PHYSICAL ASPECTS OF IMAGE FORMATION 



59 



n. STRAY LIGHT 



When a light-beam passes through a microscope, a portion of it 

 is reflected on every surface and reverts in the source direction. If 

 this luminous flux is no longer reflected, it can be considered as not 

 taking part in image formation. But if this beam is reflected anew, 

 a portion of it is reverted to the image, forming a halo of stray light. 

 Figure 1.75 shows diagrammatically the luminous paths just described. 



Objective 



Imoge Aq 



Fig. 1.75. Scallered light refieclcd by (he lenses t)f ;in objective. 



Let us now consider a narrow pencil of rays originated from the 

 luminous source A. After passing through the objective's element O^ 

 it is reflected at // on to the element 0[, next at KL on to the element 

 Oi and returns to the image A^^. The light beam // I'G' shares in the 

 formation of the actual image ^y, but the beam KL K'L' gives rise 

 to a stray-light halo in the plane of the image A'^y. This halo is not 

 very pronounced. Yet scattered light reflected on the curved sides 

 of elements Oi and 0[ have to be considered, the result being that 

 reflection losses become quite heavy when the instrument has many 

 surfaces. Such losses impair image illumination and produce stray light. 

 Intensity of the stray light may attain and even exceed 5 to 7 per cent 

 of the intensity of the normally illuminated field. Low-contrast, imaged 

 objects may vanish. Therefore reducing to a minimum the stray 

 light reflected between the lenses of the instrument is essential. The 

 light reflected by a glass surface can be reduced by coating it with 

 a thin film of suitable thickness (Fig. 1.76). Reflected light is origi- 

 nated by the interference of rays such as a and Z?: it is least at wave- 

 length Aq if the product ne equates /lo/4. Wave-length 2o from the 

 minimum is selected, the wave-length for which the receiver exhibits 

 utmost sensitivity. For instance, for the eye, Aq is selected close to 



