A coherence approach to phase-contrast microscopy: theory.

Optical coherence theory is used to describe image formation in a telecentric optical system. By assuming a weakly interacting object and by considering points that are not too far from the optical axis, an optical transfer function description is obtained for imaging both the phase and the amplitude components of the object. A dimensionless coordinate system is identified to allow the transfer functions to be expressed independently of the details of the imaging system. Phase-contrast imaging is found to have an essentially coherent behaviour when the coherence length is a factor of 15 larger than the system resolution, and that the coherent region of the illumination therefore does not need to encompass the object.