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Specific detectivity, or D*, for a photodetector is a figure of merit used to characterize performance, equal to the reciprocal of noise-equivalent power , normalized per square root of the sensor's area and frequency bandwidth.
Specific detectivity is given by D ∗ = A Δ f N E P {\displaystyle D^{*}={\frac {\sqrt {A\Delta f}}{NEP}}} , where A {\displaystyle A} is the area of the photosensitive region of the detector, Δ f {\displaystyle \Delta f} is the bandwidth, and NEP the noise equivalent power in units. It is commonly expressed in Jones units in honor of Robert Clark Jones who originally defined it.
Given that noise-equivalent power can be expressed as a function of the responsivity R {\displaystyle {\mathfrak {R}}} and the noise spectral density S n {\displaystyle S_{n}} as N E P = S n R {\displaystyle NEP={\frac {S_{n}}{\mathfrak {R}}}} , it is common to see the specific detectivity expressed as D ∗ = R ⋅ A S n {\displaystyle D^{*}={\frac {{\mathfrak {R}}\cdot {\sqrt {A}}}{S_{n}}}}.
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.