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Converging beams for distortion-free imagery in transfer holograms

In order to obtain a distortion-free image when a transfer hologram is replayed by a diverging beam, the original reference beam must be the precise conjugate of the replay beam, i.e., it must converge towards the future location of the replay source. Off-the-shelf collimating mirrors have too long a focal length to achieve this, and suitable custom-built mirrors are heavy and prohibitively expensive. Several methods are suggested for obtaining a suitable beam, including a new type of varifocal pellicular mirror made by stretching a metallized plastic membrane over a circular drumhead and creating a partial vacuum behind it, producing a concave mirror of high optical quality with adjustable focal length.