CRITICAL VERSUS KOHLER ILLUMINATION 295 



16. CRITICAL VERSUS KOIILER ILLUMINATION 



Phase microscopy occurs in a Zernike system whether the microscope 

 is adjusted for Kohler or for critical illumination. The following simple 

 considerations indicate that it is remarkal)le that the two systems of 

 illimiination should produce under any circumstances similar contrasts 

 in the image. In Kohler ilhunination the coherent light from a point 

 in the condenser diaphragm emerges from the substage condenser as a 

 substantially plane wave which extends over the entire object plane and 

 which subsequently forms in the image plane an "Abbe" picture of the 

 entire field of view. The complete picture of the object is formed by 

 the superposition of the Abbe pictures produced by the light from the 

 various points in the opening of the condenser diaphragm. On the other 

 hand, with critical illumination, the coherent light from a point in the 

 source is focused in a highly localized manner upon a small portion of 

 the object plane. The object plane is the scene of a localized, circular 

 diffraction image of the point in the source. This diffraction image is 

 reprojected by the objective into the plane of the eyepiece as a mod- 

 ulated, but still localized, diffraction image of the point in the source. 

 Consequently the image of the object is constructed in a piecewise 

 manner by the light emanated from the various points in the source. 



Some consideration will show that the elementary theory of phase 

 microscopy does not apply to critical illumination. Suppose, for ex- 

 ample, that the object is a ruled grating. Since the coherent light from 

 a point in the source is concentrated upon one or more of the rulings 

 and not upon all of the rulings, the deviated and undeviated waves which 

 leave the grating will be vastly different from the case of Kohler illumina- 

 tion. Hence the usual simplified vector models for phase microscopy 

 fail. 



A general theory for phase microscopy with critical illumination can, 

 however, be formulated. This theory shows that the total energy 

 density in the image plane can become similar with critical and Kohler 

 illumination provided that the source of illumination is broad and fairly 

 uniform, provided that a large field of view is illuminated, and provided 

 that the speeds of the lamp and substage condensers are high. These 

 conditions are well fulfilled in the normal usage of a microscope. To 

 the extent that one or more of these three conditions are violated, phase 

 microscopy with critical and Kohler illumination will produce different 

 distributions of energy density in the plane of the image. The fact 

 that the phenomena of phase microscopy are not necessarily similar 

 with Kohler and critical illumination is an advantage rather than a dis- 



