Passive 3D Projection – Part 4
This is Part 4 of a series of blogs discussing passive 3D projection systems.
The current discussion is focused on passive 3D systems that use polarization as the means to separate the right from the left images. Previously I discussed (in Part 3 – HERE) the role of the 3D Source, the 2-way HDMI Splitter and the 3D Processors. Also in that earlier blog I discussed some general characteristics to consider when selecting the projectors to be used for a do-it-yourself (diy), dual projector passive 3D setup. For this part of the series I continue the discussion by providing specific details for making the selection of a suitable pair of projectors.
Below is a repeat of the illustration, previously showed in my earlier blogs, depicting the functions that make up a passive 3D dual projector System.
As I noted in my previous blog, 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.
As mentioned in my previous blog there are two different types of polarization that are frequently used for 3D projection systems (Wikipedia article on polarization in general is HERE and an article specifically on Polarized 3D Glasses is HERE).
Linear Polarization – The simplest form of polarization is linear polarization and linear polarizing filters for use with the projector are modestly priced and readily available as are 3D glasses with linear polarizing lenses. When using linear polarization the orientation of the polarization is offset 90 degrees (i.e., orthogonal) on one projector from that on the other projector. The viewer’s 3D glasses must then have polarizing filters for their lenses that have the matching right and left orientation as used with the projectors providing the right and left images respectively. The two most commonly used configurations of the orientation of the polarization are as illustrated below:
It appears that IMAX has used both 0o/90o and the 45o/135o polarization orientation over the years for their IMAX 3D theaters (film based and digital).
The advantages for linear polarization (vs. circular) are:
a. filters for the projector are easier to produce (i.e., lower price – but usually not a major factor) and are more widely available;
b. when the axis of the projector’s filters and those of viewer’s glasses are perfectly aligned the 3D crosstalk (blocking of light intended only to reach the other eye) should be a little better; and
c. silver screen materials frequently retain linear polarization better than circular polarization resulting in less 3D crosstalk.
Circular Polarization – Without getting into the detailed physics, light can be considered an electromagnetic wave that has both electric field and magnetic field components which propagate outward together from the light source in the form of waves. For linear polarized light these two type of waves are ‘in phase’, or synchronized with each other. However, for circular polarization the electric field wave is shifted such that it is 90 degrees (i.e., 1/4 wavelength) out of phase with the magnetic field wave. The direction the phase is shifted (i.e., +/- 90 degrees) for the electric as compared to the magnetic waves determines whether the resulting light is right-hand vs. left-hand circular polarized. For the details see the section on “creating circular polarized light” in the Wikipedia article – HERE.
It is important to understand that a circular polarizing filter is made up of two elements. The first element is a linear polarizer and the second element is a quarter wave plate (also called a quarter wave retarder). The light first passes thru the linear polarizer and then passes thru the quarter wave plate which introduces a quarter wavelength delay for only one of the two fields.
Circular polarization is used by RealD for their commercial digital cinema 3D projection systems and the wide availability of RealD glasses provides a low cost source for those home theaters that use this circular polarization for their passive 3D projection system.
The most significant advantage of circular polarization over linear polarization is the 3D crosstalk level does not change when the viewer tilts their head toward the side while with linear polarization the minimum crosstalk can only be achieved when viewed with the 3D glasses perfectly level.
Sources for Linear Polarizing Filters and 3D Glasses
The following are a few examples of sources for polarizing filters (either glass or acrylic) and polarized 3D glasses. A web search will find many additional sources.
polarization.com (polarizing filters/sheets and waveplate retarder film)
SilverFabric (professional quality polarizing filters designed for use with 3D projectors)
Berezin Stereo Photography Products (projector filters and 3D glasses)
The 3D Market (polarizing filters and 3D glasses)
Filter Size and Mounting
The required physical size for the polarizing filter will vary depending on the specific projector being used including the front diameter of the projector’s lens, the distance from the projector’s lens to the filter and also to where you set the zoom lens on the projector (if zoomed for maximum image size on the screen a larger filter size may be necessary as compared to when the zoom lens is set for minimum zoom). The polarizing filter must be large enough such that rectangle shaped projected image fully passes through the filter without cutting off the corners or sides of the image. Also the mounting bracket for the filter should to be adjusted such that the projected image can more-or-less centered with the filter.
Frequently diy builders create their own brackets to hold the filters in front of the projector’s lens. If you use a projector with a lens barrel that extends from the front of the projector, rather than having the lens recessed back into the projector’s case, you may be able to find a filter holder intended for use with a medium or large format camera that can be adapted to attach the polarizing filter to your projector. Such filter holders from manufacturers as Cokin, Holga, and Lee Filters are available from most large camera stores and online dealers such as B&H Photo and Video. Alternatively, if your projector uses a manual slide-on lens cap, you may be able to cut out the whole central portion of the lens cap (or a spare lens cap you have purchased) leaving the outer ring that slides onto the outside of the lens barrel with just a small portion of the original front of the lens cap. The polarizing filter can then be mounted (e.g., glued) onto the large hole created across the front of the modified lens cap to create a filter holder that can easily slipped on the projector and rotated to the desired orientation (i.e., for when using a linear polarizing filter).
When using such projectors (i.e., that output non-polarized light) the polarizing filter will typically reduce the light output by approximately 50%. Therefore, you should make provisions on at least one, or better yet both of the two projectors to have the ability to easily remove the filter from the light path for when you are using the projector for normal 2D viewing. Some diy builders have used either a hinge or a slide arrangement to easily move the polarizing filters into or out of position in front of the projector’s lens. Ideally you would be able to use either of the two projectors for viewing normal 2D video. By sharing your 2D viewing time between the two projector you can more or less equalize the number of hours of use on the lamps of the two projectors. This will help maintain similar light output for two otherwise identical projectors using lamps of approximately the same age.
I discussed in my previous blog (HERE) some general characteristics to consider when selecting projectors for a diy dual projector 3D setup. Not covered in that earlier discussion is an additional major factor that must be taken into account when selecting suitable projectors for a polarized passive 3D projection system. Certain types/technologies of projectors produce unpolarized light while other types of projectors project light that is linear polarized. Those that do produce polarized light do so as the result of the specific projection technology being used. The following sections discuss the implications and consequences of this projector characteristic (and much more to come in my next blog).
Projectors with Non-Polarized Light Output
Of the three major digital imaging technologies (i.e., DLP, LCD and LCoS) used for modern video projectors, only DLP does not require a polarizing element within the optical light path. As a result DLP projectors put out non-polarized light. While both LCD and LCoS technology (such as JVC’s DiLA and Sony’s SXRD technology) inherently require internal polarizers. However, the Panasonic LCD based projectors with “smooth screen technology” include an optical element within their internal light path subsequent to the polarizer that has the effect of depolarizing the light. As a result, when I talk about projectors that do not produce polarized light that discussion is limited to DLP projectors plus those Panasonic LCD projectors with the “smooth screen” feature.
In some ways projectors in this category are the most straight forward to work with when attempting to created a passive dual projector 3D system using polarization since all one needs to do is add a polarizing filter in front of the lens on each of the two projectors. With linear polarization this means using two identical filters with one rotated with the axis of its polarization shifted 90 degrees from the other filter (such as with a 45o/135o setup describe earlier). The filter orientation needs to be the same as for the polarized 3D glasses you have selected.
If circular polarization is being used, then different filters are required for the two projectors with one filter having right-hand circular polarization and the other filter having left-hand circular polarization. Many diy builders of passive 3D systems using circular polarization use glasses designed for use with RealD commercial cinema 3D system and the circular polarizing filter configuration (i.e., which projector gets which filter) needs to be consistent with the configuration of that used for the lens of the 3D glasses.
I discussed in my previous blog (Part 3 of this series) that ideally the selected projectors will have a lens shift adjustment. For projectors that lack such an adjustment, the projector must be centered left-to-right with the screen and there is only one specific vertical position for a given screen size/location and screen-to-projector distance that will produce a image on the screen with the correct geometry (i.e., a perfect rectangle shape that aligns with the sides and top/bottom of the screen). If the first such projector is mounted at the specific location that is needed to obtain correct image geometry then a second identical projector will not be able to be mounted at that correct location and the results will be incorrect image geometry. When one projector is mounted above the other, then the projector at the non-ideal location will have keystone distortion with the projected image being a trapezoid rather than a perfect rectangle. While it certainly is best to use projectors with a lens shift adjustment to optically achieve correct image geometry and positioning with alignment of both the left and right images, a compromise can be to use the electronic keystone adjustment found on many projectors that lack a lens shift capability. Use of electronic keystone correction should generally be avoided for home theater applications because it degrades the image quality due to the digital processing/scaling being used will introduce some undesired image artifacts.
Many, but not all, DLP projectors do not offer a lens shift adjustment. If you already own one of these models that lack a lens shift adjustment and if you need to minimize costs by using that existing projector as the starting point for creating a passive 3D dual projector system, then you may want to purchase a second identical (or very similar) projector. One of the two projectors should be installed at the optimum position (i.e., where you do not have geometry issues) and you can use that projector for your most critical 2D viewing. In this case you could then install the 2nd projector just above or just below the first projector and use that projector’s electronic keystone adjustment and zoom lens to achieve as close as possible an alignment of the image from the second projector with that from the first projector. This second projector would then only be used when viewing of 3D programs for for less critical 2D viewing. If purchasing both projectors for creating a new passive 3D system, then I strongly suggest selecting models with a lens shift adjustment as this will provide a ideal solution.
To finish up this part of the blog I will mention a few moderate priced projectors that output non-polarized light and based on their specifications appear to have the features, including lens shift, that would make them well suited for a dual projector passive 3D system. All of the following models that I have listed provide full HD 1080p native resolution. I have included some models that are now discontinued but may be available at reasonable cost as used or refurbished units. This list is not intended to be comprehensive as there are other past and current models that are be suitable for a dual projector passive 3D system. Also there are many budget-through-moderate priced DLP projectors being offered that do not have a lens shift adjustment (i.e., offer only an electronic keystone adjustment), but if you are on a very tight budget these could be considered, but are less than ideal for a dual projector passive 3D system. Click on the links below for the full projector reviews.
BenQ W6000 – This 1080p DLP model was discontinued last year, but you may be able to find them available through some dealers. This model has a 1.5X zoom lens as well as lens shift. It does require a fairly long projector-to-screen throw distance (as compared to many LCD and LCoS projectors). It is a fairly bright projector which helps overcome the light loss inherent with 3D projection system.
BenQ W7000 – Although this new 1080p DLP model is a 3D ready projector, a pair of these could also be used in a dual projector passive 3D setup. Like the earlier W6000 it replaces, the W7000 includes a 1.5X zoom lens and lens shift as well has fairly high lumens of output making it suitable for a dual projector passive 3D setup.
Optoma HD8200 – This 1080p DLP model was first introduced about 3 years ago and used or refurbished units may be found at reasonable prices. This model has a 1.5X zoom lens as well as lens shift. While it is does not put out the lumens of the above BenQ DLP models it would be adequate for use with smaller screens having most gain or larger screens that have high gain.
Optoma HD8600 – This is another 1080p DLP model that was first introduced about 3 years ago as a relative high-end model and used or refurbished units may be found at reasonable prices. This projector offered optional lenses for different throw distances while the standard lens was a 1.25X zoom. This projector has lens shift and is capable of producing a fairly bright image in some of the non-calibrated, but usable modes (i.e., overall more lumens output than the Optoma HD8200 but less than the two BenQ models above).
Optoma HD8300 – This is a current model 1080p DLP projector that is 3D-ready but two of these could be used for a dual projector passive 3D setup. The projector has a 1.5X zoom lens and have a limited range vertical lens shift (no horizontal lens shift). Note that on the first page of Art’s review (link above) the specification chart lists “lens shift: none”, but multiple owners have reported that this model does indeed have vertical lens shift. This model puts out a moderately bright picture in best mode, but does not offer the relatively high lumens modes offered by the above BenQ models.
Panasonic AE4000 – This is a popular 3LCD home theater projector that has been around for 2+ years, but you many still find it available from a few dealer or as a used or refurbished unit. This model uses Panasonics “smooth screen technology” to create a more film-like look by virtually eliminating visible pixel structure. The optical element within the projector that provide “smooth screen” also has the side effect of removing the polarization of the light coming through the projector’s LCD micro display panels. This model has a 2X zoom lens with lens shift and also with lens memory. While this models light output is not especially impressive in best mode, it does offer brighter, but less accurate, modes that place it brightness in the same league as some of the DLP models above, but well below the bightest of the above DLP models.
My next blog will continue the above discussion by covering projectors that inherently output polarized light and discuss how to use such projectors in a dual projector passive 3D system.