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| Half HD Reflecting Stereoscope (aka Mirror Stereoscope) for viewing side by side pairs on computer monitor. Mirror box widens inter-ocular distance to 10". Carl Pisaturo, 2012 |
My recently completed Half-HD reflecting stereoscope (above) is a 3D viewer which bears an obvious resemblance to its 19th century cousin, the Holmes stereoscope (right).
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The Holmes uses magnifying lenses to allow the source material, actual photographic stereo pairs about the size of a postcard, to be viewed from a close distance, thus giving a good field of view. This compact approach works well with high resolution material like a good photograph - equivalent to perhaps 300-500 PPI (pixels per inch) .
The Holmes approach starts to break down if the quality of the source material drops lower than around 300 PPI. If you've used a Holmes stereoscope or similar device to view reproductions of old stereocards found in many books, you know what I'm talking about: the magnification makes half-tone dots visible (exaggerated example at right). It starts looking more like modern art than a smooth photographic image.
And if some sort of LCD display (e.g a smartphone or computer monitor) were used as the source material for a Holmes viewer, the magnification probably will make the RGB pixels visible (exaggerated example at right). Not good!
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I say probably because as of 2012, such displays are well below the requisite 300 PPI, but this may change in the next several years.
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As one fast forwards into the digital photography age and thinks about the fundamental issues of building a quality digital stereoscope, 2 key principles are obvious...
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The first is image density, essentially how many PPI (pixels per inch) can be placed on a flat surface. Higher PPI media, like slides, photographic prints and "super-displays-of-the-future", allow physically smaller stereoscopes. Conversely, lower PPI media, like the typical HD computer monitor, necessitates physically larger stereoscopes because the eyes need to be farther away to see a smooth image.
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The second key principle is image quality: the total number of pixels in an image. This is obviously key to the viewing experience: the more pixels the better. 1 megapixel per image is in the "pretty good" range, and 10 megapixels per image is in the "amazing" range.
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An HD computer monitor is the obvious candidate for a digital stereoscope as of 2012 because of its low cost and high (1920 x 1080 pixel) pixel count. This allows a pair of 1 megapixel images (960 x 1080 pixels each). That's pretty good image quality. The downside is that such a 22" HD monitor has low-ish image density (100 PPI), necessitating about 20" of distance from the screen for the pixels to melt into smooth imagery.
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The key challenge in making such a viewer arises from the fact that human eyes are 2.5" apart and the monitor's images are 10" apart. Unfortunately, we cannot "spread out" our vision beyond parallel. That's why a mirror arrangement is required. The mirrors make it as if our eyes were 10" apart, and that allows 3D perception of the large images on the monitor.
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For good optical performance, typical mirrors can't be used because the path would be distorted passing through the supporting glass. Instead, "front surface" mirrors are employed. These have a very accurate reflective surface on the used side. CNC machined slots in the polycarbonate holder plates ensure good geometric accuracy of the mirror system and its placement relative to the monitor.
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Compared to 3D TV
The advent of reasonably priced 3D TVs has dramatically shifted the digital stereo photography viewing landscape, rendering mirror stereoscopes niche items which offer slightly higher image quality.
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The above reflective stereoscope has effectively the same pixel count as a passive 3D TV, but since it has zero ghosting and requires the viewer to be properly aligned, it presents superior image geometric quality. The lack of polarizing filters or shutter lenses also gives the reflective stereoscope superior brightness and sharpness. Furthermore, as higher resolution monitors become affordable, the reflective stereoscope can readily employ them, thus increasing its quality advantages proportionally.
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The reflective stereoscope's cons include an aspect ratio is roughly square whereas the 3D TV is "widescreen"; only one viewer at a time compared to several with 3D TVs; bulky (though interesting and readable) form; and fragile mirrors which need to be protected from dust and fingers.
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