PHYSICAL ASPECTS OF IMAGE FORMATION 



43 



A = 0-6 //, £ = 0002 //. As with a small black disk, evolvement of the 

 imaged line in terms of condenser aperture is shown in Fig. 1.52. An 

 imaged thin hne is always very similar to the image in coherent illumi- 

 nation and it is equation (1.8) that should be used in microscopy 

 except when fluorescence is involved. As condenser aperture is in- 

 creased, diffraction fringes become less prominent and the image 

 becomes finer while retaining the same contrast. 



Image of thin streak of light on black ground in coherent illumination 



The image is not the same as in incoherent illumination. In co- 

 herent illumination, the diffraction-pattern minima are zero minima 



^0 ^0 



Fig. 1.53. Image of Ihin slreak of light on black ground in coherent illumination. 



(Fig. 1.53, curve 2). Perception of a thin streak of light on black 

 ground is also a matter of luminous flux. Extremely fine streaks of 

 light are perceivable, provided the luminous flux emitted by them 

 be adequate. 



Edge of area imaged in coherent and partially coherent illumination 



Referring again to Fig. 1.42, the curves are those shown in Fig. 1.54. 

 Curve 1 applies to coherent illumination, curve 2 to partially coherent 

 illumination when the condenser N.A. is the same as the objective's. 

 The diffraction fringes are clearly visible in coherent illumination and 

 impair the image sharpness along the area edges. Increasing aperture 

 of the condenser diaphragm causes these fringes to vanish. 



Periodic object in coherent and partially coherent illumination 



Referring again to the object shown in Fig. 1.44, the luminous 

 lines consist of narrow parallel slits, evenly spaced and of equal size. 



