Specific detectivity is given by D ∗ ∗ = A Δ Δ f N E P , where A is the area of the photosensitive region of the detector, Δ Δ f is the bandwidth, and NEP the noise equivalent power in units [W]. It is commonly expressed in Jones units ( c m ⋅ ⋅ H z / W ) in honor of Robert Clark Jones who originally defined it.