Figure 12a Test structure built as a brick arch with 
loading arrangement and the first sheet 
of patterned paper attached. 
  
Figure 12b Horizontal displacement pattern. 
  
Figure 12c Vertical displacement pattern. 
related to the transverse sensitivity by the ratio of 
the distance separating the two camera positions to 
the distance from the camera base to the object plane. 
The technique has also been applied to small 
‘specimens with very small deflections by using a 
pattern of titanium dioxide pigment applied through 
an electron microscope grid with 40 or 80 holes per 
mm. Figure 13 shows the discontinuities in the fringe 
pattern at a welded junction between steels of 
different expansion coefficient, when heated between 
exposures. Displacements of a small fraction of a 
fringe interval (25 pm) are detectable. To improve 
sensitivity even further, the grid frequency may be 
effectively multiplied up to the 3rd and 4th harmonics 
of the pattern by bringing the camera system closer, 
to a distance equal to the appropriate fraction of the 
distance for tuning to the fundamental frequencye 
References 
T Burch J M and Forno C, "A High Sensitivity Moiré 
Grid Technique for Studying Deformation in 
Large Objects" Optical Engineering, 1975, 14, 
No 2, pp 178-185. 
8 Forno C, "A high sensitivity photographic 
technique for studying thermal strain at 
elevated temperatures" NDI International, 1977, 
June, pp 121-124. 
5  WHITE-LIGHT SPECKLE PHOTOGRAPHY 
C Forno (9) 
In some of the work previously outlined(7) a brief 
examination of the possibility of dispensing with a 
specially prepared grid pattern on the object was made. 
Reference to the original papers should be made for an 
account of the first part of the work, but more 
recently some successful experiments have been carried 
out on improved methods of making use of random 
variations of reflectance on the surface of the object, 
in the same way that laser speckle effects have been 
used to locate points on an otherwise featureless 
surface (3,5,6). A similar mask in the camera system 
to that shown in Figures 10 and 11 is employed. The 
surface to be studied is coated with retroreflecting 
paint containing tiny reflective glass spheres which 
act as point sources and give records on the film of 
small grating-like images. 
  
Reconstruction from a double exposure 
negative of a stencilled specimen 
area, 145 x 80 mm, of a large welded 
pipe section, showing differential 
thermal expansion. 
Figure 14 
Deformation caused by à vertical load 
applied to à steel bowl. One fringe 
is equivalent to 140 am displacement, 
approximately.