ETCH TECHNIQUE 23 



5.42 The Transmission Method 



Figure 5.8 shows one form of the transmission metliod of examining Z-cut 

 etched surfaces. A parallel beam of light passing normally up through the 

 bottom poHshed surface and the top etched surface of a section will be bent 

 by refraction only at the etched surface, as in A. Each facet refracts the 

 light by ordinary laws of refraction, and the whole group acts similarly to a 

 single refracting surface at this angle, as in B.^ The resultant effect of all 

 three sets of facets is shown in C (where a lens is added for focusing the 

 light beam). If the incident beam is not normal to the bottom surface 

 there is an additional bending of the beam at this surface. If the incident 

 surface is not polished (or rendered optically flat, with a cover glass and im- 

 mersion fluid, for example) the diffusion at this surface will mask or com- 

 pletely destroy the desired effect.^" 



5.43 The Pinhole Transmission Method 



Figure 5.9 shows the pinhole form of the transmission method, as applied 

 to the examination of Z-cut etched surfaces. Here a section with a top, 

 etched surface is illuminated from below through a small hole with a wide 

 angle of illumination. The light radiates upward in all directions from the 

 pinhole, and in passing through the upper etched surface is refracted by a 

 single set of etch facets as in A. With the eye placed above the pinhole 

 (and section), certain of these rays will fall on the eye. The eye then sees a 

 virtual image of the pinhole P displaced to P', elevated from the level of P, 

 and along the line of the ray which enters the eye. The effect of a group of 

 facets is similar to that of a single prism, as in B." The resultant effect of 

 all three sets of facets of a Z-cut section is shown in C, where the section is 

 viewed from directly above and no optical system is shown. Only the three 

 virtual images of the pinhole are seen and they are located down in the quartz 

 (roughly two-thirds of the way down). 



Though the desired effect is due entirely to the top, etched surface, the 

 nature of the bottom surface may cause a deleterious masking effect, which 

 must be considered in the design of an mstrument. Due to the diffusing 

 effect of irregularities in the top surface it may act somewhat as a screen upon 

 which the extended light source shown in Fig. 5.9A, B may be imaged by the 

 pinhole. This extraneous image occurs if the bottom surface is polished, 

 and to some extent if the surface is semi-polished, strongly etched, or oily. 



* See footnote 8. 



" Similar optics hold if the section is illuminated from the etched side instead of the 

 polished side. 



" See footnote 8. 



