viewing systems

Free viewing

This is the cheapest method of viewing, but also requires the most practice and dedication.  Images can be viewed cross-eyed, or parallel.  Most people are unable to diverge their eyes and therefore cannot stereoscopically fuse images that are separated by a larger distance that their intraocular distance.  Cross-eyed viewing is easier for most people but also can be difficult due to the neurological linking of accommodation and convergence.  The main benefits of free viewing are that it can be done anywhere, anytime with no viewing aids and no image ghosting.

Optical aids


Included here are optical devices that permit the delivery of images to each eye with tolerable accomodation-convergance disparity.  The device can be as simple as handheld mirror or quite complex and bulky (some of the large mirror-based stereoscopes used for photogrammetry or old two-wall radiograph viewers).  I carry a small mini-Wheatstone viewer - essentially two small periscopes laid on side to spread one's effective intraocular distance - in my bag that can be used to quickly view images on any computer screen or on radiographs that are printed in parallel view format.

Wheatstone principle

One of the easiest stereoscopes to make is one using a single mirror.  2 computer screens, radiograph viewing boxes, or pictures are placed at 45o to each other with one image laterally reversed.  A carefully sized mirror is then placed on the plane between the two images - this superimposes the images optically in the right viewing position.  If at all possible, a front-silvered mirror should be used to eliminate ghosting. 

Mirror based stereoscope

Despite the ease of use of digital displays, I believe that if a dedicated user will gain the most information from an optical film viewing system at the moment.  This is for two reasons - the lack of ghosting in a well designed optical system and the wider dynamic range of transparency film as compared to prints or screen displays.

Liquid crystal shutterglasses

Liquid crystal shutterglasses are a neat device.  The glasses are formed of liquid crystal panels that alternately blank out in sync with the left and right images being displayed on the screen.  The ability to drive the glasses and alternate left/right images on the screen (page-flipping) results in a smooth flicker-free stereo appearance if the speed is high enough.  I find 120 Hz to be comfortable flicker-free stereo, but there are people who can comfortably look at 60 Hz stereo as well as those that need rates of 140 Hz+ for comfort.  Also, if shutterglasses are used in a room with florescent lights, the interference between the shutterglasses and the florescent tubes will cause major flicker.
 As long as the images aren't given a horrible amount of parallax, the accomodation-convergance discrepancy is largely dealt with.  Ghosting is also an issue with the shutterglasses, especially in high contrast regoins of the image.

Shutterglasses are a reasonably cheap (US$40-100+) solution.

Polarized light systems

IMAX has used these in projection systems for years with success.  Standard linearly polarized glasses have polarizers oriented at 135o for the left eye and 45o for the right eye.  Similar polarizer orientation is used for each of the projectors and a silver non-depolarizing projection screen needs to be used.  There are a number of other polarizing displays available such as the VREX micropol filter (horizontally interlaced alternate polarization) or the Z-Screen from Stereographics (active screen shutter with passive circular polarizing glasses).  The advantage of polarized systems is that they do allow multiple viewers easy access to the picture.  The drawbacks are ghosting which is still present and the need for users to wear eyeglasses.

Autostereoscopic displays

Autostereo is the ability to deliver separate images to each eye without requiring the use of viewing glasses.  At present, there are two main methods for achieving this - use of a barrier to block light destined for the contralateral eye, or use or a lenticular lens to direct light into the chosen eye.  Autostereoscopic displays are available from two to nine different viewing zones, but for each additional viewing zone, the effective resolution of the image is degraded.  The other drawback with autostereoscopic displays is the requirement for the user to be in a fairly well defined "sweet spot" or else the image will be displayed in mono or pseudostereo - head tracking devices can overcome this, but are cumbersome and expensive at present.  Both raster barrier and lenticular displays do suffer from ghosting which can be significant when high contrast images are used.

Displays are currently quite expensive and do have technical deficits, but these are being addressed by developers.  Solutions such as the DTI and Sharp screens use a switchable raster barrier and can be used for conventional mono work as well.

Where picture quality is important in an autostereoscopic screen, the SeeReal lenticular autostereoscopic screens offer the clearest picture with the least ghosting at the present time.

Emerging technology

Technology will continue to develop and there are some interesting ideas being worked on.  Holographic displays, elemental lens arrays and true volumetric displays are all being developed presently.  One of the most interesting developing technologies uses your retina as the primary projection screen - I'd still feel uncomfortable at having two projectors aiming for my eyes but can see the potential.

Anaglyph

The anaglyph is the use of color (usually red-left, cyan-right) to code for left and right images.  The images can be printed, displayed, or projected on just about any medium and a simple set of glasses with the appropriate lenses is used to direct the appropriate image to the appropriate eye.  "Retinal rivalry" is the conflict that arises in the brain by having the two different colors used for the same object.  Apparently, this does not bother a lot of people - I find anaglyphs unusable for any length of time.\

home