Screens for 3D Projection – Part 2
This blog is a continuation of my previous blog. These discussions focus on factors that need to be considered when selecting a projection screen for use in a home theater with a 3D projector. While many of the screen characteristics equally apply to selecting a screen for regular 2D projection, there are some factors that are either more important when it comes to 3D or apply only for the case of 3D.
Screen Characteristics (continued from Part 1 in my prior blog)
Retention of Polarized Light – This characteristic of projection screens is only applicable to 3D projection and even then it only applies for screens used with certain types of 3D projection system.
This is one of those additional factors that needs to be considered in selecting a suitable projection screen for 3D viewing in a home theater environment as compared to limiting the screen considerations to only supporting conventional 2D viewing. First you need to know what technology your 3D projector uses to separate the projected right and left images and if the projected light is polarized, or not. There are two types of 3D projection systems where the ability of the projection screen to retain the polarization becomes significant (as described below).
1. The first case is where retaining the polarization of the projected light is absolutely essential because the 3D projection system uses polarized light, with different orientations, to separate the left and right images and the viewer must wear polarized 3D eyeglasses (sometime called passive 3D glasses since they contain no electronics). The light may either be linear polarized, where the polarization orientation for the right vs. left images is offset by 90 degrees (e.g., vertical vs. horizontal -OR- vertical +45 degrees vs. vertical –45 degrees). This technique is frequently seen for 3D presentations in digital IMAX theaters. The other polarization approach is to use circular polarization with an opposite direction for the polarization orientation used the right vs. left images (i.e., right hand circular vs. left hand circular polarization). This is the technique is used by RealD for 3D presentations in commercial digital cinemas. Only projection screens that retain very near to 100% of the polarization are suitable for use with such 3D projection systems (i.e., those that inherently rely on polarization to separate the right from the left images).
2. The second case is for screens used with active 3D projection systems where the projected light is polarized, but for which polarization is not used as the means to separate the right from the left images. In this case the orientation of the polarized light if fixed for all projected video. This broad category of 3D projection systems are those that sequentially alternate the right and left projected images and for which viewers are required to wear liquid crystal (LC) active shutter eyeglasses for viewing 3D programs. However, many and perhaps most of the currently available and announced consumer 3D projectors that use active 3D projection technology, do not project polarized light. The ability of a screen to retain polarization makes no difference in those cases where the light from the projector is not polarized (or technically is randomly polarized). For example, 3D projectors using DLP technology (e.g., Sharp XV-Z17000 or Optoma HD33) do not project polarized light and thus the ability of the screen to retain polarization makes no difference. Where it may make a difference is when the 3D projector is using LCoS or LCD technology. Projectors using LCoS and LCD display chips inherently contain a polarizing filter. Such projectors will project polarized light unless the projector also includes a subsequent optical element within the projector’s light path that de-polarizes the light. Projectors known to project polarized light include all of the JVC 3D models (e.g., DLA-RS60), which use LCoS technology. As of this time I have not been able to confirm if the 3D Sony and Mitsubishi LCoS (SXRD) projectors emit polarized light or not. Also it is not confirmed if the soon to be released Panasonic PT-AE7000 or expected Epson LCD 3D projectors will emit polarized light, or not.
The reason all of this can be important for at least some active 3D projection systems is because all 3D active shutter glasses also include a polarizing filter element (this is inherent to how the liquid crystal shutter lenses work). If the light being reflected off of the projection screen is polarized and has the same orientation as the polarizing filter within the 3D active shutter glasses then there will be relatively less light loss as the light passes thru the glasses. For the case where the light reflected by the screen is non-polarized then the polarizing filter within the 3D active shutter glasses will lose approximate 50% of the light. The final case is where the light reflected by the screen is moderately to highly polarized, but with the opposite orientation from the polarizing filter within the 3D active shutter glasses. In this case the glasses can lose most of the light to the point of being unusable. If you use the 3D active shutter glasses sold by the manufacturer of the 3D projector, there should be no possibility of having this situation (i.e., with severe light loss) since the glasses and the projector should have the some orientation for the polarization. However this can be a potential issue when using 3rd party ‘universal’ 3D active shutter glasses which might have the opposite (i.e., incorrect) orientation.
Screens for use with Passive Polarized 3D Projection Systems
For screens intended for use with passive polarization systems (i.e., that rely on polarization to separate the right from the left images), the screen must retain close to 100% of the polarization otherwise visible 3D cross-talk or ghosting will occur. This generally means that you must use a ‘Silver Screen’ material made specifically for use with such passive 3D projection systems For such ‘silver screens’ manufacturers frequently publish an “extinction rate” or “extinction ratio” which is a measure of the ability of a given screen material to retain the polarization of the projected light. This value is normally measured by projecting a highly polarized light onto the screen material then using a light meter to measure the light level thru a polarizing filter that is oriented the same as the projected light then measured again thru the polarizing filter oriented opposite to that of the projected light. Thus, for example, if a manufacturer specifies a screen to have an extinction ratio of 100 that means that when viewed thru a polarizing filter with the matching orientation the light passing thru the filter will be 100 times more than when the filter has the opposite orientation. In this example, that means a viewer will see the 3D cross-talk image only 1% as bright as the main image that is intended for that eye. The best extinction ratio I have seen being specified from the major consumer screen manufactures (e.g., Stewart or Da-Lite) in on the order of 150, which would produce a cross-talk image that is 0.67% as bright as the main image or stated another way approx. 99.3% of the polarization is retained for the main image. Most manufacturers normally only specify a extinction ratio for screens intended for use with polarized 3D projection systems and the specified value for extinction ratio normally applies to linear polarization (i.e., extinction ratio for circular polarization may be lower). Also note that the only consumer priced passive 3D projector currently available is the LG CF-3D while very high-end ($50,000+) dual projector passive 3D projection systems are also available. However, it is possible to put together a do-it-yourself dual projector passive polarized system at moderate cost (this will be discussed in a future blog). Suitable silver screens will be discussed again in a future blog when discussing passive 3D projection systems. Silver screens have a reputation for not being well suited for conventional 2D projection due to a tendency to have “hot spotting” and a limited viewing angle. As a result some consumers using a passive 3D projection system have installed two screens with the silver screen used only for 3D viewing. The typical arrangement for this is to use an electric drop down screen mounted just in front of a fixed screen. Some manufacturers realizing the growing consumer interest in 3D have over the past year introduced screen materials claimed to be better suited for both 2D and passive 3D projection. Such screen materials, while not optimal for either 2D or 3D, have the potential of providing an acceptable single screen solution for some consumers building a passive 3D capable home theater. Stewart’s dual use screen material is called Silver 5DTM (so called because 2D + 3D = 5D). Da-Lite’s dual use screen material is called Silver Lite 2.5TM.
Screens for use with Active 3D Projection Systems
Now moving on to screens for use with those active 3D projection systems that do project polarized light. As noted above, the only brand of active 3D consumer projectors that I have specifically verified to project polarized light are the JVC 3D projectors, which project horizontally polarized light. This does not mean that other brands of current or future LCoS or LCD 3D projectors do not, or will not, also project polarized light. Rather, I have no confirmed information as to whether they do or do not project polarized light (I will post if or when I have such information).
Now let us consider the case where the projector emits highly polarized light and you are using 3D active shutter glasses that have the same polarization orientation as projector. In the case of JVC both the projector and the JVC shutter glasses are horizontally polarized. This arrangement will work with virtually any screen material since the light reflected from the screen will either be non-polarized or will contain some ratio of polarized and non-polarized light. If using a screen that results in a total loss of polarization (or nearly so), such as typical for most gain 1.0 matte white screens, the glasses will lose approximately 50% of the light as it passes thru the glasses’ internal polarizing filter. However, if a screen material retains even a moderate amount of the polarization, for example has an extinction ratio of 3, then when viewed thru the 3D active shutter glasses the image 3D image on the screen will appear noticeably brighter. This is because the polarizing filter within the 3D active shutter glasses will pass more of the incoming light (as a result of the incoming light having a polarization orientation that matches the glasses’ polarizing filter). Thus using a screen material that retains at least some polarization can provide what is in effect a boost to the screen’s gain, as compared to a screen that does not retain any polarization. The following table shows the effective gain multiplier that would apply vs. the screen material’s extinction ratio.
For example, if you are using a screen with a gain of 1.5 and with an extinction ratio of 4 then the theoretical effective gain for 3D viewing would be:
(2D gain) x (screen gain multipler) = 1.4 x 1.6 = 2.24
Or in other words the image as viewed thru the 3D active shutter glasses would be more than twice as bright as compared to using a reference gain 1.0 matte white screen that retains no polarization. However, remember this boost in effective gain only works if you are using a projector that emits polarized light and if your screen retains at least some significant level of the polarization. Also this represents a theoretical maximum boost in effective gain and in the real world the actual effective gain increase may be somewhat less.
Unfortunately virtually no screen manufacturer routinely publishes the extinction ratio for screens except for those ‘silver screens’ they market specifically for use with passive 3D systems. Therefore, you cannot rely on the manufacturer’s web sites or specification sheets to tell you which of their screen materials retain significant polarization. Also it appears that manufacturer’s may change the specific composition of a given screen material, without impacting its 2D gain, or other performance parameters listed on the manufacturer’s spec. sheet, and as a result a screen material sold under a given name today may have a different extinction ratio (which are not specifying) than the screen material sold under this same name just a few months earlier. I have tested a set of sample screen materials, that I had received from Da-Lite about 2 years ago, for their ability to retain polarization. I cannot say with any certainty that the current version of these screen materials will perform the same as the version that I evaluated. With that qualification the following list groups these screen materials into their ability to retain polarization. Note that none of these screen materials have a really high extinction ratio (i.e., probably none greater than perhaps 5) and none would provide the very high extinction ratio (i.e., 100 or greater) as would be necessary for use with passive polarized 3D projection systems. Also I did not perform a measurement with a light meter, rather the ratings in this list are just my subjective estimates.
|Da-Lite Screen Material||Polarization Level Retained*|
|HD Progressive 0.6||none|
|HD Progressive 0.9||none|
|Matte White (gain 1.0)||none|
|High Power (gain 2.4)||none|
|High Contrast Audio Vision (gain 0.8)||low|
|High Contrast Da-Mat (gain 0.8)||low|
|Da-Mat (gain 1.0)||low|
|Audio Vision (gain 1.0)||low|
|HD Progressive 1.1||moderate|
|Pearlescent (gain 1.5)||moderate|
|High Contrast Matte White (gain 1.1)||moderate|
|High Contrast Cinema Vision (gain 1.1)||high|
|Cinema Vision (gain 1.3)||high|
|Video Spectra 1.5||high|
* Even those screen materials listed as ‘high’ for retaining polarization may only have an extinction ratio on the order of 4, while those listed as ‘low’ will have an extinction ratio that is probably on the order of 1.5 to 2, and those listed as moderate may have extinction ratios of perhaps 2 to 3. Again these are only intended as rough estimates.
My next Blog will give a little preview of the upcoming CEDIA Expo trade show, that starts next week (several of us from Projector Reviews will be attending). After the next 2 or 3 blogs that will be associated with the CEDIA Expo, I plan to come back with a wrap up of the discussion on Screens for 3D Projection and then continue the discussion 3D projection technologies.