5 - Optical systems and light sources

Summary

A typical optical system for recording transmission holograms of a diffusely reflecting object is shown in fig. 5.1; one for recording a reflection hologram is shown in fig. 5.2.

A simpler system for making reflection holograms is shown in fig. 5.3. This arrangement is essentially the same as that described originally by Denisyuk [1965] in which, instead of using separate object and reference beams, the portion of the reference beam transmitted by the photographic plate is used to illuminate the object. It gives good results with specular reflecting objects and with a recording medium, such as dichromated gelatin, which scatters very little light.

Making a hologram involves recording a two-beam interference pattern. The principal factors that must be taken into account in a practical setup to obtain good results are discussed in the next few sections.

Stability requirements

Any change in the phase difference between the two beams during the exposure will result in a movement of the fringes and reduce modulation in the hologram [Neumann, 1968].

In some situations, the effects of object movement can be minimized by means of an optical system in which the reference beam is reflected from a mirror mounted on the object [Mottier, 1969]. Alternatively, if the consequent loss in resolution can be tolerated, a portion of the laser beam can be focused to a spot on the object, producing a diffuse reference beam [Waters, 1972].