Passive 3D Projection – Part 5
This is Part 5 of a series of blogs discussing passive 3D projection systems.
For these blogs I am 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. In my most recent blog (Part 4 – HERE) I discussed the different types of polarization, sources for polarizing filters, and projectors with non-polarized light output. Also in these earlier blogs I discussed some general characteristics to consider when selecting the projectors to be use for a do-it-yourself (diy), dual projector passive 3D setup. For this 5th installment of this series, I am discussing the use of projectors that inherently project polarized light and how these can be used in a diy 3D passive linear polarized dual projector system.
Below is a repeat of the illustration, previously showed in my earlier blogs in this series, depicting the functions that make up a passive 3D dual projector System.
The following illustration was also presented in a previous blog and repeated here for convenience. It depicts the two most common alignments used for linear polarized passive 3D systems.
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.
Projectors with Polarized Light Output
The goal for a passive 3D projection system, that uses polarization to separate the right from left image streams, is to configure the system such that the light arriving at the screen from one projector will have the opposite polarization from the light arriving from the other projector. However, using projectors that already have polarizing filters internal to their light engines can represent a challenge to creating a diy 3D passive projection system.
There will always be some loss of light as it passes through a polarizing filter. If the incoming light is already polarized with the same orientation as that of the filter then this will produce the minimum light loss. If the incoming light is polarized but the polarizing filter has a different orientation, then the amount of light loss will increase in proportion of the amount of the misalignment. If the incoming light is unpolarized (as discussed in my previous blog for DLP projectors) then there will be an approximately 50% loss of light as it passes through the polarizing filter. Since all LCD and LCoS based projectors already have an internal polarizing filter, the ideal arrangement would be to use external filter configurations that will minimize any additional light loss.
Projectors Without Polarization Alignment
Let’s first consider the worst-case scenario. All LCD (e.g., Espon’s 3LCD models) and LCoS (e.g., Sony’s SXRD and JVC’s DiLA) projectors inherently must include internal linear polarizing filters. This is necessary to create the images when using the LCD and LCoS micro-display chips. These projectors contain three micro-display chips (i.e., for red, blue and green primary colors) with a polarizing filter associated with each. Most and perhaps all of the Epson 2D 1080p projectors using their 3LCD design have the orientation of the polarization of the green primary color rotated 90 degrees from that of the red and blue colors. This is the case for all recent vintage Epson 2D models as the 6100, 6500UB, 7500UB, 8100, 8350, 8500UB, 8700UB, 9350, 9500UB, and 9700UB. However, it appears that at least the new Epson 3D models 5010 and 6010 (and probably the 3010) have the polarization aligned with the same orientation for the 3 primary colors. At least some of the Sony 2D 1080p SXRD projectors (e.g., VPL-VW50) have been reported to also have their green output with the opposite polarization orientation as compared to the orientation for other primary colors (e.g., red and blue). It appears (but not yet confirmed) that at least some of the current Sony 3D SXRD projector models also have their polarization orientation not aligned for the three primary colors.
As noted in my previous blog, Panasonic 3LCD projectors (that use Epson supplied LCD micro-display chips) that include their “Smooth Screen” feature to reduce pixel visibility include an additional internal optical element in the light path after the internal polarizing filters. This optical element used for Panasonic’s “Smooth Screen Technology” appears to effectively de-polarize the light resulting in projected images that are not polarized.
The real question is can a projector that does not have internal polarization alignment be successfully used in a diy dual projector 3D system that uses polarization to separate the right and left images. Some hobbyists have attempted to use projectors, such as above 2D Epson models, by adding external polarizing filters with the filter on the first projector oriented at 45 degrees and the polarizing filter on the second projector oriented to 135 degrees. These Epson projectors have each of their 3 primary colors oriented at either 0 degrees or 90 degrees. Note that when 0 deg. is indicated in this blog I am referring to a vertical orientation and when 90 degrees is indicated this refers to horizontal polarization. So by placing the external polarizing filter at either 45 degrees or 135 degrees, the orientation for the green polarized light coming from the projector would be shifted 45 degrees in one direction from the orientation of the external filter and the orientation of the red and blue polarized light coming from the same projector would be shifted 45 degrees in the other direction. The result from this 45 degree misalignment of the polarization for all 3 primary colors will cause a significant loss of light (perhaps around 50% loss), thus dimming the projected image. However a second, and potentially even more serious issue, has been observed by hobbyists that have attempted this approach in that they have found color shifts and lack of color uniformity in the resulting image. Some non-critical viewers may consider such color uniformity issues as minor and be willing to live with them in order to have 3D capability, especially if they already own one (or two) of the projectors of this type. What I am attempting to do in this series of blogs is to steer the reader toward a more optimum solution for diy dual projector passive polarized 3D setups and therefore with this goal in mind I cannot recommend, for anyone not already owning such projectors, the use of projectors without alignment of the polarization for all 3 of the primary colors. Note that in a future blog in this series I will discuss how such projectors might be used for a passive 3D setup that does not rely on polarization as the means for separating the right from the left image streams.
Projectors Having Polarization Alignment
This category of projectors are those that project linear polarized light where the orientation the polarization is the same for all 3 of the primary colors. JVC DiLA (LCoS) 1080p projectors, both earlier 2D and current 3D models (e.g., DLA-RS25, DLA-RS45, DLA-X70, etc.) have the polarization for all 3 primary colors aligned with the same orientation. In the case of recent and current JVC projectors, their projected light has a horizontal polarization (i.e., 90 degree orientation). The new Epson 3D projectors (using 3LCD based light engines such as the model 5010) also have the polarization for all 3 primary colors aligned. I will use the JVC projectors as the basis for this discussion since I have the most familiarity with their use for a passive 3D setup. These JVC projectors have a horizontal polarization with their projected image. In order to create a linear polarized passive 3D setup external filters are necessary to provide the result of the polarization being offset 90 degrees between the image coming from projector #1 versus that coming from projector #2. There are the following two approaches to go about doing this with use of external filters:
1. Modify the polarization orientation of one of the two projectors (Projector #2 in this example) to vertical, thus creating a 90 degree offset in the orientation of polarization of the unmodified image from projector #1 and the modified image from projector #2.
2. Modify the polarization orientation for both projectors to created one image with a 45 degree orientation and the other image with a 135 degree orientation (thus still retaining the required 90 degree offset in orientation between the two projected images).
One might assume the above required changes in polarization orientation would be accomplished using an simple external polarizing filter. However, this is not the correct method to alter the orientation of light this is already linear polarized. Instead a wave retarder plate (filter), or more specifically a half-wave retarder plate, is placed in front of the projector’s lens to rotate the polarization to the desired orientation. One big advantage of using a half-wave plate to rotate the polarization (as compared to an external polarizing filter) is there is only very slight light loss with the wave plate while a external linear polarizing filter whose orientation is offset from that of the projector by 45 degrees will result in perhaps 50% loss of light. The combination of the half-wave plate and a recent vintage JVC projector (i.e., DLA-RS40) has been shown to be an effective setup for creating a linear polarized passive 3D projection system. An external half-wave plate can be mounted in front of the lens of each of the projectors and the wave plate oriented to create a 45/135 alignment configuration (discuss in my Part 4 blog of this series). This 45/135 degree arrangement is the alignment currently used in digital IMAX theaters. As a result inexpensive 3D glasses sold for use in digital IMAX theaters will be able to be used with this setup.
If instead you were to use a 0/90 degree configuration then a half-wave plate would only need to be placed on one of the two projectors and that wave plate rotated to produce a vertical polarization alignment to complement the other projector’s normal horizontal orientation. Of course 3D glasses with a matching 0/90 degree configuration would be needed for this latter setup.
One supplier offers pairs of filters that are being sold specifically for use with LCoS projectors what have the linear polarization with all 3 colors aligned (basically JVC projectors). The Advisol LSAR-L-50 filter pair includes the filters (referred to as SPAR filters) needed for the two projectors. Each of these filters combine a half-wave retarder plate with a polarizing element. The wave plate is oriented to rotate the polarization to 45 degrees from one projector and to 135 degrees from the other projector (i.e., the digital IMAX arrangement). Each of these filters are made up of a polarizing element plus a wave retarder plate (rather than just the half-wave plate alone) to ensure the light that is sent to the projection screen remains highly polarized in order to minimize 3D crosstalk/ghosting. The total light loss thru these Advisol filters is specified at 20% (or 80% of the incoming light is transmitted through the filter). One user has reported that his tests using these filters with a JVC DLA-RS40 projector resulted in a total light loss, including losses from both the Advisol filter plus the light loss through the polarized IMAX style 3D glasses, of only about 25%. Advisol also sells mounts for their filters. Advisol is based in Israel and filters based on the Advisol design are sold by dealers including SilverFabric (Germany) and Inition (UK) without specifically using the Advisol name. Advisol has no dealers in the US and thus US customers must order the filters directly from Advisol or one of the overseas dealers. These are high quality glass filters and are fairly expensive (approx. $1000 for the pair). If interested, it is best to contact Advisol (link above) or one of their dealers for advice on the filters for use with your specific pair of projectors.
NOTE: While the two projectors used in a dual projector passive 3D setup are each operated in a normal 2D mode, the JVC projector model (DLA-RS40) that was used for the above testing with the Advisol filters is a 3D model and when used with the JVC active shutter 3D glasses has more than 80% total light loss (as compared to 2D from that same projector). This is a typical value (80% to 85%) for the light loss for active 3D projection systems. Thus the dual projector passive 3D setup described above will produce a 3D image that is perhaps 4 times as bright as using one of these projectors in its normal active 3D mode. This emphasizes one of the potential benefits from going with a dual projector passive 3D system and that is to provide very good 3D image brightness rather than the much dimmer images typical with an active 3D projection system.
A lower cost alternative to the above glass filters is to use a wave plate and/or polarizing filter made from plastic (polyvinyl) film. A source is Polarization.com which sells such films by the foot or by the sheet. For example, their PF030 fully laminated linear polarizing film that is 17 inches wide sells for $35 per linear foot and their WFLA-OG2 half-wave plate retarder film is sold in 11.5 inch by 14 inch sheets and priced at $34. With these films the hobbyist will need to create their own mounts to hold the film in place in front of the projector’s lens. This alternative provides a very cost effective solution for diy hobbyists.
The above discussion has focused on the use of linear polarization and for my next blog in this series I will expand the discussion to include circular polarization plus other topics related to passive 3D projection.