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System provides good images of small objects

A Heidenhain (GB) product story
Edited by the Engineeringtalk editorial team Nov 12, 2007

The camera develops a large depth of field as optical magnification is minimal, meaning that 3D object images are 'sharp' on all surfaces.

SmartDrive has been helping to make museum collections available digitally.

Dennis Murphy, SmartDrive's Managing Director, said "Typically the majority of museums can only display around 10% of their collections due to lack of space, as an alternative photographs and books are usually published but by definition only reach a limited audience".

"If artefacts could made available as high-resolution digital images they would reflect a moment captured in time and if the original is lost or damaged could be used as good representation or substitute".

"Also digital images have the huge advantage of being more portable, providing almost instant access for schools, colleges and academics alike".

One of the problems of traditional photography, particularly when dealing with small objects, is that high magnification creates a small depth of field.

To solve this problem the SatScan, which uses standard, high resolution lenses, was used along with the SD Corrstitch correlation image stitching software, to create images of almost unlimited resolution and size.

Atypical manageable image would be 23,000 by 23,000 pixels in 24 bit colour.

The camera is mounted vertically and tracks across the object, taking numerous images which are then mapped and stitched together to create one large image.

Using this system the camera develops a large depth of field as optical magnification is minimal, meaning that 3D object images are 'sharp' on all surfaces and the problem of parallax is eliminated, as every part of the object being scanned is being viewed from the same angle.

A further advantage of this approach is that the resulting image has uniform scale, making research work based on the images more valid.

The problem faced by SmartDrive was one of achieving highly accurate positioning to enable the SD Corrstitch to work effectively.

"The initial thought was to construct a highly rigid frame on which to mount the camera and to position the camera in X and Y using high-precision ballscrews", Murphy said.

"The problem with this solution was both cost and scale-ability, as we have already had enquiries for scanners measuring 4 x 4m, ballscrews are simply not an option".

The eventual solution proved to be a relatively low-cost one, using a simple modular framework on which to mount the camera and stepper motors and belt drives to position it.

This gave an initial positioning accuracy of +/- 100 microns, which was overcome by the use of Heidenhain linear encoders on the X and Y axes to give a true reading of the camera carriage position to within a micron.

"While the obvious solution was to use highly precise and very expensive mechanical components to construct SatScan, the use of Heidenhain linear encoders means that we can reduce the manufacturing cost of each system by choosing relatively inaccurate mechanical components and then compensate for this by implementing the precision encoders to deliver a more accurate, yet cost-effective, scanning solution" Murphy said.

"Now we simply position the camera where we think we want it to be and take an X and Y co-ordinate from the encoders to provide a 'base point'".

"An image is then taken and this, along with the co-ordinates, is stored in memory".

"The camera is then moved to the next position and the process repeated until the entire object has been scanned".

"Once these images are stored the stitching software examines the crossover of each image and processes the overlap, merging the individual images into a single photograph".

"We have worked with the Natural History Museum in developing this system and in one meeting we were asked to scan a fossil-bearing rock".

"The resulting photograph had such detail that they actually found another fossil in the rock, which the naked eye had missed completely".

"Once a collection is imaged using the system anybody can have access to a particular object".

"Schools, for example, could have DVDs that show the 'real fossil bones' of a dinosaur rather than graphical representations".

"The options are endless".

SmartDrive's next development step with SatScan is to create a vertical scanner for use in the restoration of expensive works of art.

This will be equipped with infra-red and visible spectrum cameras as well as laser to measure the depth of paint on the canvas.

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