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DNP enters HOE market with photopolymer.

During his presentation at the Holo-Pack * Holo-Print[R] 2010 conference in November, Dai Nippon Printing's (DNP) Kenji Ueda focused his remarks on DNP's development work on the use of photopolymer holograms for document protection, notably bank notes. However, he also mentioned that it was DNP's intention to use the same photopolymer for HOEs. Details of this work can now be revealed. It appears that DNP is using its photopolymer volume hologram film with its superior mass production attributes to develop holographic optical elements for the reduction of laser speckle in video and data projectors.

Lasers are a favoured light source for projectors, delivering superior image sharpness, colour and contrast compared to the more common LED or mercury lamp projectors, thanks largely to the beam control and specificity of the output colour. Mercury is also an environmentally dubious element to use. But one issue to overcome in laser projectors is that laser speckle causes scintillation on the display surface. Holographers are very familiar with the phenomenon because they see it all the time in their recording studios whenever laser light is reflected off objects.

The light scattered on the light source unit and screen that serves as the irradiated surface interferes with each other, and is prone to generate speckle noise, which appears as a fine mottled splashing, which is in turn the cause of scintillation. DNP has used volume hologram film photopolymer to develop a technology for reducing this speckle noise.

HOE Replaces Spinning Diffuser

Speckle noise is generated by the light source unit and the screen. The conventional solution is to insert a revolving diffuser panel into the light path and by projecting averaged interference patterns onto the screen, it has been possible to reduce the speckle noise originating in the light source unit. With speckle noise generated on the screen, however, it has been possible to reduce this by rotating the screen, but spinning big-screens is impractical.

So with currently available technology it has only been possible to reduce speckle noise generated by the light source unit, but with this newly developed technology it has become possible to effectively reduce speckle noise generated on the screen, and as a result, lasers can be used with a variety of devices including rear-projection and front-projection displays.

Multi-Function HOE for Projectors

The new technology developed by DNP utilizes the features of volume holograms that reproduce the same image in the same position no matter which point on the hologram the light beam is incident on. By irradiating the laser beam on various points of the hologram with an optical scanning device, such as a scan-mirror, the angle of incidence on the images reproduced on the screen constantly changes, and generates numerous interference patterns. As a result, the interference patterns are averaged, and it is possible to reduce speckle noise. Also, as the hologram can shape and homogenize the laser beam, optical components such as a diffuser or a lens array are no longer necessary. As it is possible to obtain a speckle noise reduction effect, regardless of transverse mode or coherent length of laser, it is possible to apply this technology to a variety of lasers.

As the technology can be freely designed to suit the reproduced image configurations for various applications, it can be applied not only to displays and projectors, but to sensors, such as 3D scanners that can capture clear configurations with reduced speckle noise, and image devices.

The physics behind this development was described in a recent paper by Kurashige and Ishida et al. titled 'Despeckling method with variable speckle generator utilizing photopolymer film' (Proceedings of IDW 10--International Display Workshop -p. 1471-1474 (2010)). This paper contains the recording set up for the master diffuser, as shown in the diagram.

The paper cites several previous references to de-speckling methods, including a 1994 Texas Instruments patent (US5313479) for a spinning diffusion element and a 3M patent (US6466368) for a diffuser laminated to a back projection screen. DNP's HOE method appears to be easier to implement and probably lower in cost. The company is now approaching projector manufacturers and hopes to start shipping samples of its HOE scanning VPH screen from early 2011.
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Publication:Holography News
Date:Dec 1, 2010
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