CHAPTER III 



Interference Microscopy in Transmitted 



Light 



1. OBJECT AND FUNDAMENTALS OF INTERFERENCE MICROSCOPY 



The object of interference microscopy is the same as that of phase- 

 contrast microscopy: it consists in detecting transparent objects re- 

 maining unseen not owing to their minute size but to their lack of 

 contrast in the remainder of the field. Phase-contrast microscopy 

 provides outstanding sensitiveness when observing very minute path 

 differences; the imaged objects conform accurately when the latter 

 are narrow. This type of method is based on taking effect on the 

 direct light without altering the light diffracted by the detail. This is 

 not feasible unless the detail observed is small: otherwise, the light 

 it diffracts is coalesced with the direct light to such an extent that 

 segregating readily the two phenomena is not possible. Furthermore, 

 the phase-plate occludes the objective to a greater or lesser degree, 

 this giving rise to a diffraction halo edging the images. 



Interference-microscopy fundamentals are different. The phase- 

 shifting object is placed in an interferometer and alters the optical 

 path of the light-rays passing through it. As in any interference 

 phenomenon, an optical-path change correlates a light intensity change 

 rendering visible the transparent object. Owing to its very principle, 

 interference microscopy does not discriminate between wide and narrow 

 objects: phase-shifting objects are seen regardless of shape and di- 

 mensions. 



However, these advantages are often offset by the instrument's 

 complexity and the adjustment skill required. 



2. FUNDAMENTALS OF TWO-WAVE INTERFERENCE MICROSCOPES 



Interference-microscopy fundamentals may be outlined as follows 

 (Fig. 3.1). The condenser-originated hght-ray SM originates at M the 

 two rays B and A. The branching taking pkice at M is brought about 



94 



