Holographic interferometry and recording media
Recording media
Materials used to record holograms are much less sensitive than those used for conventional photography, but the resolution is much higher. Silver halide media such as Agfa 8E75 (red sensitive) require an exposure energy of 0.015 mJ/cm2. Ilford film has a similar sensitivity. These media require development and further processing using wet chemicals. They are more sensitive than photopolymers, facilitating use of split beam configurations.
Photopolymer materials such as those provided by Litiholo are considerably less sensitive: they require an exposure energy of 20 to 30 mJ/cm2. These plates are sensitive to red, green, and blue. No development process is needed. Any remaining light sensitivity may be bleached out by illuminating the plate with white light following laser exposure.
To avoid long exposure times, it is expedient to make the holograms using the Denisyuk single beam reflection method, with the object close to the holographic plate. Such holograms may be viewed in laser light of the original wavelength or in white light from sunshine or from a spotlight.
Holographic interferometry
The deformation field can be revealed by making a double exposure hologram, one exposure with the undeformed object and one exposure with the deformed object. The fringes that appear on the final hologram are due to interference of light from the two versions of the object. Interpretation of fringes may be done via the static zero order fringe method or the dynamic fringe counting method as outlined here. The hologram retains a 3D image of the object itself as well as the fringes which represent 3D deformation. The object itself may be removed. The fringes are frozen fringes in contrast to the live fringes discussed below.
If one does a bending experiment, keep in mind the holographic method is sensitive to all components of displacement. Consider a horizontal cantilever beam or bar bent by a downward force at one end. The largest component of displacement is downward. It can be measured using the dynamic fringe counting method in which the observer moves the observation point vertically. The observer then considers a particular point on the bar and counts the fringes that pass by during a sweep of known angle
In bending, one may also investigate the deformation along the length of the bar. This deformation is directly related to the bending strain. Observe the hologram in a direction for which the fringes become very sparse and disappear. Consider the direction orthogonal to the bar in the horizontal direction. Consider the angle of the illuminating laser beam with respect to the orthogonal direction. The appropriate observation direction will be an equal angle from the normal in the opposite direction. Now move the observation point horizontally a known angle. Fringes will appear. Consider their meaning in the context of the interpretation equations.
Live fringes
If one makes a single exposure hologram and views the object through that hologram, fringes can be studied in real time as the object is deformed. If silver halide media are used, the plate is developed in a tank with transparent sides to allow viewing of the hologram without moving it. If photopolymer is used, leave the hologram plate as it was held during exposure. Do not bump or disturb the plate because even a tiny motion will generate fringes.
Photography of holograms
Images taken with a cell phone camera will look grainy. The reason is laser speckle which is due to interference of light from asperities on any object with a rough surface. The speckle is more noticeable if the camera pupil admits a small bundle of rays from the object. Cell phone cameras have a much smaller lens than a dedicated camera, so laser speckle is prominent. Furthermore, contrast can be high in holographic images, especially if the hologram's illumination is much brighter than the room lights. The high contrast can saturate the image in the region of the hologram. If zoom capability is available in your camera, try zooming in so the bright region fills the frame. If you use a camera with auto focus, the camera may not focus properly because the holographic image occupies a range of depth. Use manual focus.
A photograph of the hologram allows interpretation by the static zero order fringe method but not the dynamic method. The angle of the camera lens with respect to the object surface is needed; also the angle of the laser illumination used to make the hologram.