Spinning Mirror 'camera'

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Spinning Mirror 'camera'

Postby Andrew Davie » Wed Feb 07, 2007 10:33 pm

I quote from http://www.omega.com/literature/transac ... scan2.html

"Detectors Options
The earliest thermal imaging systems featured a single detector and a spinning mirror that scanned the image coming through the lens of the camera and focused the pixels of the two-dimensional image on the detector in sequence. The electronics that captures data is synchronized with the mirror so that no thermal information is lost or garbled. One of the problems with the single detector approach is dwell time. Scanning a 120 x 120 pixel image with a spinning mirror does not give any single pixel very much time to register a reading on the detector. "

This seems almost like a reverse spot-scanner type of camera. The image is scanned onto a detector, rather than a detector responding to a light source scanned over the subject. Has anyone played with spinning mirror-type cameras, as described above?

I'd be very interested if anyone can explain the mechanism via which a 120 x 120 image is scanned by a spinning mirror onto a single-pixel detector.

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Postby Stephen » Mon Feb 12, 2007 8:02 am

Since no one has yet replied on this subject, I just thought that I would mention that several NBTVA members have successfully used the Weiller mirror drum for camera and display purposes. As you suggested, each of 32 mirrors has a slightly different cant to move a different line segment of the picture across a stationary aperture, behind which is an appropriate photosensor, thereby scanning the entire picture with a revolution of the drum. Peter Yanczer has an excellent description of the Weiller drum on his website at http://www.televisionexperimenters.com/mirrdrum.html .

A mirror type of optical scanning element for display purposes is advantageous because a single point light source that replaces the photosensor in the camera has its entire light output transferred to the display screen. There is no wasted light. The disadvantage to the Weiller drum is that it requires a separate mirror for each scanning line. This results in a large heavy optical scanning element that has 32 adjustments to make and maintain in alignment. John Logie Baird described a nicer mirror type of optical scanning element in his British Patent GB374,564 filed on 9 April 1931.

In one of his arrangements he describes a mirror unit that has several canted mirrors mounted on a revolving pyramidal support element that direct light from a modulated light source onto several canted stationary mirrors that in turn direct the light onto the screen. The number of scanning lines comprise the number of rotating mirrors multiplied by the number of stationary mirrors. For instance, you could use four mirrors mounted on a four sided rotating pyramidal support in combination with eight stationary mirrors.

With this arrangement, there are only twelve mirrors total to align and adjust. The rotating element has only four mirrors, so there is very little moving mass. Ernest Traub came up with a similar but more complex arrangement a few years later and I believe that it may have been put into production. See the Yancer site at http://www.televisionexperimenters.com/mhalytrb.html .

See Baird's patent at http://www.taswegian.com/NBTV/images/GB374564A.pdf
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Postby Panrock » Mon Feb 26, 2007 9:53 am

I built a Mihaly-Traub camera and similar monitor many years ago, using a modulated laser in the monitor. I had serious problems with raster distortion when using a central rotating polygon and reverted to a simple rotating plane mirror. I don't know how well the original M-H units can have worked.

All that remains of this apparatus today is one of the mirror arrays less its central mirror, in poor condition. Picture attached.

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Traub scanner.

Postby Stephen » Mon Feb 26, 2007 10:46 am

That is pretty impressive, Steve. Did you design your scanners based on just pictures of the original or the patent? I have posted Mr. Traub's patent for the polygonal mirror unit at http://www.taswegian.com/NBTV/images/GB425552A.pdf for reference.
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Postby Panrock » Mon Feb 26, 2007 7:53 pm

No Stephen, I didn't see the patent at the time (before the days of the internet) - but it would have been most useful.

A problem I encountered was that the path distance between the polygon and the mirror array effectively varied as the polygon rotated and the beam swept across it. Maybe an elliptical array could have compensated for this. Also, using the lenses at my disposal, vignetting by the mirror elements was another problem; that's why I used a laser.

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Traub scanner.

Postby Stephen » Tue Feb 27, 2007 4:38 am

Yes, Mr. Traub in fact indicates on page 3, column 1, lines 6 through 9 of his patent that arranging the fixed mirrors along a hyperbola is preferable, probably for the reason that you mention.
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