Holography is most frequently thought of as a method of photography that results in three-dimensional images of the object being photographed. It is certainly true that this is the most visually spectacular aspect. But holography can also be used as a powerful tool for the investigation of a variety of photochemical and photophysical processes. These experimental techniques rely on the fact that small spatial modulations of a material's optical properties (index of refraction and absorption coefficient) can deflect an incident light beam into another direction. By following the growth or decay in intensity of this deflected beam, one can follow the underlying photochemical and photophysical processes producing the changes in optical properties. If a CW laser is used to produce the hologram one can use the technique to investigate solid state photochemistry. If a pulsed laser is used one can investigate a broad range of time dependent processes; energy transfer, diffusion, rotational relaxation, charge transport etc. Compared to conventional spectroscopic techniques the holographic method shows various advantages. So for example it is a highly sensitive zero-background technique and permits free choice of detection wavelength and detection beam intensity.
As a result of information obtained using the holographic technique as a scientific tool, one can also find new classes of materials for the recording of holograms. This is the way in which two-photon four-level systems for hologram recording process gated on and off with an auxiliary source, and can be read with the infrared recording laser with no erasing of the hologram.