# Obliquity factor in Huygens principle

What is the derivation of the obliquity factor (intensity proportional to 1+cosø,where ø is the angle between direction of wave propagation and wavefront tangent at some point on the envelop) used in the field of wave optics under Huygens principle?

• Look at Kirchhoff's diffraction formula which contains parameters arbitrarily assigned in the Huygens equation. en.wikipedia.org/wiki/Kirchhoff%27s_diffraction_formula – Farcher Jul 16 '17 at 12:11
• The mathematical derivation can be found in Godman "Intro to Fourier Optics". However, as stated here it turns out that this factor has no simple "quasi-physical" explanation. – Semoi Nov 24 at 22:17