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Getting it Bright Enough

This blog is the follow-up to my earlier blog from before the CEDIA show.  That earlier blog was from August 31 and was titled “Screens for 3D Projection – Part 2”.  This new blog will be my wrap up, for now, on projection screens for 3D, but I’m certain there will addition future blogs that will cover the topic of screens. I had noted in my earlier blog that JVC 3D projectors have been confirmed to project linear polarized light with a horizontal orientation.  I had the opportunity to check one of the new Epson 3D projectors (the 6010 as I recall) during a demo and found that it was also projecting polarized light with all 3 of the primary colors (i.e., red, blue and green) having the same orientation (I don’t recall if the orientation was vertical or horizontal).  In any case, the discussion in the earlier blog about the potential for getting an effective boost in 3D image brightness through the use of screen that retains at least some significant amount of polarization should equally apply to home theaters equipped with JVC or the future Epson 3D models. As for screens that are specifically being sold for use with passive 3D projection systems, where polarization is used as the means to separate the right from the left image streams, I noted a few screen manufacturers have recently introduced new “silver screen” materials, or when questioned at CEDIA indicated such new screen materials are in the works.  This new generation of “silver screen” materials are claimed to also be more acceptable for 2D projection as well as having the high extinction ratio required for polarized passive 3D projection).  This means these new screen material achieve an extinction ratio of 100:1 or greater (see my early blog for a discussion on extinction ratio).  I have just recently received a sample of the Da-lite Silver Lite 1.5TM screen material and have requested a sample of the Stewart 5D screen material which are two such screen materials that are already available. One question that frequently comes on on-line forums when discussed projector-based home theater system is what size and gain of screen is need to produce a bright enough image with a given projector.  I’m providing the following information in the hope of providing some guidance to assist those of you attempting to select a projector and screen (size and gain) for your home theater.  I will first make 2 assumptions that are based on a fairly ideal home theater arrangement.

First – the home theater has total light control.  In order to get really low black levels and a high contrast image you must eliminate virtually all sources of light except for that coming out of the projector.  If your screens appears white or grey with the projector turned off and with the room lighting as you have it for movie viewing, then that is as near black as you will ever see once the projector is turned on.  Just remember that projectors can project light and they cannot project dark.

Second – the home theater has wall and ceiling painted a dark color in order to reduce light being reflected back toward the screen.

Now let’s get into the most important characteristics for  projector and for screens necessary to achieve and adequately bright image in your home theater. Projector’s Light Output My first golden rule for projectors is to never assume the lumens listed in the manufacturer’s specification sheet has any relationship to reality for using that projector in a home theater application.  Just about all projectors will have a mode that is intended to provide the maximum possible light output.  This mode may be called such things “dynamic” or “vivid” and while it usually will produce a lumens output near that specified by the manufacturer for that model of projector, this mode is normally not suitable for use in a home theater environment.  Such high light output is typically achieved by sacrificing accurate color, grey scale, color temperature and black level.  Some projectors when properly calibrated for critical viewing may produce a lumens output of perhaps a much as 80% of the lumens in the maximum brightness mode.  However, other projector may produce less than 30% of the manufacturer’s specified lumens output once calibrated.  So the bottom line is you need to find out what is the actual light output of the projector you already own, or are considering to purchase.  One good way to do this is to read the reviews Art has posted here on Projector Reviews where you will find out what were the actual measured lumens once the projector was calibrated to the D65 video standard.  One other point to consider is the lamps used in video projectors (not applicable for those few projectors that use LEDs instead of a lamp) will dim with use.  A decrease in light outputs of 25% to 30% is typical during the first few hundred hours of use and the rated life of a bulb (specified by the manufacturer in hours) is for when the bulb is predicted to lose 50% of its original light output.  Therefore, a projector that is measured in “best mode” to put out, for example, 600 lumens may only put out perhaps 450 lumens after 500 hours of wear on the lamp and eventually only 300 lumens as the bulb approaches what the manufacture considers the end of the bulb’s life (typically somewhere between 2000 and 4000 hours).  So it is a good idea to assume at least some bulb dimming (e.g., 30%) when attempting to forecast how bright an image you can expect with a specific projector and screen combination.  Also another factor to consider when reading reviews of projectors is to note where the projector’s zoom lens was set, or if the projector was position for short, mid, or a long throw distance.  A projector’s zoom lens will normally have greater light loss when set for a long throw distance vs. when set for a short throw distance.  Or expressed another way, a zoom lens will have more light loss when set to it maximum focal length (i.e., min. picture size) vs. when set to minimum focal length (max. picture size).  Projectors having a wide ratio zoom lens (e.g., 2X zoom) will normally have a greater change in lumens output over the zoom range as compared to projector with a more limited zoom range (e.g., 1.2X zoom).  Most projector measurements provided here at Projector Reviews will have the zoom lens set to a mid-throw position but will provide information to indicate how much the light output will increase at shortest throw and how much the light output will decrease at longest throw.  So you will need to determine for a given projector and screen size if your mounting location for the projector will place it at a short, mid or long throw position.  Some manufacturer’s web sites provide a projector setup calculator that will show the throw range supported by the projector’s zoom lens for a given screen size and/or you can download the owner’s manual which will have a section on the projector setup/installation that will provide the required information. The table below shows, for a few popular projector models that have been reviewed here at Projector Reviews, the manufacturer’s specified lumens and also the measured lumens in high lamp mode once the projector was calibrated for the most accurate colors, grey scale, etc. (ie., the “best mode”) as well as info on the projector’s zoom lens ratio. All of these lumens measurements were performed with the projector positioned at a mid-throw distance. See the full review (click on the projector model) for the complete set of measurements.
Projector Model Rated Lumens Zoom Ratio Calibrated Best Mode Lumens
Epson Home Cinema 8350 2000 2.1X 467
BenQ W6000 2500 1.5X 866
Panasonic PT-AE4000 1600 2X 430
Mitsubishi HC9000D 1000 1.8X 701
Sony VPL-VW90ES 1000 1.6X 588
JVC DLA-HD250 1000 2X 758
As you can see from the above table there can be a significant difference in the manufacturer’s claimed lumens as compared the actual measured lumens once the projector is calibrated for the best picture. Screen Size and Gain Next let us consider the two most important characteristic of projections screens that impact the image brightness. Size matters when it comes to projection screens.  If you double the size of a screen, for example going from a 80 inch diagonal size to a 160 inch diagonal size, the larger screen will have 4 times the area and as a result (given everything else equal) will produce an image that is only ¼ as bright as with the smaller screen.  Screen gain (as described in my earlier Part 1 Screen blog) will provide a brighter image to the viewers seated within the optimum viewing cone (i.e, toward the center of the screen and not too far toward the sides).  Higher gain screen typically have narrower viewing cone as compared to lower gain screens.  Manufacturer’s web sites will typically specify the nominal gain for the screen materials they offer.  This will be the peak gain for when the viewer is seated directly in front of the screen center and the projector is also centered horizontally with the screen.  High gain screens may have a large enough drop in gain as the viewer moves toward the side of the room that you may want to consider that reduced gain when estimating how bright you actual image will be.

Estimating Image Brightness

The industry recommendation for the image brightness in a light controlled home theater environment is approximately 16 foot lamberts (with a minimum of 12 foot lamberts).   This was discussed in one of my earlier Part 1 Screen blog (HERE).  So how can you estimate in advance if the projector and screen setup you are considering will satisfy the goal of providing the recommended 16 foot lamberts (or more).  For this you will need to know the following information:

- Estimated actual projector lumens output (from reviewer’s measurements)

- Screen size (aspect ratio and diagonal size in inches – HD format screens have an aspect ratio of 1.78 (to 1) while “scope” screen have an aspect ratio of approx. 2.4 (to 1).

- Estimate of screen gain for your primary viewing locations (from screen spec. sheet)

For 3D projection the effective lumens, when accounting for both the projector's light ouput but also subtracting for the light loss through the 3D glasses, will be far less than for viewing 2D video with the same projector.  For 3D systems using active shutter 3D glasses the effective 3D lumens is typically about 20% as much as when using the same projector for 2D viewing.  Therefore, it is generally a good ideal to increase the goal for the 2D brightness (from the screen) from 16 foot lamberts up to at least 25 foot lamberts  so that when used for 3D projection there will still be at least an effective 5 foot lamberts available (also considering the light loss through the 3D glasses). I have attached an Excel spreadsheet file (below) that can be used to calculate the estimated image brightness, in foot lamberts, once you enter the above basic information.  If you have Microsoft Excel (part of Microsoft Office) installed on your PC simply download and open the attached file with Excel.  If your PC does not have Excel installed, you can download an Excel viewer from Microsoft .  Just download and install the Excel Viewer program and then you should be able to open the attached spreadsheet file. Click to download the Excel spreadsheet >>>>> ProjectionCalc ______________________ For my next blog I will be discussing DLP projector technology for 3D

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