Bleaching the Emulsion
The amplitude of the waves corresponds to the contrast between the fringes. The wavelength of the waves translates to the shape of each fringe. Both the spatial coherence and the contrast are a direct result of the laser beam's reflection off of the object.
Turning these fringes back into images requires light. The trouble is that all the tiny, overlapping interference fringes can make the hologram so dark that it absorbs most of the light, letting very little pass through for image reconstruction. For this reason, processing holographic emulsion often requires bleaching using a bleach bath. Another alternative is to use a light-sensitive substance other than silver halide, such as dichromated gelatin, to record the interference fringes.
Once a hologram is bleached, it is clear instead of dark. Its interference fringes still exist, but they have a different index of refraction rather than a darker color. The index of refraction is the difference between how fast light travels through a medium and how fast it travels through a vacuum. For example, the speed of a wave of light can change as it travels through air, water, glass, different gasses and different types of film. Sometimes, this produces visible distortions, like the apparent bending of a spoon placed in a half-full glass of water. Differences in the index of refraction also cause rainbows on soap bubbles and on oil stains in parking lots. In a bleached hologram, variations in the index of refraction change how the light waves travel through and reflect off of the interference fringes.
These fringes are like a code. It takes your eyes, your brain and the right kind of light to decode them into an image. We'll look at how this happens in the next section.