LASER TRANSMISSION HOLOGRAMS We saw in Chapter 1 how the interaction between a reference beam and a beam disturbed by passing through a transparent object generated a stationary interference pattern that could be recorded in the form of a hologram. A laser transmission hologram is made with both reference and object beams incident on the light-sensitive material from the same side, and when scientists talk of holograms, this is the type they usually mean. The object beam may have been reflected from an opaque object or transmitted through a transparent one: there is no essential difference in the nature of the wavefront. Both are examples of diffraction by the object; both contain the entire information about the shape of the object, and this information is encoded in the hologram in the same way. There are numerous ways of providing the two beams, one of the simplest being that shown in Figure 4.1a. In this arrangement, the mean angle between the object and reference beams is some 45°–50°. The spacing of the interference fringes at this angle is about 1.4 times the wavelength, somewhat less than 1 μm (10−6 m). To record so fine a pattern demands a grainless or at least ultrafine-grain recording material. The traditional material has usually been a photographic (silver halide) emulsion.