About radiance equation in Radiometry

[dammage]

Homework Statement

According to some pages on web, differential formula for calculating radiance L is :

L = d²P / ( dA*cos(a)*dw)

L : radiance
P : power flux
A*cos(a) : projected area
w : solid angle

Please tell me why does power P has derivaties two times d²P ?

Homework Equations

The Attempt at a Solution

 

[ideasrule]

Because radiance is power per unit area, per solid angle.

 

[dammage]

Ah, I understand now. Thank you !