144 7. OPTICAL TECHNIQUES
Table 7.1: Features of strain gauge and optical measurements
Strain Gauge Measurements Optical Measurements
Moderate equipment cost, high per-
measurement cost
Higher cost equipment/software, moderate
per-measurement cost
Signifi cant preparation and measurement time Preparation and measurement time can be short
Small number of very accurate and reliable
measurements
Large number of moderately accurate
measurements that can be averaged
Stress calculations are relatively compact Stress calculations often quite large
Modest capabilities for data averaging and
self-consistency checking
Extensive capabilities for data averaging and
self-consistency checking
Relatively rugged, suitable for fi eld use Less rugged, more suited to lab use
Sensitive to hole-eccentricity errors Hole center can be identifi ed accurately
7.2 HOLOGRAPHIC INTERFEROMETRY
e concept of holography was introduced by Dennis Gabor in 1947, for which development
he was recognized by the Nobel Prize in 1971. e process involves illuminating an object by
coherent monochromatic light in an interferometer. Light scattered by the object forms an in-
terference pattern when mixed with reference light from the same light source. e pattern can
be recorded on very fine grain photographic plate. e fine grain of the photographic plate allows
the interference pattern to be recorded on a wavelength scale such that both phase and ampli-
tude data are recorded. When the resulting photographic image is subsequently illuminated by
a reference light, a virtual image of the original object can be observed. is is the basis for the
process used to create the holographic images that are in common use in credit cards, security
seals and novelty items.
In the classical technique, the “recording medium” is a photographic plate. is works
effectively, but is inconvenient experimentally because of the time and effort required to develop
the plate. A practical alternative approach is to use a thermoplastic hologram plate instead. is
plate does not require chemical treatment and is “developed” in place by electrostatic means. In
addition, after the contained fringe pattern has been recorded, the plate can easily be erased and
reused for subsequent measurements.
Figure 7.1 illustrates the holographic technique used for hole-drilling measurements. It
is a variant method that makes a hologram by exposing a thermoplastic recording plate to ref-
erence and scattered object light and then developing the plate. After hole-drilling the plate is
exposed again to the same light sources. e hologram then displays a pattern of live light and
dark “fringes” that resemble a contour map. Figure 7.2 illustrates an example holographic fringe
pattern. e fringes indicate lines of constant surface displacement between holographic expo-