PHYSICAL ASPECTS OF IMAGE FORMATION 



33 



multifariousness of images met with in microscopy, the results shown 

 refer to typical objects similar in nature to the basic details of micro- 

 scope specimens frequently met with. 



Small black disk on white ground imaged in incoherent illumination 



The object now considered is a black disk, small in relation to the 

 objective-originated diffraction disk. Figure 1.38 shows the distribution 

 of light-intensities in accordance with image diameter. The pecked 

 line curve is related to the geometrical image of the object. Intensity 



Fig. 1.38. Small black disk on white ground, imaged in incoherent illumination. 



is zero within a circumference of diameter A'^^B'^^ (diameter of the 

 geometrical image of the dark disk). Outside of this area, field illumi- 

 nation is uniform, its intensity being, e.g. I^. Curve 2 shows the 

 intensity distribution in the actual image of the dark disk. Owing to 

 diffraction, the intensity I^, in the image centre, is not zero; there 

 are no breaks and the image is wider. The image contrast may be 

 defined by the expression: 



7 





(1.3) 



If h = 0, i.e. the image centre is black, the contrast is: 



7 = 1. 



The image contrast equates the contrast of the object itself and 

 is then maximum. The contrast dwindles as A increases. There is 

 no contrast when L equates /j. Contrast values are, therefore, in- 

 cluded between the minimum value and the peak value 1. In all 

 the following applications, it is assumed that the observer's eye 

 functions as if it was a perfect instrument. In normal daylight 



