Specific detectivity, or D*, for a photodetector is a figure of merit used to characterize performance, equal to the reciprocal of noise-equivalent power (NEP), normalized per square root of the sensor's area and frequency bandwidth (reciprocal of twice the integration time).
is the bandwidth, and NEP the noise equivalent power in units [W].
) in honor of Robert Clark Jones who originally defined it.
[1][2] Given that noise-equivalent power can be expressed as a function of the responsivity
, it is common to see the specific detectivity expressed as
It is often useful to express the specific detectivity in terms of relative noise levels present in the device.
is the wavelength of interest, h is the Planck constant, c is the speed of light, k is the Boltzmann 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),
Detectivity can be measured from a suitable optical setup using known parameters.
You will need a known light source with known irradiance at a given standoff distance.
The incoming light source will be chopped at a certain frequency, and then each wavelength will be integrated over a given time constant over a given number of frames.
Next, an average signal and rms noise needs to be measured from a set of
Next, emitting area must be converted into a projected area and the solid angle; this product is often called the etendue.
This step can be obviated by the use of a calibrated source, where the exact number of photons/s/cm2 is known at the detector.
If this is unknown, it can be estimated using the black-body radiation equation, detector active area
This ultimately converts the outgoing radiance of the black body in W/sr/cm2 of emitting area into one of W observed on the detector.
The broad-band responsivity, is then just the signal weighted by this wattage.
where From this metric noise-equivalent power can be computed by taking the noise level over the responsivity.
Now, the detectivity is simply the noise-equivalent power normalized to the bandwidth and detector area.
This article incorporates public domain material from Federal Standard 1037C.