Many environments could be right for projector-stacking, but you may not be aware of it. First, let's take a look at what projector-stacking is, and then we can examine what you need to know before attempting it.

Projector-stacking is different from edge-blending—another multi-projector technique we often hear about—in that rather than using two or more projectors to create a wider-than-normal display, projectors are stacked vertically to create a brighter, single display of more conventional width. You are combining the lumens of each projector to create one image that is double, or nearly double, the brightness that could be supplied by a single unit.

ISE Epson Projector Stack Screen
At this year's recent ISE trade show in Amsterdam, Epson used a pair of stacked 30,000-lumen laser projectors to effect a bright image on a large screen over their booth.

As an example, I was recently working on a proposal for a church and they needed 14,000 lumens of intensity to provide an acceptable image on the size screen they are using. However, a 14,000-lumen, high-definition laser projector was outside the price range of the client. Fortunately, by stacking two 7,000-lumen HD laser projectors, we could get them where they needed to be without breaking the bank. This is because the premium associated with projectors at 10,000 lumens and above remains high, while the cost of those below that brightness continues to drop.

This is a clear example of how budget can be a factor when determining a projection solution; however, sometimes the purpose of the projection system can be the most important factor. With some 3D systems, for example, two projectors are required to provide a true stereo image. In some 3D configurations, four projectors may be used to provide enough brightness while also providing the necessary stereo image, using two stacks of two projectors.

The idea of stacking multiple projectors is not a new one, having been around since the days of low-brightness, three-gun CRT projectors. What has changed, though—along with the increased brightness of today's digital projectors—is that the ease of setting up a successful projector stack has gotten much, much easier. Most manufacturers even provide their own proprietary software or systems to help the installer or end user accomplish this task. Some high-end manufacturers provide camera-based alignment to help make the process even easier, and most better commercial projectors will provide geometric correction, warping, and other blending tools built right into the menu system to help you navigate what is still a somewhat complex operation. If that does not provide you with enough options, there are many third party companies that also provide solutions, which can be helpful if you have projectors from different manufacturers. However, best practice is to ensure that you have matching projectors whenever possible. Differences in color, brightness, processing, etc., can lead to a lot of trouble when trying to match projectors.

NEC Projector Stack
Stacked projectors can sometimes be placed directly on top of one another, but various rigging and racking systems for aerial mounting are widely available.

You may have seen videos over the last few years of impressive architectural projection displays using projection-mapping. Many of these are accomplished using both stacking and blending in order to provide the bright, vibrant images that amaze audiences.

Is Projector-Stacking Right For You?

So, what do you need to know about projector-stacking before deciding if it's right for your environment or attempting to do it yourself?

(1) First, figure out what size image you need for your space. There are a couple different methods to accomplish this, but I follow the one that is based on furthest viewing seat away from the screen (measured in feet), then divide by 8, which gives us the screen width (divide by 5 for home theaters). A simple calculation of dividing by 1.78 (for 16:9 aspect ratio) or 1.33 (for 4:3 aspect ratio) can provide the height.

(2) With the screen size information in hand, calculate the square footage, then divide the number of lumens you think you might need by the square footage and multiply by the screen gain (start with 1.0). This will provide a foot-Lambert measurement. The general consensus is that if you have a little ambient lighting, 40 ft-L will work for you, but if you have high ambient light that you cannot control or that needs to be on during projection, then 60 ft-L would be better for your needs. Keep increasing the lumens in the first calculation until you get the ft-L measurement that is right for your space. If your needed lumens are above 12,000, then projector-stacking might begin to make sense for you.

(3) Should you choose to go with projector-stacking, understand that you will need enough power available at the mounting location for both projectors.

(4) Ensure that you have the same signal and source coming to both projectors. One easy way to do this is to use a small 1x2 distribution amplifier to distribute the same signal to both projectors. Or, you can go the traditional route of running two discrete signal cables from the same source to each projector. This can build in redundancy, which is not a bad thing.

(5) Make sure that the projector model you are looking at is compatible with whatever stacking system or software utilities the projector manufacturer provides.

(6) Find the mounting solution that is right for your situation. Are you mounting from a catwalk or truss? Are you ceiling- or wall-mounting? Know the weight that needs to be supported and understand that hanging equipment over people requires a specific skill set and knowledge base, such as working load limits, etc. If you are not qualified and/or educated in doing this safely, hire a professional!

(7) Read up on proper stacking procedures before you install so you know the process of blending the two images together into one.

This Epson instructional video shows how to use geometric correction tools to manually align a projector stack, but manufacturers often offer camera-based utility software today to automate and speed the process.

Even if projector-stacking isn't right for your current installation, keep it in mind for situations that may arise in the future. Small churches (1,000 members or fewer) generally shouldn't have to worry about using stacks, but medium-size churches might want to look into it if audio/visual excellence is part of their priorities.

Projector stacking doesn't have to be overly complicated, but it can become so if you don't properly understand it. The goal is to provide the right brightness without any ghosting or other image issues in as economical way as possible. If you follow the best practices and procedures, the results can be extraordinary.

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