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December 2007

Light Wars in Your Sanctuary

By Thomas Stewart

Ambient Light vs. Projector Output & Screen Gain/Contrast


The world of Rear Projection Technology has and still remains somewhat of a mystery to many in the church audio visual field. This article is designed to help clear the air and take the mystery out of this very effective way to get your message to the church body. Rear projection has many advantages over a standard front projection video system. Some of the advantages are; better image contrast and image resolution, wide viewing angles, hidden projector as well as projector noise reduction. In addition rear projection systems are able to project with higher ambient light in the sanctuary (better contrast ratios), the use of first surface glass mirrors or mylar mirrors to reduce space requirements behind the projection screen, and a stable image with the use of a rigid substrate, (glass or acrylic rear screens).


Image Contrast/Resolution
Most rear projection screens used in churches today are of the rigid rear diffusion technology type. This type of rear screen when tinted neutral grey offers better image contrast levels used in high ambient light installations. Typically the rigid rear projection screen is mounted in an opening on both sides of the sanctuary stage and the rear projection room acts like a shadow box to block out stray light behind the screens. A good rear projection diffusion screen will give you the highest resolution possible to create a clear image. When viewing projected images, the human eye prefers screen images with high dynamic contrast, simply meaning, deep blacks and bright whites. The blacker the blacks and the whiter the whites, the more pleasing the image fidelity will be.


Wide Viewing Angles
A neutral grey rigid rear projection screen with a screen gain of 1.0 has a wide viewing cone of up to 120 degrees. So in other words the congregation can be seated at a 60 degree angle each side of the center of the screen and still see a high quality bright and uniform image.


Hidden Projector/Noise Reduction; A major advantage to a rigid rear projection system in a church is the ability to hide the projector in the enclosed space behind the screen. Many churches have cathedral ceilings which do not lend themselves to hanging a projector out in the middle of the sanctuary and projecting front projection in harsh ambient light conditions not to mention the lack of aesthetics involved. Rigid rear projection offers a clear high contrast image that is noise free and has a minimal impact on the architecture of the sanctuary.


Single/Dual/Wall Mount Mirror Systems
Mirror systems come in three (3) major configurations. The first type would be the Single Mirror System that can be a stand alone system or mounted to the projection room wall behind the rigid rear projection screen. This mirror system serves the purpose of “folding the light path” from the projector once to cut down on the throw distance required by the projector lens. So the projector is actually placed behind the screen in the direction so the lens is pointing toward the back wall of the projection room and the image is “folded” to reflect off the mirror toward the back of the rigid rear projection screen.


The Dual Mirror System is used when the space behind the screen is more limited and the light path is “folded” twice. The geometry of “folding” the image twice needs to be left up to the experts who will design mirror and projector mounting systems to meet your space and image size constraints.


The Wall Mount Mirror System consists of a framed single mirror with wall mounting brackets that is usually mounted on the back wall of the projection room and “folds” the image once, but doesn’t require a mirror and projector stand.


There are two types of mirrors that can be used with a rear projection system. The first and most popular is the First Surface Glass Mirror, (the silver coating is layered on the front surface that faces the projected light path, unlike your bathroom mirror which is layered on the back surface reflecting through the glass. This type of mirror is 94% reflective so most of the projected light is getting to the back of the rear projection screen. The other type of mirror surface is Coated Mylar. A high quality mylar surface is about 89 to 91% reflective but has an advantage over first surface glass in the fact that they come in larger sizes. The larger the desired image size the larger the mirror or mirrors will need to be. Again leaving this part of the rear projection system design in the hands of the audio visual experts is very much advisable.


The importance of Ambient Light Control in the Sanctuary
The more ambient light (both natural and sanctuary lighting) you can direct away from the rear projection screen the better the image contrast will be. So having the ability to dim and redirect the lights near any screen surface is an important feature when designing or remodeling a church sanctuary. With new construction it is advisable to get a lighting designer involved in the design of the sanctuary lighting. There are many rules to a successful visual installation related to rear projection technology in the church and some of the more important ones are:


Knowledge of the “Ambient Incident Lumens” (by using an incident light meter), which measures ambient light in foot lamberts. A very acceptable foot lamberts reading in and around the screen surface would be 10 to 15 foot lamberts of incident ambient light.


Know the projector brightness in lumens; Projectors are all rated by their Ansi Lumen output (i.e. 1,000 ansi lumens to say 10,000 to 20,000 ansi lumens) depending on the technology and your budget. Generally speaking the higher the lumen output of the projector the higher the cost. Also, the larger the desired image size (square feet) of the screen the greater the demand for more lumens from the projector, (you are spreading the light over a greater surface area therefore needing more brightness from the projector). This same thought holds true related to the incident ambient light in the sanctuary, i.e.; the more incident ambient light the greater the need for a high brightness projector, (higher ansi lumen rated projector).


The importance of the right rear projection screen peak gain and color/contrast
Peak Gain of a rear projection diffusion screen refers to the measurement of the amount of projected light that flows through the screen and is seen by the viewer in the sanctuary. An example of a higher gain rear projection screen would have a gain value of say 1.5 to 2.0 gain and the lower gain rear screens will have a peak gain of 0.60 to say 1.00 gain. A general rule of thumb would be to say that higher ansi lumen rated projectors will need lower peak gain rear projection screen to present the best image possible and conversely lower lumen rated projectors will need higher peak gain rear screens. Remember that image size is important here related to projector brightness output, i.e.; larger image sizes need brighter projectors and higher peak gain screens. The trade off in higher gain screens is a narrower viewing angle and a possible hot spot toward the center of the image. When looking at purchasing a projector for a rear screen application it is best to consult the projection screen professionals to get the best possible screen for your installation.


Neutral density grey rear projection screens can be a big help in fighting light wars in your sanctuary by enhancing the image quality giving the viewer better contrast when viewed with higher (15 to 20 foot lamberts or more) ambient light near the screen surface. Again the blacks are blacker with a translucent grey rear screen which increases the dynamic range of the image. One rear projection screen actually has the optical layer embedded between two sheets of glass similar to the laminated process of the windshield on your automobile. The advantages are many and some of them are; This method dramatically increases the amount of projected light moving through the screen enhancing brightness and image quality, while reducing unwanted backscatter, (reflected light back toward the projector).


This makes the image uniform and eliminates visible hot-spotting from the projector. The combination of the screen layer and the glass substrate create extraordinary image contrast. Add to this an anti-reflective (AR) coating to reduce unwanted reflections and the image quality is excellent even in higher ambient light environments.


The standard rigid rear projection screen is either a laminated film on the viewing side or the optical coating is flow-coated also on the viewing side. The substrate of these screens can be either be acrylic or glass depending on the need. The key to an excellent projected visual system is maximizing “The Total Environmental Dynamic Range”. This term coined by a projector manufacturer refers to the actual contrast ratio achieved in a venue such as a church sanctuary. The battle over ambient can be best fought by reducing ambient light near the rear projection screen or screens so that the congregation can read but still have a high contrast screen or screens to view during the service. It has been said “The Battle Belongs to The Lord”, and this is very true, but your audio visual professional and projection screen manufacturer can be of great assistance when it comes to “Light Wars in your Sanctuary”.

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