PHYSICAL ASPECTS OF IMAGE FORMATION 



37 



line xv' (Fig. 1.42), and curve 2 (Fig. 1.43) shows the light-intensity 

 distribution in the image in the presence of diffraction, which also 

 does away with any sudden break in intensity when changing over 

 from the imaged illuminated area to the black one. Shaded-off 



Fig. 1.43. Area edge imaged in incoherent illumination. 



illumination straddles the geometrical boundary and extends on either 

 side of it but soon becomes hardly perceptible. The smaller the 

 angle a the sharper the image outline. 



Periodic object imaged in incoherent illumination 



Let us take as periodic object a Foucault test divided into alternately 

 light and dark parallel lines of identical length and width (Fig. 1.44). 

 Such an object can be likened to some periodic structures occasionally 



Fig. 1.44. Periodic object (Foucault test). 



met with in microscopy. If the dark lines are absolutely black as in 

 Fig. 1.45, the contrast equates unity. If not (Fig. 1.46) the contrast 

 decreases and may have any value from 1 to 0. Let I^ be the intensity 



Fig. 1.45. Light distribution in the Fou- 

 cault test. 



Fig. 1.46. Foucault test with contrast 

 smaller than unity. 



