(Last update:  Nov. 2004)

CHAPTER 7:   3D PROJECTION

Fig. 7.1a  A collection of polarizing glasses and clip-ons.  Prices range from 50 cents for cardboard frames and (sometimes warped) lenses, to about $15 for fancy units (upper left).  See Berezin's store.	Fig. 7.1b.  Polarization schematic.  The beam coming back from the screen is polarized as indicated.  This matches the standard for polarized viewing glasses.  The screen must preserve polarization.  "Silver metallic" or aluminum-flake paints do this fairly well.

SLIDE PROJECTION

The projection setup for twin 35mm frames consists of two 35mm slide projectors mounted vertically.  One holds a reel with the Left Images in it, the other holds the Right Images.  Vertical mounting eliminates lateral keystone distortion in favor of small vertical keystoning.  The former is more objectionable in 3D projection.  Each lens is equipped with a linear polarizer (from a camera store).  Good quality polarizers are desirable in order to minimize ghosts (where the left eye picks up a little of the right eye signal, for example).  The ratio of light that gets through when the projector polarizer and the polarizer in the glasses are aligned, to that when they are crossed, is called the extinction ratio.   This should be large.  If the two polarizers don't go really black when crossed, try a different brand.  You can get technical and measure extinction with a light meter.  Try for 6 or 7 stops as a goal, though 5 will work for most images.  For reference, really top quality camera polarizers have about 8.5 stops of extinction.

Extinction (or its opposite, undesirable ghosting) is a strong function of the polarization preservation properties of the screen that is used.  Until its recent discontinuance at Da-Lite, the Super Wonderlite Silver Lenticular was the most common screen used in pro-am (i.e. non-theatrical) slide shows.  Much brighter and more ghost-free screens are available (see TechNote:  A Frame for a Polarization Preserving Screen (Stewart Silver 400) ).  You can even make your own screen by painting a flat surface with an aluminized paint.  I had good success with Krylon Silver Metallic spray paint (#1406).  You must wear a high quality gas mask and protective clothing.  The aluminum dust gets everywhere.  Spray-painted screens can be overly directional, so that you have to sit right in front (only).  Small warps in the surface or variations in the coat can cause very large dodging or burning effects in the images.  Try a fairly rough surface like wood or sanded polycarbonate, and hopefully you can get a nice uniform finish.  I was not successful in uniformly painting a screen more than about 3 feet wide.  The price is right if your painting technique is good.

Fig. 7.2  Twin Ektagraphic projection setup.  Good optical quality polarizers are mounted onto the front of the projection lenses with electrical tape (so that they can easily be removed for non-3D slide shows).  A home built stand provides adjustments for vertical and lateral pointing.

Once the screen is set up, the right (top) projector's polarizer is adjusted so the left eye goes dark (i.e. no right-eye beam is picked up by the left eye), and vice versa for the left (lower) projector.  The projectors are pointed so the frames overlap as exactly as possible.  

Projection of the stereo slides that come from monolithic stereo cameras is also possible.  One can either use vintage stereo projectors (available used on E-bay from time to time), or consider a modern unit from RBT or Brackett.  These latter devices are built in small quantities and are very expensive (~$3000), costing much more than a pair of Ektagraphic projectors.  The Ektagraphics have greater lens selection, but the Bracketts are more self-contained and have fewer alignment issues.  Bracketts are brighter than Ektagraphics (when both use their standard f3.5 lenses).  An Ektagraphic with f2.5 lenses is probably just as bright as a Brackett (maybe even a little brighter), outputting about 1600 Lumens.

Fig. 7.3a  A RBT stereo mount projector.  See the 3D Concepts store.	Fig. 7.3b.  The Brackett stereo mount projector.  Two of these can be combined for dissolves (if you are rich).  See the 3D Concepts store.

Slide projection using polarized beams is quite effective.  But (like anything else in life) there are  

Positives:

• High resolution color 3D images!

• Images on screen can be made fairly large (but see Negatives).

• Depending on the screen material, viewing angle is decent.  Viewers can sit outside the screen width (by about a factor of 2 or so in the back, and still get a good effect).  For example, for a 6 foot screen width (Da-lite Lenticular) the best seating is in an area about 12 feet wide, starting about 6 to 8 feet back from the screen.

and Negatives:

• People must wear polarizing glasses and keep their heads vertical while viewing (or else you get ghosts).

• There is about 2 stops of light loss due to the polarizers.  This can be made up for somewhat (0.5 to 1 stop) by using a brighter, more directional screen.  Viewers will then have to sit closer to the projection axis.

• Slides should be CAREFULLY REGISTERED  (see below).

• Good auto-focus or quick manual focus is necessary.  Both images must be in focus.  As a projectionist, unless you blink one eye shut, it is difficult to which image may be out of focus.  

FOCUS

You can glass mount the slides and set the projectors to manual focus.  This works, but is tedious.  Glass mounts are ever so slightly less sharp (because of the necessary anti-newton ripples in the glass), absorb about 15% of the light, and are difficult to make dust free.  But if you want edge-to-edge sharpness this is the way to go.  Some people put  "projection slide dupes" in such mounts for slide shows, keeping their originals in an archive.  It is better to shoot multiple originals, if possible, because dupes, though the may (or may not) have fairly good color and saturation, lose a substantial amount of resolution (see Film vs. Dupes ).

You can use a higher f-stop projection lens (like f3.5 - more depth of field) to compensate for lack of film flatness, and then use slide mounts without glass.  However, because the polarizers already cut the projection beam by a factor of 4 in intensity, going from f 2.5 to 3.5 (where another substantial fraction of the beam is lost) may cross the threshold of brightness acceptability.

REGISTRATION

The slides coming back from your photolab will be not be registered for 3D projection.  You will need to remount your slides making the requisite vertical adjustments and stereo window settings (see chapter 5).  This must be done manually by removing the film chips from their mounts and re-taping them into new glass or glass-less frames.  This tedious process may be aided by building yourself a mounting jig that can hold and move the chips while you view them through a 3D viewer.  Such jigs are described in Ferwerda, "The World of 3D".  Below is a picture of an accurate jig.

Given that the slides are properly registered and that the projector pointing is adjusted, then you must be sure that the individual slides drop into the film gate accurately (i.e. the same) every time. The projectors should have positive, active, slide registration clamps in the gate.  Kodak Ektagraphics (not Carousels) are fairly repeatable in registration.

Fig. 7.4.  SLIDE MOUNTING MACHINE Two slide mounts (lower faces) are held in clamps.  The right film chip is fixed where you want it.  The left chip is held in a low angle clamp.  Its position is adjusted using fine screw-threaded actuators (3: for horizontal, vertical and rotational motion).  The image pair is viewed with a disassembled slide viewer, or, for highest accuracy remotely with input into a computer.  When the registration position is attained the left chip is taped down.  Both faces are removed and the slide mount is closed.  This works with glass mounts as well as non-glass mounts.

DISSOLVE?

You may have seen the nice smooth 2D slide shows that are made by cross-dissolving between two projectors holding an odd - even sequence of slides.  Instead of the abrupt transitions of a typical slide advance, the dissolves are pleasing and can be long and slow or near-instantaneous.  Often such "multi-media" shows are set to music, giving a nicely paced presentation with almost motion-picture-like sequences being possible.

Dissolved 3D presentations are feasible, though instead of 2, you need 4 projectors (or a 4 lens RBT or Brackett projector).  This works well for some slides, but inexperienced viewers may have SERIOUS DIFFICULTY in fusing cross-dissolved stereo pairs that have a different depth perspective or a different stereo window setting.  Dissolved 3D works for sets of slides that are similar in perspective, color, and texture.  If they are not, at the mid-dissolve point your brain does not know which image to fuse and this can cause confusion and strain.  I have found that fade-to-black projection (requiring only two projectors that can be light modulated) works almost as well to smooth out the presentation.  It provides pace (rather than the annoying snap one gets from just pushing the slide advance button on the Ektagraphics).  Fade up and down is much easier for people not accustomed to 3D to fathom.  Although I have successfully presented dissolved 2D wildlife sound-syncs to many many audiences, I would be reluctant to try out a fully dissolved 3D projection program on anything but an audience at a 3D photography convention (or the like).  

DIGITAL PROJECTION

Using digital projectors for large venue presentations has several great advantages:

• The registration can be done in software (see the discussion of digital registration methods in Chapter 6)!!!!!  NO MORE PAINFUL HAND-MOUNTING AND DUST INFECTED MICROMANIPULATIONS.

• The media are archival in the sense that the images won't fade or gather dirt.   Slides in normal projectors last about 3 hours (by test: 1 hour for Kodachromes, ~3 hours for Fujichromes).  In very bright Xenon or Metal Halide slide projectors, like the SuperNova 5000 or the Telex Caramate C2000, you may expect deterioration after an hour, or so, of projection time for Fujichrome.  It would be archival suicide to put Kodachromes in such a projector.

• The projection is in focus corner-to-corner.  In slide projectors, if the film warps and low f-number optics are used, the images will go fuzzy at the edges.  Glass mounts avoid this but are tedious to assemble.

• Digital projectors use Xenon or Metal Halide lamps and are brighter than slide projectors.  Projectors are rated in ANSI lumens (a 9 point average of Lux, or lumens per square meter).  An Ektagraphic III with an EXR or EXW "high brightness" lamp, and an f2.5 lens, is roughly equivalent to a 1600 ANSI Lumen digital projector.  Therefore a digital projector with substantially more lumens, like 3000 or more, will be noticably brighter (but most digitals require the projector's contrast to be turned way down and this reduces actual light output).  This illumination difference can be important in large screen presentations, since the polarizers take such a toll (a factor of 3 to 4) on the light.  Digital projectors have whiter light (higher color temperature), and so reproduce blues better.  Side-by-side, slide projectors look muddy-yellow and a good digital projector sparkles with snap!

• Computer images are delivered to two projectors with crossed-polarizers over the lens, just as in figure 7.2.  Delivery is usually via a "dual-head" graphics card like the NVidia equipped GeForce4 MX440, which can drive two projectors at 1024 x 768, or whatever resolution is native.  You just send a side-by-side 2048 x 768 image to the monitor outputs, after appropriate display setup.  An advantage of digital image manipulation is that you can use various transitions (barn doors, wipes, fades), not just the standard flip, cross-dissolve, or fade-to-black that are possible with slide projectors.

• Twin digital projectors can do 3D animations and even video.  The former can be orchestrated with standard animation programs (like paintshop pro), although at 2048 x 768 (say) per frame, animations will be memory and CPU intensive applications.  Video is beyond the scope of this guide, although there is a lot of interest in this as well.  Some video systems use 640 x 480 interlaced field-sequential projection (with shutterglasses).  But recently there has been interest in  twin digital projection by making side-by-side videos.  

The disadvantages of digital projection are:

• Good digital projectors are more expensive than slide projectors.  On the other hand, mechanical slide projectors are not being made anymore.  While they still can be found, a new Ektagraphic costs $800 and a 2000 lumen XGA (1024 x 768) DLP digital projector costs about $1500.  Of course to do dissolve you need 4 slide projectors and 2 digitals.  For this application, there is not too much difference between slides and XGA.

• The resolution currently available at a "reasonable" price (< $2K each) is only 1024 x 768.  This is marginal.  My tests (see technote: Optimal Resolution for 3D )  indicate that the total slide projection package (good film, good lenses, careful technique,  glass mounts, polarizer, etc.) can give a resolution equivalent to  about 2000 line pairs across the screen.  Hollywood film moguls are quoted as wanting 4K x 2K resolution before fully endorsing digital projectors for theatrical (35mm-equivalent) movie projection, so I suspect this estimate isn't out of line.  There is only one announced native 2K x 1.5K digital projector at this point in time, the 150 pound JVC QXGA DILA unit that sells for over $200K.  However, there are mitigating factors (higher contrast at the pixel to pixel level in digital projectors, the fact that most viewers sit more than one screen width back, etc.), so I think that 3D presentations that are just as good as slides will be possible with 1600 x 1200 digital machines, that 1400 x 1050 digital projectors will be very very good, and 1024x768 stereo shows will look fine if you don't sit too close. 

• You need two of these expensive projectors to do polarized presentations (just as you need two slide projectors).  Most digital projectors are not fast enough to do high-refresh-rate shutterglass shows.  In any case shutterglass systems lose a factor of 2 or so more light than do twin polarizer rigs.  

• Most digital projectors are for computer graphics, and do poorly with "film".  In late 2004 better units started coming out with the boom in home theatre.  LCOS and DLP and some LCD home theatre projectors with better than 2000:1 contrast and color scaling for linear film-like output are now available. 

For further discussion and comparison data see the technical note on  Projector Light Output.

For large screen projection digital is the way to go.  Slides are dinosaurs, though admittedly, there is still some who will never forego the "look" of film.

Here is how digital projection is done:

Fig. 7.5a  Dual LCD projectors (ViewSonic PJ1060, 1024 x 768, 2200 ANSI lumens) produce two cross-polarized LR  images that are  overlaid onto a Da-Lite lenticular Super-Wonderlite silver-vinyl) screen.  The projectors are driven by a Matrox G400 dual head monitor in a PC.	Fig. 7.5b  Closeup of the front of the projectors.  Polarizing screens (from Edmunds Scientific) have been cut and taped over the lenses.  The polarization axes are +45 and -45 degrees off vertical.  Setup by S. Jones, Center for Integrated Plasma Studies, University of Colorado.

In choosing a digital projector, you must go to a showroom with a linear polarizer in hand and test each unit out carefully.  NOT ALL DIGITAL PROJECTORS CAN BE CROSS POLARIZED.   This is because most LCD and DILA (LCOS) units use internal beamsplitters, or polarized beamsplitters (to enhance contrast).  So also to 3 chip DLP projectors.  The output beams of such systems are polarized, sometimes in strange ways.  If the projector beams are polarized, the only systems that will work effectively are ones where all three colors emerge from a  polarizer placed over the lens with nearly equal intensity.  Note that plus and minus 45 degrees of polarization is required because that is the standard for theatrical polarizing glasses.  Figure 7.6 illustrates two LCD beam polarizations that will work.

Fig. 7.6a.   If the output beam is vertically polarized, then 45 degree polarizers will pass all three colors in the same amount with 1/2 the power.  By rotating this picture 90 degrees you can see that a projector with a horizontally polarized output beam will also work.  In fact, if all three colors are co-polarized, any output angle will work.	Fig. 7.6b  Many LCD projectors have the colors coming out at different angles.  Here is one example that works.  Green is vertical and Red-Blue are horizontal.  The 45 degree polarizers pass a 1/2 power green and a 1/2 power red-blue signal, leading to equal power in R, G, and B.  If the RB is vertical and G horizontal, this would still work, as will the situation with a different color mix.

Projectors with other polarizations may not work.  For example, if R is +45, B is -45 and G is vertical.  Then one eye sees all B and 1/2 power G, etc.  The best way to test is to set the projector on pure white, and hold the polarizer over the lens at 45 degrees.  Do you still get pure white, though at about 1/3 the power  (Note 1/2 is lost by geometry, and some more is lost because the polarizing material is not perfectly clear)?    

Single chip DLP projectors generally produce a randomly polarized output beam.  Thus placing a polarizer at any angle over the lens will not induce a color shift.  However, to be safe, you should always test before buying.  DLP projectors use a micro-mirror device to produce the image.  Since light projected off a mirror tends to preserve polarization, randomly polarized light from a metal halide or xenon lamp retains this state of polarization after reflection.  It also passes through a color wheel and lenses that usually will not affect the polarity of the beam.  Placing a polarizer in front of the lens will produce a beam polarized at the angle of the polarizer.  There is a factor of two loss in beam power (plus a little more for the polarizing material absoption), because for each moment the random beam is lined up with the polarizer, there is another moment that it is perpendicular (or crossed out).

Note:  Three chip DLP projectors use a beamsplitter  instead of a color wheel.  While the color is better, unlike mirrors, beamsplitters do polarize light.  Test required.

I have not yet had the opportunity to extensively test LCOS projectors (DILA).  There are many implementations of this new technology.  The JVC  DLA G15U (Dukane 9015) did not work with an over-the-lens polarizer. 

In summary,  digital projection has the great advantages of flatness of field, high brightness, non-destructive projection, setup ease (no reels), and digital (vs. tedious mechanical) stereo registration.  Digital images will not approach those from film projection until higher resolution and contrast, approaching 1600 x 1200 and 1000:1, respectively, are available.  As of November 2004, 1400x1050 and 2000:1 are common (and close to the goal).  However, it is useful to remember that the human eye can only resolve a certain level of detail (about 2000 line pairs across a 6 foot screen 6 feet away).  As people sit further back, the required resolution, to totally meet that of the human eye, falls off linearly.  If everyone is 3 or more screen widths back, for example, a 1280 x 1024 projector is just about "perfect".  On the other hand, the further back you sit, the smaller the immersion you experience in the scene, sort of defeating the purpose of 3D.