Filters & Glasses

The polarizing filters on each of the projectors are necessary to delineate left/right eye vision.  The goal is to view each of the images projected by the two projectors with a different eye, which is then interpreted as depth in your brain.  For our system, we used linear polarization (as opposed to circular).  The glasses are polarized at inverted 45 degree angles.

At first, we purchased plastic polarizers which we cut to size and super glued to the fittings that were provided to us by the Geowall consortium.  This setup had several problems.  The super glue smudged the edges of the filters, and more importantly, the heat from the projectors melted the filters.  When they were melted they lost polarization.  It was also impossible to rotate the polarizing filters without turning the entire lens fixture, which made it very hard to maintain a tight focus while orienting the filter correctly.

After the plastic filters failed, we purchased two Hoya 72mm linear polarizing filters (designed to reduce glare for professional photography).  Heaven.  First, the fixtures that the Consortium provided us were specifically designed for these filters.  They literally just snapped into place, and then the whole thing just slides onto the lens fixture of each projector.

The Hoya filter and the mounting device separated

The Hoya filter snapped into place

Once in place, the lens itself can rotate without moving the focus-fixture, making calibration much easier.  Since the filter is glass, it is not melted or depolarized by the heat of the projectors.

Calibrating the lenses is a simple task of using a pair of glasses to eye-ball the position at which the least amount of light gets through.  The system does not filter out 100% of the light, so don't worry if you have a ghost image in of the opposite eye on the screen.  This ghosting does not undermine the stereoscopic image.

Calibrating the filters to the glasses

For our system, the top projector is the left eye.  This means that we don't want the right eye to see it.  To make this happen, simply hold the right eye of a pair of glasses in front of the lens and turn the filter until the least amount of light can be seen through the glasses.  Do the same thing for the other eye on the other projector.

We purchased our polarized glasses at Reel3d.com.  The more you buy, the cheaper they are, per-pair.

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