TECHNICAL
Photothermoplastics
Thermoplastic is a material for producing surface relief thin phase holograms. The thermoplastic material repeatedly softens and hardens when heated and cooled. The material has a multilayer structure on a substrate of glass or film. It consists of three thin layers: doped tin or indium oxide (a transparent conductor), polyvinyl carbazole sensitized with trinitro-9-fluorenone (photoconductive organic polymer) and staybelite Ester 10 (thermoplastic substance, a resin). The material has high sensitivity over the whole visible spectrum.
The thermoplastic resin is first positively charged in the dark by a corona discharge device which uniformly moves over the thermoplastic plate at a constant distance. A uniform negative charge is thus induced on the photoconductive coating on the surface. The plate is then exposed to interference pattern which periodically alters the conductivity of the conductive layer. Electrons travel through the conductive layer and are attracted to the positively charged plate but cannot pass through the photo conductor.
The electrostatic field is further increased by recharging the surface (after blocking the light) once again by the corona discharge as in the first step. The charge pattern creates a spatially periodic static electric field of 1-10V/sqcm. The film is developed by passing a current through the conductive layer, heating the plate and softening the thermoplastic film. The softened film deforms under the static electric field, becoming thicker in the unexposed areas and thinner at the illuminated areas (where the field is higher). As the film cools to the room temperature, the thickness variation gets frozen in.
The complete charging, exposing and developing cycle takes less than a minute. The plate can be erased by flooding it with light and passing a current pulse through the conductive layer which heats up the thermoplastic layer and resoftens it. Before the next exposure, the plate is blasted with compressed air or dry nitrogen to cool the thermoplastic layer to the room temperature.
The material is optically inert when not charged, so there is no degradation from exposure to heat and actinic radiation. The stability of the developed thermoplastic hologram is excellent. The main drawback of the material is the requirement of a complex apparatus for controlled charging and development. The material is ideal for holographic nondestructive testing of materials.
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