1. Field Of The Invention
This invention relates to holographic optical elements and, more particularly, achromatic holographic optical elements and to a method of making the same.
2. Background Of The Invention
The term "hologram" is used to describe a photographic process that is becoming increasingly commonly used. Unlike ordinary photography, which consists of a recording of a three-dimensional scene as a two-dimensional image, the holographic process does not record an image of the object being photographed but, instead, records the reflected light waves themselves as they form an interference light pattern with a reference beam originating from the same source. Thus, the hologram contains all the information characterizing an object through which the light waves have passed or from which they have been reflected or scattered. If the object is an optical element such as a lens, matched filter, or diffraction grating, a hologram thereof will produce all the phenomena and attributes of that optical element; representative characteristics of holographic optical elements are focal point, scale size, and deflection angle respectively.
Conventionally, the hologram comprising the holographic optical element is made by using a laser of a particular wavelength, termed the construction (C) wavelength; and the playback (PB), or reconstruction wavelength of the holographic element is accomplished using a laser of the same or other wavelength. In conventional holography, if a laser of a different wavelength is used for the playback, the essential character of the holographic optical element, including deflection angle, focal point, and scale size, will change.