Embossed Holograms V. N. SANKARANARAYANAN Hindusthan Photofilms Mfg. Co. Ltd. Ambattur, Madras-600 058 and S. RAMAKRISHNAN, R. S. SIROHI AND S. K. SESHADRI Indian Institute of Technology, Delhi, Madras-600036 (Received October 24, 1991) ABSTRACT — A method is described to mass-produce embossed holograms on thermo-plastic films. We have started by recording a rainbow hologram on photographic plate, contact printed this on a photoresist film and used thisphotoresist relief image for making a metallic stamper. This stamper was then used to stamp out hologram on PVT films. 1. INTRODUCTION Embossed holograms have found numerous applications in commercial displays and as security devices. In these applications, a white-light- viewable hologram is mechanically printed on a thermoplastic film with the help of a metallic “Stamper” containing the hologram. The stamper itself is produced through electro-forming from are relief hologram recorded on photoresist. The details of the process are given in ref. (1). It is clear, therefore, that the important steps in the production of emboseed holograms are 1) recording of hte holograms on a photoresist film and 2) the stamper making. Even though the first step would appear fairly straightforward, in practice, recording of holograms on positive photoresists encounters some serious problems. Since commercially available photoresists are blue- sensitive, they require exposures using Argon Ion or He-Cd lasers, which emit the suitable wavelengths. At typical power levels of the above two lasers, this needs long exposures which are undesirable for holographic work. An attractive alternative to this is the recording of the original on a fast photographic emulsion and then transferring this image on a photoresist film through contact printing. This procedure has been followed by several workers [2]. The main advantage of this method is the availability of and ability to use powerful incoherent sources like the mercury are without the stringent requirements of vibration isolation etc. If the origianl hologram is an amplitude hologram, the replica which is the photoresist relief image, may even show increased efficiency. The objective of this work has been two-fold. First, the production of a rainbow hologram on a photoresist through contact printing, and second the production of a metal stamper from the photoresist image. The Following stages were involved in this process : Master amplitude hologram on holographic plate Rainbow hologram on holographic plate Copying the rainbow hologram on photoresist Metallisation Stamper making by metal forming Stamping out copies on a thermoplastic