Passive 3D Projection - Part 6

This blog post continues the discussion of do-it-yourself (diy) passive 3D projection systems that use two conventional front projectors. In my previous blog (i.e., Part 5 of this series), I discussed using projectors that inherently project polarized light as well as the role of external filters needed for use with such projectors. For this new blog, I will address one comment received to that previous blog as well as continue the discussion on passive 3D projection systems that uses polarization as the means to separate the right from the left images that make up the stereoscopic image pair that creates the 3D effect.

Follow-up to a Comment Received on Part 5 Blog

I received a comment on my previous blog concerning filters that are being offered specifically for use with LCD projectors that have the green light component polarized (i.e., with linear polarization) with a 90 degree different orientation from the red and blue light components. In my previous blog I discussed some less than fully successful attempts by some hobbyists to use such projectors by using conventional external linear polarized filters oriented at 45 degrees on one projector and at 135 degrees on the other projector. The comment that I received was pointing out that the Israel-based company Advisol is offering filters specifically designed to deal with the 90 degree offset in the green polarization on such LCD projectors. Advisol describes their StereoPol^TM filters as:

“Advisol was first to introduce high-efficiency polarizing filters for LCD projectors. These filters are based on Advisol’s proprietary StereoPol™ technology, and achieve ~65% optical transmission, compared to about 40% in conventional polarizers. The use of StereoPol™ filters has a dramatic impact on passive 3D displays design, since it allows the use of much smaller and less expensive projectors to achieve a given image brightness.”

The StereoPol^TM filters are sold in matched pairs. These are part of the Advisol series of “SPAR” filters. See my previous blog (i.e., Part 5) for information on the availability of Advisol filters. I have not been able to find any first-hand accounts from a hobbyist or professional reporting on the results obtained when using the current generation of the StereoPol^TM filters with LCD projectors. Perhaps these filters do offer a viable solution for using such LCD projectors for a dual projector passive 3D system. However, I have seen reports that an earlier generation of these filters resulted in 3D images with color uniformity issues, but I cannot independently confirm how well the current generation of these filters will perform.

A Short Recap of this Series of Blogs

Shown below is an illustration showing the functional components that make up a passive 3D projection system. PART 3 of this series of blogs discussed the “3D Source Device”, “2-Way HDMI Splitter”, “3D Video Processor (3D-to-2D Demultiplexer)”, and covered some general characteristics of projectors that are to be used with passive 3D dual projector system. PART 4 of this series discussed the general characteristics of linear and circular polarization, sources of polarizing filters and 3D polarized glasses, filter size and mounting, and the use of projectors that do not project polarized light. PART 5 of this series discussed the use of projectors that inherently project polarized light. As I noted in my previous blogs in this series, some of the information used for this discussion is based on information provided by Rob Stewart who has experimented with a number of diy dual projector alternatives for passive 3D.

Circular Polarization for Passive 3D

While back in Part 4 of this series of blogs there was some discussion on the use of circular polarization, the focus of my discussions up until this point has been on the use of linear polarization. For general information on circular polarization I suggest you go back and read Part 4 of this series of blog and well as the articles linked from that blog. As discussed in a previous blog, circular polarization is the most widely used technique for passive 3D in commercial cinemas (i.e.., those cinemas using 3D systems provided by RealD). For the Wikipedia article on RealD click HERE. A significant advantage often noted for circular polarization, as compared to linear polarization, is the viewer need not maintain their head (or actually the 3D glasses) perfectly level in order to obtain the minimum 3D crosstalk/ghosting in the image. A significant disadvantage of circular polarization is that it is more challenging for a projection screen material to retain a high level or circular polarization as compared to retaining linear polarization. This can lead to a higher level of 3D crosstalk/ghosting with circular polarization as compared to a perfectly aligned linearly polarized passive 3D projection system. With the commercial passive 3D system from RealD they also apply proprietary video pre-processing of the 3D images, prior to them being projected, to help cancel out the 3D crosstalk. I am not aware of any consumer video processor that could be placed between the 3D video source and the two video projectors to provide an equivalent function. Therefore, if using circular polarization for a diy passive 3D home theater project, it becomes critical to select a projection screen material that retains as much of circular polarization as possible. You may find it useful to review my earlier blog on Screens for 3D Projection (HERE). Even when a quality passive 3D screen material has been selected (i.e., a silver screen material that is specifically designed to retain polarization), experimentation conducted by one hobbyist has indicated that achieving an acceptable level of 3D crosstalk/ghosting when using circular polarization may not be possible for the more critical viewers. A few points concerning the use of circular polarization may be helpful before getting into the specifics of using it for a diy 3D passive projection setup.

1. Circular polarization is created by first making the light linear polarized then passing that light through a quarter-wave retarder plate (filter).

2, The orientation of the linear polarized light (i.e., at the projector before passing the light through the wave retarder plate) is important since this orientation needs to be compatible with the orientation of the linear polarizing element within circular polarized glasses being worn by the viewers.

3. If you install a circular polarizing filter backward it will act as a linear polarizer (so they must be installed correctly in order to produce circular polarized light).

4. For the following discussions I’m going to assume the goal is be able to use the widely available RealD 3D glasses with any diy circular polarized passive 3D projection system. For compatibility with the RealD 3D glasses, the filter system used at the projector (combination of any internal polarizing filter plus any external filters) will need to produce vertical oriented linear polarized light before passing that light through the quarter-wave retarder plate that creates the circular polarization.

If you are using a pair of DLP projectors for a passive 3D setup (i.e., or any projectors that do not inherently project polarized light) then a circular polarizing filter need only be added in from of the lens of each projector. A “right-hand circular” polarizing filter is mounted onto one projector and a “left-hand circular” polarizing filter is mounted onto the other projector. As noted in an earlier blog, a circular polarizing filter is created by combining a linear polarizing element and a quarter-wave retarder plate element. There are several options for obtaining the needed circular polarizing filters for use with the projectors. For experimentation you can simply purchase a pair of RealD glasses (or keep the glasses when you attend a movie presented in RealD at your local cinema). The lenses from the glasses can be removed and fitted in front of the projector’s lens. In this case another pair of RealD glasses can be worn for viewing the projected 3D video. While the lenses removed from the glasses are not ideal (e.g., shape and size) for use with a projector, they can provide a very low cost tool for the hobbyist to experiment with circular polarization. Another economical source for circular polarizing filters is to use filters made from vinyl (plastic) film. Such economical circular polarizing films are available from a few on-line sources including polarization.com . Higher quality and more expensive glass circular polarizing filters designed specifically for use with projectors are sold by a number of companies including “The 3D Market” which sells a pair of 5 inch by 5 inch circular polarizing filters for less than $400. When used with this type of projector (i.e., which does not produce polarized light), you will typically lose 55% to 60% of the light in passing through the circular polarized filter that you have mounted in front of the projector’s lens. If you are using a pair of projectors whose light output is already being (linear) polarized within the projector, then simply adding an external circular polarized filter will not normally be the correct approach. Let’s take the case of projectors, such as those from JVC, where all three of the primary colors have the same orientation for their linear polarization. For JVC projectors this means that their projected light has horizontal linear polarization as it comes from the projector’s lens. This not a compatible orientation for use with the RealD 3D glasses. However, to made these JVC projectors work correctly with RealD 3D glasses the projector’s native linear polarization orientation needs to be rotated 90 degrees to vertical before being converting to circular polarization. One relatively straight forward technique that has successfully been used to handle this situation is to use a set of two filters in front of each projector’s lens. The first “filter” is a half-wave retarder plate (or two quarter-wave retarder plates) which is physically rotated to shift the light coming from the projector from its original horizontal polarization to a vertical polarization. Note that using a half-wave retarder plate to rotate the orientation of linear polarization was previously discussed in the Part 4 blog of this series. The vertically polarized light coming out of the properly oriented half-wave retarder plate can then be passed through a conventional right-hand circular or left-hand circular polarizing filter. The circular polarizing filter contains a linear polarizing element that must also be oriented vertically (done by physically rotating the filter) to match the orientation of the vertical linear polarized light coming out of the correctly oriented half-wave retarder plate. Thus, each projector would be outfitted with an external filter “sandwich” made up of a half-wave retarder plate followed by a circular polarizing filter. When the each of the two filters within this sandwich are correctly oriented, the total light loss through the filter sandwich will be on the order of only 20%. So even with the additional modest light loss in going through the RealD 3D glasses, the overall image brightness will still be very good for viewing 3D programs (e.g., 60% to 70% as bright as viewing 2D from the same projector). This is much better (i.e., much less light loss, thus a brighter image) as compared to any consumer active 3D projection system where 3D video, when viewed through the active 3D glasses, will typically only be 15% to 25% as bright as 2D using the same projector. If you were to find a LCD or LCoS based projector whose projected light is already vertically aligned for all three of the primary colors (I don’t know of any specific projector that does this), then there would no need for the half-wave retarder plate. Thus for such a hypothetical projector, only an external circular polarizing filter would be needed (i.e., right-hand circular for one projector and left-hand circular for the other projector). The orientation of these external circular polarizing filters would need to be physically position such that the filter’s internal linear polarizing element is set to a vertical orientation. In the above cases, you can align the external filters by viewing a projected image while wearing RealD 3D glasses and then physically rotating each of the projector’s external filters to obtain the brightest image (done for one projector and one eye at a time). This should also produce the minimum 3D crosstalk. Finally, how about the case for a LCD or LCoS based projector with linear polarized ouput but where one of the three primary colors has a different orientation from that of the other two primary colors (e.g., most Epson LCD projectors). The above described technique of using a wave retarder plate plus a circular polarizing filter, or just a circular polarizing filter alone, will not work correctly since it cannot correct for the single color being misaligned. As I described at the beginning of this blog there are filters made specifically to produce a linear polarized output from such LCD projectors while aligning the orientation of the polarization for the 3 colors. The manufacturer’s web site, listed near the beginning of the blog, indicates approximately a 35% light loss with their StereoPol^TM filters. While not having any first hand experience with these filters, it would seem to me that it would be possible to use filter “sandwich” consisting of a StereoPol^TM filter, followed by a half-wave retarder plate (adjusted to rotate the linear polarization to vertical), and finally a circular polarizing filter to produce a circular polarized output that would be compatible with RealD 3D glasses. However, the combination of the 3 filters that would make up this external projector filter “sandwich” would probably have a total light loss on the order of 50% (plus the additional loss for the 3D glasses). So while it may technically be possible to use such projectors in a circular polarized passive 3D projection system, this does not appear to be the most practical setup.

Final Thoughts on Circular Polarization

Readers of this blog should understand that it can be much more difficult (or perhaps impossible) to achieve acceptable levels of 3D crosstalk/ghosting when using circular polarization as compared to linear polarization for a diy home theater passive 3D projection system. This is due, for the most part, to the more limited ability of projection screens to retain circular polarization, as compared to linear polarization. RealD is successful with using circular polarization for passive 3D presentations in commercial cinemas by pre-processing the video for 3D crosstalk reduction, but this technology is not currently available for use at the consumer level. At this point linear polarization should be considered as both easier to implement and offering better overall performance (i.e., less 3D crosstalk/ghosting), as compared to circular polarization, for a diy home theater passive 3D system. __________________________