HOLOGRAPHIC INTERFEROMETRY

Holographic Interferometry:

Remember that when trying to obtain high fringe visibility, the (Reference beam/Object beam) ratio should be reduced to 1~2.

There are three basic types of holographic interferometry:

• Live Fringes - Single exposure - Holographic image must be located exactly over object
• Double Exposure - Fixed fringes for given load - Object no longer necessary, hologram can be moved
• Vibrations (Periodic) - Single exposure with live fringes - can display modal patterns. Note that being able to get holograms of vibrating objects does not violate the stability requirement of holography; the vibrating object stops at each extremum of its path, and that is when the holographic fringe is formed.

Basic Problems

• Sensitivity Vector g : here g is vector bisecting illumination and viewing directions. There is a special difficulty called the "sign problem" associated with g. It is hard to determine the sign of g; is it going in or out? What measurement is obtained after an object displacement d ? g·d, which is a scalar value, so need three different holograms to get full 3-dimensional information.
• Sign problem - solved by introducing carrier fringes or using phase-stepping. This can be viewed as introducing a known phase change in either space or time.
• Localization of Fringes - parallex problem; fringes can be localized on image by using lenses to image object onto holographic film.

Reference:

Vest, C., Holographic Interferometry, Wiley, 1979. A classic work, still widely used.

Send Mail or Comments: matthysd@vms.csd.mu.edu

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