Reflection holograms and the replication thereof

A method of forming a reflection master hologram from a transmission hologram comprising by the steps of

• (1) dividing the transmission hologram into a first number of parallel notional strips,
• (2) placing a reflection master parallel to the transmission hologram,
• (3) dividing the reflection master into a second number of notional strips parallel to each other and to the notional strips of the transmission hologram,
• (4) arranging a laser to provide an object beam for scanning in turn each notional strip of the transmission hologram and to provide a reference beam,
• (5) arranging a shield or the like between the transmission hologram and the reflection master, the shield being formed with a slit or other beam-narrowing means parallel to said strips so that the object beam transmitted through the transmission hologram impinges upon a band of a notional strip of the reflection master,
• (6) directing the object beam through an optical system comprising a cylindrical lens and a slit or other beam-narrowing means both extending parallel to said strips and located on the opposite side of the reflection master from the transmission hologram, the lens being located at such a distance from the reflection master that a beam of light passing therethrough (in the absence of the beam-narrowing means) would be diverging from the focal point of the lens and of sufficient width to cover the whole of a notional strip in the reference master, the object beam and the reference beam being arranged to meet at an angle in the same band of a notional trip of the reflection master,
• (7) exposing said band by exposure to both said beams with reference to a scanned first notional strip of the transmission hologram,
• (8) altering the positions of both said beam-narrowing means and altering the direction of the object beam to scan the next adjacent notional strip of the transmission hologram,
• (9) exposing the next adjacent band of the same notional strip of the reflection master with reference to said next adjacent strip of the transmission hologram and repeating the process until the first notional strip of the reflection master has been exposed in bands with reference to each notional strip in the transmission hologram, the number of bands in a notional strip of the reflection master being equal to the number of notional strips in the transmission hologram,
• (10) altering the positions of the reflection master and the transmission hologram and repeating the exposure process until each notional strip of the reflection master has been exposed bandwise with reference to each notional strip of the transmission hologram, and
• (11) processing the exposed reflection master to produce the reflection hologram.

A method of preparing a copy of a reflection master hologram, which is divided into a plurality of sequential parallel notional strips extending from one side or end to the other, interference fringes present in said strips being substantially in the form of parts of concentric shells, and such strips of the reflection hologram containing information derived from the whole of the transmission hologram, comprising the steeps of placing photosensitive holographic material in contact with the hologram, placing in contact with the photosensitive holographic material a holographic optical element which comprises a number of notional strips which is a fraction of the number of notional strips in the reflection hologram, each notional strip in the holographic optical element containing light modulating fringes such that when the holographic optical element is placed in the assembly light entering each strip in the holographic optical element is converged to a focal point which is the same distance from the reflection hologram as a converging lens used in its preparation and covers one notional strip in the reflection hologram, subjecting the resulting assembly to a first exposure to laser light to expose a number of notional strips of the reflection hologram moving the holographic optical element relative to the reflection hologram, subjecting the assembly to a subsequent exposure to laser light to expose further notional strips of the reflection hologram, repeating the process until all said notional strips have been exposed, the number of said exposures being integral and being the reciprocal of said fraction, and processing the photosensitive material to form the copy hologram.