Ü 
( 
A moulded chair was subjected to various loadings in 
order to examine the ways in which it deformed under 
load, to identify any possible discontinuities in 
elastic behaviour due to material properties. A 
typical pattern of deflection, produced by a 100 g 
weight is shown in Figure 5. 
  
Figure 5 Fringe pattern on under-side of plastics 
chair, due to loading with 100 g weight, 
obtained by interference holography. 
References 
3 Archbold E and Ennos A E., "Applications of 
holography and speckle photography to the 
measurement of displacement and strain" 
J Strain Analysis, 1974, 9, No 1, pp 10-16. 
4 Ennos A E and Archbold E., "Coherent optical 
technique for studying polymers" Plastics and 
Rubber: Materials and Applications 1976, 
September, pp 116-120. 
3 LASER SPECKLE PHOTOGRAPHY FOR MEASURING TANGENTIAL 
DISPLACEMENTS OF SURFACE ELEMENTS 
E Archbold, A E Ennos (3,5,6) 
When a diffusely reflecting surface is illuminated with 
coherent light from a laser, and the surface is 
photographed with a conventional camera but 
preferably using ultra-fine grain film) the record is 
overlaid with a speckle pattern that is derived from 
the many randomly-distributed surface elements. 
Although the pattern shows no regularity and appears 
to carry little information in itself, it is quite 
uniquely determined by the microstructure of the 
surface and the geometrical characteristios of the 
recording system, and if the surface moves or is 
distorted, the pattern changes. Two successive 
exposures, made in rather the same way as the double 
exposure in "frozen fringe" holography, will give a 
doubled pattern in regions where small displacements 
in the plane of the surface occur. 
A structural component of a measuring machine was 
found to have developed a crack in a welded joint, 
and a 35 mm camera loaded with Agfa-Gevaert 10E75 
holographic film was used to make a double exposure, 
before and after loading the structure, as shown in 
Figure 6. 
90 
    
  
35mm Camera 
‚Laser 
Illumination 
  
Figure 6 Laser photography in use io study movements 
near a crack in a load-bearing box section 
structure. 
After processing the film, a narrow beam of 
monochromatic light is directed through a point on the 
film corresponding to each point on the structure 
to be examined (Figure 23. 
  
Figure 7 The displacement is measured from a speckle 
pattern derived from the double exposure film. 
The speckle pattern scatters the beam into & cone and 
if there is doubling of the speckles, the brightness 
of the patch of scattered light is modulated into a 
series of bright and dark bands (Figure 8) whose 
angular relation g to the direction of the 
beam is given by 
sin g = nA/d, 
where n is an integer, À is the wavelength of the 
light, and d is the "doubling" separation on the film.