Given that noise-equivalent power can be expressed as a function of the responsivity R (in units of A / W or V / W ) and the noise spectral density S n (in units of A / H z 1 / 2 or V / H z 1 / 2 ) as N E P = S n R , it is common to see the specific detectivity expressed as D ∗ ∗ = R ⋅ ⋅ A S n .