1. Field of the Invention
This invention relates to a method of reproducing a reflecting type hologram used as an optical element.
2. Description of Related Art
Holograms are regarded as optical elements, such as lenses and prisms, which attributes to their producing of diffracted waves according to patterns of diffraction gratings. Holographic sinusoidal gratings and holographic lenses are well known for holographic optical elements (HOE). For example, in examining or measuring of spherical surfaces and aspherical surfaces by an interference method, a holographic optical element is used in place of a conventional interference standard to generate the same reconstructed wave front as a surface having an ideal surface of test object. Such a holographic optical element is produced by the use of a what is called a computer generated hologram (CGH). For a detailed description of a representative example of such a "holographic type" interferometer, reference may be had to "Precision Engineering" Vol. 56, No. 4, page 745, 1980, published by Precision Engineering Society.
Highly precise aspherical surfaces, such as cylindrical surfaces, as interference standards, are hardly formed by the grinding and polishing optical glass blocks. Rather, a computer generated hologram enables production of a holographic optical element with a high accuracy for reconstructing the wave front of an ideal aspherical surface.
However, since holographic optical elements need to be large in size and since computer generated holograms are made by computer aided electron beam pattern drawing or forming, such a holographic optical element produced with the use of a computer generated master hologram is too expensive to be utilized in examining and measuring instruments, such as interferometers, practically available to a manufacturing line of optical elements and/or optical instruments.
For the purpose of providing a brief background of conventional hologram reproduction that will enhance an understanding of the method of reproducing a hologram of the present invention, reference is made to FIG. 1. Before reproducing reflecting type holograms, prepared are a master hologram plate 51, which has a desired positive pattern of diffraction grating 55 formed on a transparent base plate 53, such as an optical glass plate, and a photoresist plate 61 which has a photoresist thin layer 65 formed or coated on one surface of a transparent substrate 63. After placing the master hologram plate 51 on top of the photoresist plate 61, the photoresist layer 65 of the photoresist plate 61 is exposed to collimated light h.sub.v. Then, after the exposed photoresist plate 61 has been developed, it is immersed in an acid bath so as to remove an unexposed pattern, i.e. a negative pattern of diffraction grating 55, of the photoresist layer 65 and to chemically etch the negative pattern of the transparent substrate 63. Finally, the remaining portions of the photoresist layer 65, which are identical with the positive pattern of diffraction grating 55, are removed from the transparent substrate 63. As a result, a grating identical in pattern with the diffraction grating 55 of the master hologram plate 51 is reproduced on the transparent substrate 63.
This reflecting type hologram reproducing method is quite popular and efficient. However, reproduced holograms are poor in quality and precision for optical elements. This is because, the holograms include optical noise attributive to optical non-uniformity of the transparent plate 53.