In the field of heat transfer, intensity of radiation I {\displaystyle I} is a measure of the distribution of radiant heat flux per unit area and solid angle, in a particular direction, defined according to d q = I d ω cos θ d A {\displaystyle dq=I\,d\omega \,\cos \theta \,dA} where d A {\displaystyle dA} is the infinitesimal source area d q {\displaystyle dq} is the outgoing heat transfer from the area d A {\displaystyle dA} d ω {\displaystyle d\omega } is the solid angle subtended by the infinitesimal 'target' area d A a {\displaystyle dA_{a}} θ {\displaystyle \theta } is the angle between the source area normal vector and the line-of-sight between the source and the target areas.
