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Coherent diffractive imaging from OpSec.

At the Optical Document Security Conference (see page 4), OpSec presented details and offered samples of its new Coherent Diffractive Imaging (CDI) process, the latest development in the use of diffractive optical effects on CDs, which was first introduced under the trade name 3dcd in 1998 (see HN Vol 9, No 6). 3dcd is a joint venture between OpSec and Technicolor, and Microsoft has rights on the use of 3dcd on software program disks. The company worked with OpSec on the development of CDI and will be using it for new versions of its programs.

The most noticeable characteristic of CDI is the very bright, colourful images that can be seen on a CDI CD, even in what would usually be thought of as adverse lighting for OVDs--and as can be seen in the photo of a floating Microsoft Windows logo, which also shows that the separation of planes in a CDI image is readily captured photographically. This brightness and sharpness is because CDI in effect is making Hoxel (holographic pixel) lenses, which focus the incident light, so that even illumination with a fluorescent tube gives sharp images.


The key to CDI is the use of a Laser Beam Recorder (LBR), equipment that was developed for recording high definition or standard definition digital data. An LBR writes data by modulating a finely focused laser beam and exposing the beam onto a photo-resist coated (in most cases) on to a round glass plate, which is mounted on a turntable and spinning at approximately 54 Hz, in a spiral fashion. The beam is focused to around 400 nm width for writing DVDs and 150 nm for writing high definition data. OpSec states that it pushes LBR's very high level of optical and mechanical engineering to new limits.

OpSec modified a standard LBR to improve the fringe-writing ability by tightening the track pitch from the usual DVD specification of 740 nm down to between 74 to 148 nm. The company is now achieving 17 nm accuracy in the middle of the disc and 34 nm accuracy at 120 mm diameter, which it believes can be improved. An indication of the complexity of these requirements and finding the best design and control solutions is that OpSec started this development in 2006--a three-year process, because it aimed to use this system to create distinctly different optical effects.

The aim was to have any resolution required, with a wide range of spatial frequencies and many Hoxel (holographic pixel) shapes, so it could offer customers design effects not available with other optical methods. OpSec started with simple round diffractive lenses with a wide range of focal lengths, which were then combined with more conventional diffractive effects to produce a wide combination of diffractive imagery. They then developed a masking system to allow the lenses to be any shape. Combining these lenses with 2D grating effects gives images on which the foreground clearly stands out from the background, even when photographed.

Effects which have been developed using CDI include negative and positive lenses (aiding identification), animated and 3-D stereograms with brightly collared controlled effects, embedded micro and/or nano 2D barcodes, and laser-read imagery. The stereograms are 'frameless', so an image can be made, for example, to rotate, and not reside within the usual constraints of a frame. Barcodes can be read under a microscope, photographing the image and scanning from a display screen.

One objective of the development team was to make the design of CDI imagery and master artwork relatively simple and user-friendly, which team leader Nigel Abraham believes they have achieved through the use and development of streamPlot and stream Preview software. He concludes that CDI is fourth generation diffractive technology, having the advantages of e-beam combined with the flexibility of laser based exposure systems.

Microsoft seems to believe that the technique offers enhanced security and visual impact and will be using it for future software releases on CD-ROM. Contact: www. OpsecSecurity. com

Caption: Photo of a CDI of Microsoft Windows' logo.
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Title Annotation:Technology News
Publication:Holography News
Date:Feb 1, 2010
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