▁photos ens itive ▁region ▁of ▁the ▁det ector , ▁ ▁is ▁the ▁band width , ▁and ▁NE P ▁the ▁noise ▁equivalent ▁power ▁in ▁units ▁[ W ]. ▁It ▁is ▁commonly ▁expressed ▁in ▁Jones ▁units ▁() ▁in ▁honor ▁of ▁Robert ▁Clark ▁Jones ▁who ▁originally ▁defined ▁it . ▁ ▁Given ▁that ▁noise - equ ivalent ▁power ▁can ▁be ▁expressed ▁as ▁a ▁function ▁of ▁the ▁respons ivity ▁ ▁( in ▁units ▁of ▁ ▁or ▁) ▁and ▁the ▁noise ▁spectral ▁density ▁ ▁( in ▁units ▁of ▁ ▁or ▁) ▁as ▁, ▁it ▁is ▁common ▁to ▁see ▁the ▁specific ▁detect ivity ▁expressed ▁as ▁. ▁ ▁It ▁is ▁often ▁useful ▁to ▁express ▁the ▁specific ▁detect ivity ▁in ▁terms ▁of ▁relative ▁noise ▁levels ▁present ▁in ▁the ▁device . ▁A ▁common ▁expression ▁is ▁given ▁below . ▁ ▁With ▁q ▁as ▁the ▁electronic ▁charge , ▁ ▁is ▁the ▁w avel ength ▁of ▁interest , ▁h ▁is ▁Plan ck ' s ▁constant , ▁c ▁is ▁the ▁speed ▁of ▁light , ▁k ▁is ▁Bol tz mann ' s ▁constant , ▁T ▁is ▁the ▁temperature ▁of ▁the ▁det ector , ▁ ▁is ▁the ▁zero - b ias ▁dynamic ▁resistance ▁area ▁product ▁( o ften ▁measured ▁experiment ally , ▁but ▁also ▁express ible ▁in ▁noise ▁level ▁assumptions ), ▁ ▁is ▁the ▁quantum ▁efficiency ▁of ▁the ▁device , ▁and ▁ ▁is ▁the ▁total ▁flux ▁of ▁the ▁source ▁( o ften ▁a ▁black body ) ▁in ▁phot ons / sec / cm ². ▁ ▁Det ect ivity ▁measurement ▁Det ect ivity ▁can ▁be ▁measured ▁from ▁a ▁suitable ▁optical