The exitance of an extended surface is the rate at which it is radiating energy (in all directions) per unit area. The usual symbol is M and the units are W m⁻² . It is an intrinsic property of the...The exitance of an extended surface is the rate at which it is radiating energy (in all directions) per unit area. The usual symbol is M and the units are W m⁻² . It is an intrinsic property of the radiating surface and is not dependent on the position of an observer.
The factor \( \sin \vartheta d \vartheta d \varphi\) is an element of solid angle, dω, and the factor \( \cos \vartheta\) is needed to convert the projected area of radiance back into physical area. \...The factor \( \sin \vartheta d \vartheta d \varphi\) is an element of solid angle, dω, and the factor \( \cos \vartheta\) is needed to convert the projected area of radiance back into physical area. \[ \begin{array}{l} \pi F & = \int_0^{2 \pi} \int_0^{2 \pi} L ( \vartheta,~ \varphi ) \sin \vartheta \cos \vartheta d \vartheta d \varphi \\ ~ & = \int_0^{2 \pi} \int_{-1}^1 L ( ( \mu,~ \varphi ) \mu d \mu d \varphi \end{array}\]
