ELECTRON OPTICS 



4000 (nO 



. - •■-^*~*^^- 1 .^^ 6000(ni) 



\ ^^- - r'"~""..---' — 8ooo(ni) 



8000(ni) 



Fig. 4. The resolving power as a function of the pole piece spacing, magnetic field and excitation for 

 100 KeV electron energies. 



dotted curve shows an approximation which 

 becomes accurate for weak lenses and is 

 given by: 



CJf = 5[//(S + D)Y 



(S) 



The resolving power of the microscope is 

 theoretically limited by spherical aberration 

 (requiring a minimum aperture angle) and 

 diffraction (requiring maximum aperture 

 angle) to a minimum value (d) at an opti- 

 mum aperture angle a, given by: 



d = 0.43 C'/^XS/" 

 a = 1.4(X/C.)i/* 



(4) 

 (5) 



Figure 4 shows the variation of resolving 

 power under optimum aperture conditions 

 as a function of pole piece spacing (S), mag- 

 netic field strength (Hp) and excitation (A^7) 

 for 100 Kev electron energy. It is seen that 

 for a given field strength a flat optimum oc- 

 curs around a particular value of spacing. 



A further parameter of importance is the 

 chromatic constant (C<.) which gives the 

 variation of focal length with small changes 

 in the high voltage or lens current 



Sf = CciSV/V - 257/7) (6) 



8f must clearly be kept within the depth 



151 



