BACKLIGHTING
For transparent materials, it is possible to avoid applying a base coat and instead backlight the specimen. Apply speckles to
the surface of the material as usual, then backlight the specimen using diffuse light.
When using this technique, it is important to be careful of any feature that is not on the surface of the specimen that may
show up in the image. These will interfere with the ability to accurately measure surface behavior.
BIOLOGICAL MATERIALS
In applications involving wet tissues or other biological-like material, standard speckling techniques may not stick to the
surface well. Staining the material with an ink such as India ink can be an effective method.
Another method that has been used with success is using microbeads that bind to the specimen and provide contrast to
create a speckle pattern.
UNDERWATER
With any specimen that is submerged in water or other fluid, the main challenge in speckling is making sure the pattern will
not degrade over time. Be sure the technique used is sufficiently waterproof when doing these tests.
INHERENT PATTERNS
Some materials such as wood or concrete display an inherent pattern. These patterns may be used for correlation if they
have sufficient contrast, although they will often provide noisy results and an applied pattern is still usually optimal.
PROJECTION OF A PATTERN
For shape measurements, a speckle pattern may be projected onto the surface using a computer and projector. For this
method, room light should be controlled to give high contrast.
Note that because a projected pattern does not stay with a moving surface, this technique is only useful for shape
measurement. Displacements will not be accurately calculated.
TEXTURES
Some specimens exhibit an apparent speckle pattern due to an inherent texture; examples include sand, rough metal, and
concrete. Extreme caution must be used when using these textures as a pattern; because the shading comes only from light
and shadow, it may often be inconsistent between the left and right camera, or may change in unexpected ways when the
specimen moves or deforms. Because of these issues, this technique should only be used when alternatives are not
available.
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