The six key things to think about when designing a front projection system

by Nyal Mellor May 22, 2013

We at Acoustic Frontiers love to use projection systems. They are really the only way to get wide enough viewing angles to properly replicate the immersive cinematic experience in your home.

There are two main types of projection system:

• Front projection. This is the type of setup used in 99% of theaters and is where the projector is in front of the screen.
• Rear projection, where the projector is behind the screen. This type of design is used infrequently, typically when the viewing environment has high ambient light.

Putting together a front projection system is not as simple as just picking a projector and screen based on reviews. For best results it must be engineered.

This article discusses some of the key factors that must be taken into consideration when designing a front projection system. A future blog article will discuss rear projection systems.

1. Screen type is appropriately selected for the viewing environment.

Screen material. There are three main types of screens in usage today:

1. White, non-acoustically transparent screens. We seldom use these unless doing a retrofit in an existing theater where the speakers are located outside of the screen boundary. Ideally these screens need fully light controlled environments with no windows or at least black out roller shades on all windows.
2. White, acoustically transparent screens. These are our favorite type of screen as they allow the center speaker to be located behind the screen where it belongs
3. Black, light rejecting screens. These screens reject light that hits the screen at off axis angles. They are great for use in rooms that have poor light control or where the client wants to keep some lights on during room usage. An example would be someone who likes to watch sporting events on their big screen. It is more fun and more practical to watch these events with ambient light on, especially when entertaining.
   
   

Usage.  A large projection screen is optimized for high definition content sourced from blu-ray discs and may not look it’s best with DVDs, TV or other low quality sources. Many HTDV stations are poor quality sources due to visible artifacts arising from excessive video compression, data losses during broadcast and error correction. Many people also find the sensation of watching news on a large projection screen odd, because presenter’s heads can be 3ft in size! In cases like this a good solution is to use a flat panel for casual viewing and low quality sources and a projection screen that drops down over the TV for blu-rays.

Aesthetics. We think that fixed projection screens visually dominate a room. This may be fine in a dedicated theater but in a mixed use media room we prefer using motorized screens that can be retracted into a ceiling or soffit when not in use.

2. Aspect ratio is appropriately selected to match typical viewing habits.

We like to design our systems so they match the content that our  clients typically watch. Usually this means a 2.35:1 aspect ratio screen but on occasion we might suggest something alternative such as 16:9 if they want to use their screens mainly to watch sporting events or foreign movies. See our blog article for more information.

Projectors have two key variables which must be taken into account when laying out the system: throw ratio and lens shift. These metrics should be provided by the projector manufacturer.

Throw ratio typically quoted as a range – for example 1.82-2.48:1. Throw ratio together with screen width tells you the distance from the screen to mount the projector:

Screen  width* x Throw ratio = Throw distance

* If you are using a 2.35:1 constant image height setup (via an anamorphic lens) then you need to use the 16:9 equivalent screen width, not the 2.35:1 width when factoring throw distance.

Lens shift is the other variable that must be taken into account when determining projector placement. There are two types of lens shift – vertical and horizontal.

• Vertical describes the ability to position the projector above or below the screen centerline, properly defined as a percentage of screen height. 50% lens shift is half screen height (i.e. the projector can be mounted at the top or bottom of the screen and still fill it), 100% lens shift is full screen height.
• Horizontal the ability to position the projector left or right of the screen centerline.

Lens shift ranges are frequently poorly documented by manufacturers. Many DLPs have more lens shift up than down yet only a single figure is quoted. Others will misstate the lens shift. For example BenQ’s specifications are wrong and incorrectly correlate the projector being placed level with the top or bottom of the screen as 100% lens shift capability whereas in actual fact it is 50%.

There is one other point worth mentioning and it is that many projectors have ‘sweet spots’ in their throw range where the optics are performing the best with the least chromatic aberration and light loss. We do our best to test all the projectors we represent in order to characterize their lens shift capabilities and throw ratio sweet spots. This attention to detail ensures our installations maximize the picture quality from any given projector.

3. Image brightness meets standards.

Throw distance is typically a range because most projectors have optical systems with zoom and hence variable throw ratios. If the projector is mounted outside the throw distance range then you either have an image that spills over the edge of your screen or does not fill it, neither of which are good things. It seems crazy that this would happen but believe us it does, and with frightening regularity!

The image brightness or luminance that you get from a front projection system in a light controlled environment should meet industry standards of 14ftL. Luminance is related to projector output in lumens, screen gain and screen area. We see many people try to use very big screens with projectors that have insufficient light output. The result is a dim, washed out image. See our blog article for more details on designing for luminance.

4. Viewing angles meet standards.

The angle subtended from each side of the screen to the seats in a home theater should meet industry standards. We prefer 45 degree or larger viewing angles for an experience that replicates a commercial cinema. See our blog article for further details.

5. Room decor is neutral.

The interior decoration of a home theater should not negatively detract from the movie watching experience. The job of a screen is to reflect back light from the projector towards the viewer. This reflected light will strike nearby surfaces such as floor, ceiling and wall. If these surfaces are reflective then this light will then in turn be reflected back towards the viewer. In the worst cases a ghost of the onscreen image might be visible on the offending surface. Suffice to say that dark matte paint finishes or dark fabrics are obligatory for the best visual experience.

The other consideration is the wall to which the screen is mounted. For highest perceived dynamic range in the image this wall would be black. We like mounting our screens to baffle walls, covering the front of these walls in a black acoustically transparent fabric.

Have we missed any important aspects of front projection system design? Please let us know via the comments below!

Need help engineering your home theater front projection system? Contact us now! Our favorite and most commonly used front projection brands are SIM2, JVC, Epson, Seymour Screen Excellence / Seymour AV and Screen Innovations.

About the author: Nyal Mellor
is founder of Acoustic Frontiers, a company specializing in the design, installation and calibration of high performance home theaters for enthusiasts
worldwide.

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