Using Projector Throw Ratio to Determine Audience Placement Relative to Screen
Ah, the screen. It’s a fundamental element in so many different types of events. It must be large enough and positioned properly for everyone to see projected images clearly, even from the back row. Yet it shouldn’t be so big it dwarfs other stage elements or looms over the audience members down in front. But the screen doesn’t work by itself, it has a companion element – the projector. So where should that go? It’s actually all about math.
Let’s talk about throw
A projector “throws” light onto the screen to display images. The projector’s distance from the screen determines the size of the image as well as its clarity. Too close, and the image appears tiny compared to the screen. Too far, and resolution and color suffer or the image bleeds off the screen.
Aspect ratio – screen width compared to its height – is also important because the content you plan to project will have a certain aspect ratio, typically 16:9 or 4:3. Your screen will need to be the right width and height, or your image won’t look right. And the angle of orientation also matters because off-center projection produces a skewed image.
But throw distance is the biggest factor when it comes to choosing the right size screen and placement in relation to your audience. Remember algebra? (Don’t worry – the formulas aren’t that complex.) Here’s how to calculate your screen/audience placement:
The projector’s throw ratio (TR) is measured as the throw distance (TD) relative to image width (IW). So, your basic formula is:
TD / IW = TR
You can plug in any two numbers and calculate the third. For example, most projectors have a throw ratio of 2.0 which means the projector must be 2 feet from the screen for every foot of image width for optimal image quality. You can use this formula to figure placement in two ways:
- You know the desired image size (on the screen). Let’s say that’s 6 feet. In this case, you’ll need to locate your projector 12 feet from the screen. You know that because: TD (your unknown) = TR x IW, or 2 x 6 feet.
- You know your set-up requires placing the projector 15 feet from the screen. In this case, your image will be 7.5 feet wide. You’ll need a screen that accommodates that size image. You know that because: IW (your unknown) = TD/TR, 0r 15 feet divided by 2.
Here’s another scenario. Sometimes, you may know your screen size and where your projector will be located, so you’ll need a projector that uses the proper throw ratio. Simply use the basic formula. If your image must be 20 feet wide and your throw distance will be 30 feet, then: TR = TD/IW, or 30 feet divided by 20 feet, or 1.5.
Throw ratio is inverse to image size, so a projector with a small throw ratio can project a larger image from a shorter distance.
Tip: Using a projector with a zoom lens can make positioning easier because you’ll have some leeway. For example, if your projector’s zoom ratio is 1.2, the “.2” indicates you can vary the throw distance by 20%.
Note that the images you plan to project might be PowerPoint slides with text, charts, and graphs, or a video, or a gigantic photograph, or simply a colorful pattern. The source may or may not be digital. Each type of content has somewhat different requirements, including format (aspect ratio), to produce optimal image quality, and that can also affect ideal throw distance.
Other placement considerations
Of course, it’s not all about math. There are other factors to consider, especially the size of the audience and the height of the ceiling:
How big will your audience be?
- For a small audience, you can use a free-standing screen (on a tripod). These are both adjustable and easily portable, giving you great flexibility in positioning the screen.
- For a medium-size audience, a larger fast-fold screen (with legs at each side rather than a tripod in the middle) can be a good option.
- For very large audiences of 1,000 or more, you’ll want a custom-designed set-up that will likely include rigging a truss screen. Or you can scrap the projector altogether and use an LED screen to create a truly customizable wall or panel display.
How high is the ceiling?
Ideally, the bottom edge of your screen should be between 40” and 48” from the floor if everyone is seated at floor level, between 24” and 36” if you’re using theater-style seating or you’ll have just a couple of rows or participants. Obviously, large screens require more height, perhaps as much as 10 or 15 feet.
What if you don’t have enough room?
Not every venue is perfect for designing your AV set-up. There could be columns or other architectural features that block lines of sight no matter how you try to work around them. The space may be an awkward shape. Or it might be just too small to provide the throw distance you require based on where you’re able to place your screen and projector.
No sweat. Your AV team has a simple solution for that. Two solutions, actually. You can switch to rear projection, or go with LED screens. In fact, there is a customizable, extra-creative application with LED screens called CONTENTCANVAS™ that can turn your standard stage setup into a dynamic LED wall of interactive shapes and screens.
Unusual challenges are the norm for experienced audiovisual professionals, so whatever your projection needs, your AV team should be able to find (or create) a solution that works and delivers the audience experience you envision.