On Partial Coherence and the Concept of 'Effective Source' as Used in Instrumental Optics

An expression has been obtained for partial coherence on a plane illuminated by a finite source through a condenser. For selected pairs of points, numerical values of the same have been presented graphically. The concept of ‘effective source’ has been examined; and a lower limit to the size of the actual source has been obtained which will still admit of the exit pupil of the condenser being used as an ‘effective source’.