▁setup ▁using ▁known ▁parameters . ▁You ▁will ▁need ▁a ▁known ▁light ▁source ▁with ▁known ▁ir rad iance ▁at ▁a ▁given ▁st ando ff ▁distance . ▁The ▁incoming ▁light ▁source ▁will ▁be ▁cho pped ▁at ▁a ▁certain ▁frequency , ▁and ▁then ▁each ▁w avel ength ▁will ▁be ▁integrated ▁over ▁a ▁given ▁time ▁constant ▁over ▁a ▁given ▁number ▁of ▁frames . ▁ ▁In ▁detail , ▁we ▁compute ▁the ▁band width ▁ ▁directly ▁from ▁the ▁integration ▁time ▁constant ▁. ▁ ▁Next , ▁an ▁average ▁signal ▁and ▁r ms ▁noise ▁needs ▁to ▁be ▁measured ▁from ▁a ▁set ▁of ▁ ▁frames . ▁This ▁is ▁done ▁either ▁directly ▁by ▁the ▁instrument , ▁or ▁done ▁as ▁post - processing . ▁ ▁Now , ▁the ▁computation ▁of ▁the ▁rad iance ▁ ▁in ▁W / sr / cm ² ▁must ▁be ▁computed ▁where ▁cm ² ▁is ▁the ▁em itting ▁area . ▁Next , ▁em itting ▁area ▁must ▁be ▁converted ▁into ▁a ▁project ed ▁area ▁and ▁the ▁solid ▁angle ; ▁this ▁product ▁is ▁often ▁called ▁the ▁et end ue . ▁This ▁step ▁can ▁be ▁ob vi ated ▁by ▁the ▁use ▁of ▁a ▁cal ibr ated ▁source , ▁where ▁the ▁exact ▁number ▁of ▁phot ons / s / cm ² ▁is ▁known ▁at ▁the ▁det ector . ▁If ▁this ▁is ▁unknown , ▁it ▁can ▁be ▁estimated ▁using ▁the ▁black - body ▁radiation ▁equation , ▁det ector ▁active ▁area ▁ ▁and ▁the ▁et end ue . ▁This ▁ultimately ▁converts ▁the ▁out going ▁rad iance ▁of ▁the ▁black ▁body ▁in ▁W / sr / cm ²