PHYSICAL ASPECTS OF IMAGE FORMATION 



29 



the same way. As critical illumination is a more convenient basis for 

 discussion, it will be used to investigate image coherence as it obtains 

 in microscopy. 



(a) The numerical aperture of the condenser is very low 



It is assumed that the aperture of the condenser iris diaphragm 

 is minimal. Under such conditions, every point of the source ^o 

 (Fig. 1.34) gives rise to a diffraction disk overspreading substantially 



% 



I 



i 



♦ Bl 



N;'B 



-tAo 



Fig. 1.34. Points A and B are within the condenser central diffraction disk. 



in the plane P of the specimen. In Fig. 1.34, the diffraction disk shown 

 is correlated to a point of the extended source. Let us consider two 

 pin-point details A and B in the specimen, close enough to be within 

 the condenser-originated central diffraction disk. Under the effect of 

 the incident light, A and B diffract coherent vibrations. Irrespective 

 of the objective's numerical aperture //sinw, images A'^ and B'^ of A 

 and B are coherent diffraction disks. When /zsinw is also very low, 

 A'q and ^0 are not discrete and these two points exhibit two diffraction 

 disks merging at P' . This instance is obviously not very useful in 

 microscopy since the relevant objectives do not have very low numerical 

 apertures. If nsinu increases, the images A'^ and B'^^ may be separated 

 although remaining, naturally, coherent. Therefore, when considering 

 an extended object of high contrast whose dimensions are smaller 

 than the diameter of the condenser-originated diffraction disk, it is 

 seen in coherent illumination. 



(b) The numerical aperture of the condenser is random but not low 



Investigation is restricted to a high-contrasted object, but smaller 

 than the diffraction disk of the objective. Let us take diaphragm D^ of 

 the condenser (Fig. 1.35) so that the diameter of the condenser-origin- 

 ated central diffraction disk is definitely smaller than distance AB. Both 

 points A and B are overspread by two diffraction disks from two 

 different points of 5*0. The points A and B are incoherent and emit 



