REFLECTION 1 



in the direction perpendicular to the plane 

 of the specimen (direction z) is often brought 

 out with a clarity which is not possible by 

 other microscopical methods and the reflec- 

 tion image is very sensitive to changes in this 

 direction. 



Contrast. This is high and arises largely 

 from the difference between the intensity 

 from illuminated areas and from areas "in 

 shadow" behind raised features. In addition, 

 there is a tone range within the illuminated 

 areas due to the local variation in angle be- 

 tween the surface and the axis of the imaging 

 system. The contrast is thus extremely easy 

 for the eye to interpret and this accounts for 

 the very pleasing appearance of reflection 

 micrographs. 



Resolution. Electrons are scattered in all 

 directions, evenly illuminating the objective 

 aperture the size of which therefore governs 

 directly the confusion in the image due to 

 aberrations. Of these, chromatic aberration 

 is the overriding effect due to the large 

 energy spread in the scattered beam. 



The radius of the disc of confusion is 

 given by 



d = aCc8v/v 



(1) 



where a is the semi-angular aperture of the 

 accepted beam 

 Cc is the chromatic aberration con- 

 stant of the objective lens 

 f)v is the half width of the energy 



spread of accepted electrons 

 V is the accelerating voltage 

 d can be reduced by reducing Cc , al- 

 though this appears to be limited by the 

 present design of lenses to approximately the 

 focal length, or by reducing the objective 

 aperture. There is a limitation to the latter 

 course due to the increasing effect of diffrac- 

 tion, but this is in fact never a problem be- 

 cause a reduction in a decreases the intensity 

 of the image. In practice the lowest value of 

 a is chosen which gives an image just suffi- 

 ciently bright to be focused and recorded 

 at the necessary magnification. 



Fig. 2. Cleavage fracture on mica. (After 

 Menter).© , -H © 2 = 14° Magnification 5,100 X. 



Typical values are as follows: 

 a = 1.5 X 10-^ 

 Cc =1.0 cm 

 Sv/v = 0.0015 



o 



This gives a resolution of about 200 A, and 

 this cannot be appreciably improved on. 



Equation (1) gives the resolution on the 

 axis of the system. Abaxial points generally 

 have an inferior resolution because of the 

 field chromatic aberrations. Page (10) has 

 shown the importance of these and has given 

 a practicable method for their correction in a 

 triple lens electron microscope. 



Depth of fieltl. This is given by 



D = 2d /a {=2CcSv/v) 



(2) 



The typical values above give a depth of 

 field of about 30 microns and this ensures 

 that any part of the specimen is entirely in 



221 



