Specific detectivity is given by , where is the area of the photosensitive region of the detector. Its common units are , also called the Jones in honor of R. Clark Jones who defined this magnitude.
Given that noise-equivalent power can be expressed as a function of the responsivity (in units of or ) and the noise spectral density (in units of or ) as , it's common to see the specific detectivity expressed as .
The unit Jones is now commonly used with the D* figure of merit.
It is often useful to express the specific detectivity in terms of relative noise levels present in the device. A common expression is given below.
With q as the electronic charge, is the wavelength of interest, h is Planck's constant, c is the speed of light, k is Boltzmann's constant, T is the temperature of the detector, is the zero-bias dynamic resistance area product (often measured experimentally, but also expressible in noise level assumptions), is the quantum efficiency of the device, and is the total flux of the source (often a blackbody) in photons/sec/cm².
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