So far, we have treated a hologram recorded on a photographic film as equivalent, to a first approximation, to a grating of negligible thickness with a spatially varying transmittance. However, if the thickness of the recording medium is larger than the average spacing of the fringes, volume effects cannot be neglected. It is even possible, as mentioned in Section 1.7, to produce holograms in which the interference pattern that is recorded consists of planes running almost parallel to the surface of the recording material; such holograms reconstruct an image in reflected light.

In addition, with modified processing techniques, or with other recording materials, it is possible to reproduce the variations in the intensity in the interference pattern produced by the object and reference beams as variations in the refractive index, or the thickness, of the hologram. Accordingly, holograms recorded in a medium whose thickness is much less than the spacing of the interference fringes (thin holograms) can be classified as amplitude holograms and phase holograms.

Similarly, holograms recorded in thick media (volume holograms) can be subdivided into transmission amplitude holograms, transmission phase holograms, reflection amplitude holograms and reflection phase holograms.

In the next few sections we review the characteristics of these six types of holograms. For simplicity, we consider only gratings produced by the interference of two plane wavefronts.