um 
7 CAMERA TESTING BY THE USE OF SPECKLE PATTERNS 
L H Tanner (Brighton Polytechnic) (11) 
Just as the possibilities of the use of laser speckle 
(Section 3) followed on the development of holographic 
testing (Section 2), so one of the authors of the 
work in the previous section has developed a visual 
testing method based on the observation of speckle 
generated at the surface of the film and observed in 
the plane of the diaphragm (Figure 19). 
  
  
  
LU ALL EU 
I, 
À à b: 
P Ly 
erem ~ 0.5m ss ed 
7 
Figure 19 Experimental apparatuse 
The principle is that the focusing of the laser beam 
on the film is judged by the absence of bodily 
movement of the observed pattern when lens L, is 
translated laterally. Under reasonable conditions 
the focusing may be verified to about /20 of the 
Rayleigh limit, and enables the aberration coefficients 
to be derived from observations at various field angles 
and positions within the camera aperture stop. 
Separating out the aberrations provides information on 
the focus—error term, and this is shown in Figure 20, 
  
  
7 
| 101 
SF 
se 
5+ >, 
e ó rm 
“1.0 -0ls 0 0.5 1.0 
— 5 ]-—t 1 
o IS o 
o 
° -5+ 
e -104- 
x ® Sagittal 
e 
o Astigmatism 
asl 
Focal position variation versus 6 for 
x' = O, for the sagittal direction, 
showing field curvature and film 
position errore 
Figure 20 
94 
and compared with the predicted parabolic form. This 
result indicates the great power of the method to 
relate the optical properties of the camera system and 
its mechanical adjustments to the real condition of 
focusing on the actual film. 
Reference 
11 Tanner L H, "Camera testing by the use of speckle 
patterns" Applied Optics, 1974, 13, No 9, 
pp 2026-2034. 
8 FINE TOPOGRAPHIC STRUCTURE IN POLISHED GLASS 
SURFACES 
K Lindsey (Reference 12,13) 
Surfaces for use as reflectors in the X-ray region must 
have much smoother microstructure than is necessary 
for visible light. The same applies to the reflecting 
facets of diffraction gratings for X-ray use. The wave- 
length of the X radiation is much shorter, and 
undulations in the surface of very small height(&1 nm) 
and small lateral extent (5-10 am) cause noticeable 
variations in the convergence of the beam to its 
intended focus, and show up as displaced subsidiary 
images. Moreover, these surfaces are used near the 
critical angle and variations of slope show up as the 
reflectivity varies rapidly near the critical angle. 
A method of mapping the variation of slope over 
reflecting surfaces uses the latter principle. Well 
collimated radiation of wavelength 0.15 nm is carefully 
adjusted to be incident on the surface at the critical 
angle, and the reflected beam is received on & photo- 
graphic plate placed close to the reflecting surface. 
Typical records are shown in Figure 21, and the 
yariations in intensity may be related to the areas on 
the reflector responsible. 
  
Figure 21 Topographic maps of two surfaces produced 
by the critical-angle X-ray reflection 
method. (The horizontal lines are 
instrumental artefacts). 
Alternatively, the variations in focus mentioned first 
will provide a means of testing the perfection of the 
surface, as was suggested by Ehrenberg (Reference 13). 
Examples of these secondary focal concentrations are 
shown in Figure 22, where they are superimposed on a 
slightly out of focus primary image. This method can 
be more sensitive