Carlovision: Veiwers
The 3D viewer shown above is a descendent of the Holmes stereoscope, circa. 1860, pictured at left, and works similarly: each of the user's eyes view the respective image of a side-by-side pair.

A fundamental limitation of the Holmes stereoscope is imposed by the human eyes... they are 2.5" apart and cannot diverge.  This means that the viewed stereo pair cannot be very large.

A reflecting stereoscope overcomes this size limitation with mirrors, thus allowing a high resolution monitor to take the place of the paper stereo card. 
In the reflecting stereoscope, the user's eye centerlines (dashed blue lines in diagram at right) are spread out to match the spacing of the full screen stereo pair.  This is accomplished with 4 precisely aligned mirrors (blue rectangles in diagram).  These are special "front surface mirrors" which create no optical distortion.

The slots CNC machined into the 3/8" polycarbonate plates shown below hold the mirrors.
The monitor and mirror box must be held in alignment, and must be tilt adjustable to suit different height users.  The pictured viewer also has adjustable monitor distance and balance point which was valuable in the prototyping phase. 1.5" aluminum box forms the backbone of the unit.
Reflecting Stereoscope Pros
  • no ghosting or distortion
  • bright and sharp at close range
  • easily adaptable to future superior monitors

Reflecting Stereocope Cons
  • expense and bulk of mirror box and support structure
  • dust and fingerprint vulnerability of mirrors
  • large dedicated piece of equipment
  • single user
  • square aspect ratio not ideal for panoramic material

The ultimate limitation on this type of viewer the number and density of monitor pixels.  More pixels means better image quality; higher density means smaller viewer size.  This common HD monitor (1920 x 1080) has 2 million pixels at 95 PPI (pixels per inch), which is only marginally acceptable in my opinion.  Fortunately, the situation can only get better, and the advent of the Apple Retina display probably means that compact ~4 million pixel displays will be affordable within the next few years.
3DTV-based Carlovision viewing station,
using tactile user interface and Vizio E3D320VX passive 3D TV
To the naked eye, passive 3DTVs are a mess: the view intended for the left eye and the view intended for the right eye are BOTH shown on top of each other.  The glasses fix that, causing each eye to see only it's intended view.
Price Tipping Point
Until a few years ago, 3D monitors were exotic and expensive - living mostly in corporate and government laboratories.  But as of 2012, they have come down to the people, and you can buy the one shown above for $350, not much more than a non-3D monitor.  This gets you a system where each eye sees a 540 x 1920 image... a fairly good viewing experience.  The glasses are cheap, or even free if collected from the cinema (same type as Real 3D).

Compared to building a special piece of equipment like the reflecting stereoscope described above, using an out of the box 3D-ready device like this seems like a no-brainer.  And, aside from a few caveats, it is.

Multi Person
Unlike the single-user reflecting stereoscope, at least 3 people can use this system at once.

The Fine Print
Passive 3DTVs look good if one sits far enough away and at an acceptable angle.  The minimum distance for this 32" unit is 7 feet, and that means the overall form factor of a viewing setup is big, and that the apparent field of view is correspondingly small.  The intrinsic gotcha is similar to, but worse than, the "screendoor effect" of video projectors:  the "venetian blinds effect" (described more below).   Also, one needs to stay still when watching 3D - because sideways movement produces a dissonant feeling. More information on the Vizio 3DTV.

What About Active 3DTV?
I conducted some informal tests at the local Best Buy to get a sense of the differences between passive and active 3D monitors.  Keeping an open mind, I didn't pay attention to prices and watched the demo reels of each system a few times, and was surprised to conclude that the passive 3DTVs just looked better.  Surprised, because the conventional wisdom is that, since active has twice the verticle resolution, it must be better.  Aside from winning in looks, passive sets are less expensive and the glasses are light and cheap - making passive much more suitable for public exhibition. 
works of  Carl C Pisaturo