Detectivity simply put is the inverse of the noise equivalent power. This is a key characteristic of photodetectors of utmost importance. Photodetectors of extremely high detectivity also have equivalently high ability to detect weak signals capable of competing with the noise of the detector. As the temperature of the detector decreases, the detectivity function increases. Detectivity definition varies from one author to the other but every single one of them considers the noise power of a photodetector and that of its environment.
Making use of optical setups known as professionals, the value of detectivity of a photodetector can be measured. This optical setup requires the use of a chosen light source. This light source needs to have a known irradiance value. The light source also needs to be positioned at a specific standoff distance. After chopping the designated light source at a certain frequency, the resulting wavelengths will be further integrated over a given number of frames making use of a given time constant.
While discussing detectivity, we will eventually get to consider specific detectivity. Specific detectivity is a type of detectivity which is normalized to a unit detector area along with detection bandwidth. The specific detectivity can be calculated through the multiplication of the detectivity with the square root of the product obtained from the detector area and the detector bandwidth. This specific detectivity is especially important when making comparison according to performances of the various types of detector technologies. The noise level of a detector might be reduced in order to reduce the noise equivalent power which would then lead to an increase in detectivity. This decrease in noise can be achieved by restricting the detection bandwidth. There are various formulas involved in further understanding of detectivity and its related quantities.