PHASE-CONTRAST MICROSCOPY 



71 



3. CONTRAST AND SENSITIVENESS OF THE METHOD 



Resuming the definition given in § 1.8, let us investigate the 

 bacterium contrast. The intensity /j, in the imaged bacterium is 

 = 1 + 2(/ and the intensity /a, in the remainder of the field, is unity. 

 The bacterium image-contrast is 



y = 2<r . (2.8) 



If the index ii of the bacterium is not very different from index n' 

 of the medium encompassing it, both q? and contrast y are small. 

 The contrast y, under which the bacterium is observed, can be in- 

 creased, thus improving observation. Let us assume that the phase- 

 plate Q be absorbent. This change does not alter the vibration Kg in 

 any way since the diffracted light it represents passes almost wholly 

 close to the phase-plate. Conversely, the greater the phase-plate 

 absorption the less the amplitude of the sinusoid V2 (direct light). 

 Let us denote absorption of the phase-plate by the number N that 

 divides the direct light-intensity passing through it. If A^ = 1, the 

 phase-plate is transparent; if A'^ = 10, it only lets through one-tenth 

 of the direct-light incident intensities and so forth. Then, the bac- 

 terium image contrast is given by the expression 



y=2(p^N. (2.9) 



In relation to the transparent phase-plate, image contrast is multi- 

 plied by ] N. For instance, if A'^ = 2500 path differences of ap- 

 proximately 1 angstrom may be observed with a contrast amounting 

 to 01 being still clearly visible. This shows that the phase-contrast 

 method ensures great sensitivity provided, however, that the optical 

 system be of adequate quality and, more particularly, devoid of stray 

 light. The latter, always present in a microscope, is the reason why 

 phase-plates exceeding the value N = 100 are seldom used. Table 2.1 



Table 2.1. Image contrast according to various phase-plate absorption 



RATES 



