This page dicusses the theory behind the stereo scanning camera, for construction details or for directions on its usage follow these links.A sheet fed scanner works by reading the pixels of a line sensor, producing one row of the picture. A motor then advances the paper one line and the sensor reads the next row. With the sensor vertically in the back of a camera and the motor spinning the camera around, the sensor scans columns of a panoramic image. For stereo, use the top half of the sensor for the left image and the bottom half for the right.
To create the left and right displacement, use mirrors in periscope arrangements (much like in a submarine) to give two separated views of a scene. By placing mirrors at 45 degrees in front of the lenses facing outward and then placing other mirrors at 45 degrees facing forward the lenses will view the scene from a different point from where the lenses are positioned. Having a set of mirrors on the left for the bottom lens and right for the top lens will give a left and right separation to the up and down lenses. Finally, by rotating the mirrors slightly anti clockwise will cause the horizons of the top and bottom lenses to match up.
The motor needs to rotate the camera. To do this, the output shaft of a gearbox rotates a gear which is engaged in a gear on the main camera shaft. When the motor moves, the gearbox pushes itself and the camera around the stationary shaft.
To be able to use the scanning software that comes with the scanner the gear ratio between the motor and the camera main shaft has to be chosen so that one revolution of the camera corresponds to a 'scanlength' just less than the maximum length the software can be set to.
The maximum scanlength for the PageOffice IIC scanner is 24 inches. The gearbox and gears available at the time of camera contruction only allowed one usable ratio because the gearbox provides 4:1, 16:1, 64:1 or 256:1 and the final ratio available was 64:12. It was experimentally determined that using 16:1 * 64:12, a scanlength setting of 13.5 inches corresponds to 360 degrees. When the motor is started, the camera shudders, making the first part of the scan unsable. In normal use the scanlength is set to 14.5 inches so that the defective scan start can be cropped.
The maximum scanwidth of 8.2 inches in the original scanner corresponds to the full length of the line sensor, 21 mm, which in the ScanCam corresponds to the height of both images. Simple trigonometry shows that the vertical field of view for each image
a = 2*arctan( h/(2*f))
where h is image height and f the lens focal length.
With h = 10.5 mm, and f = 21 mm, a = 28 degrees.
So vertically 8.2 inches corresponds to 28 degrees and horizontally 13.5 inches corresponds to 360 degrees. It follows that the images produced will appear vertically stretched by a factor of 7.8. To compensate, vertical compression by 0.13 is expected, but experiments show this to be 0.18. This error must result from inaccuracies in measuring the lens focal length and sensor length.
