial noise is noise across the image at any given point in time. It is perceived as an unchanging fixed pattern on top of the image. Temporal noise is noise at any point in the image over time. It is perceived as the static that moves in an image. NETD is typically the measure of both these noise types.
Responsivity is the measure of how much an infrared detector's output changes given a certain temperature change. It is typically represented as mV/K. Each element in a detector outputs a voltage that represents the amount of signal, or temperature, the element is "seeing." Higher responsivity creates better image quality. As the amount of signal goes up, the signal to noise ratio increases which, by definition, decreases NEDT. This comes at a price however. Detectors can only output a specific range of voltages. In addition, analog to digital (A/D) converters only have a specific range of voltages they can input. Increasing the responsivity decreases dynamic range as smaller temperature differences will consume the available output of the detector or input of the A/D converter. In arrays with many
detector elements, the reponsivity differs from element to element; this is one of the reasons long calibration procedures are necessary. In order for an infrared picture to be usable, the reponsivity across elements has to be normalized in post processing.
Thermal Time Consent
Thermal time constant refers to the amount of time it takes a given pixel to change from one temperature to another. Since cooled detector materials do not hold the temperature they are sensing, they have no thermal time constant. So, this parameter is not relevant to cooled detectors. Resistive bolometer elements actually change resistance based on the pixel itself becoming a proportion of the temperature it is sensing. The thermal time constant is the amount of time it takes a resistive pixel element to change from one temperature to another. The result of a long thermal a long time constant is smearing of objects, particularly of very hot or very cold temperatures, when moving in an infrared
image. The longer the thermal time constant the longer it takes to change from one temperature to the next, and therefore more smear.
Modulation Trransfer Function (MTF)
MTF is the ability of an optical system to transfer contrast at a given resolution. It is measured by looking at four bar targets. Each target has slightly smaller bars and less distance between the bars. These targets are said to have varying spatial frequencies. The targets are placed at a specific temperature that is different from their surroundings. The peak to peak output of the target against its surrounding is measured. As the spatial frequency becomes no longer resolvable, the two temperatures will blend together reducing the peak to peak output. The results from this can then be related to range performance as smaller targets can simulate objects at farther distances.
Minimum Resolvable Temperature Difference (MRTD)
MRTD is measured by also looking at a four bar targets of varying spatial frequencies. For this test, the temperature of the target is adjusted in small steps (both from hot to cold and cold to hot) toward the surrounding temperature until the operator can no longer resolve the target. The difference between the two temperatures at this point is the minimum resolvable temperature difference. This measure can relate to range performance also however since a human element is involved it can be a more subjective test. Some people still prefer to also understand MRTD of system, however, for additional performance criteria