Choosing a Video Projector

By Clay Stahlka, CTS-D, Starin Technical Solutions

There are three questions that always need to be answered before we can choose the proper projector in any space . 1 . Required image size (and video format e .g . 16:9 or 16:10) 2 . Light level in the room 3 . Distance from projector to screen Once you have this information, projector selection can begin .

Required Image Size

We should understand that there is a basic formula that determines the minimum image size requirement . Measure the farthest viewing distance from the screen location (in inches) and divide it by four . This will give you a rough idea of the minimum diagonal image size in inches . If the farthest viewer is 30-feet from the screen, here’s your calculation:

30-feet x 12 = 360 inches 360 ÷ 4 = 90-inch diagonal screen (minimum)

We probably won’t find a 90-inch screen, so let’s round up to a 110-inch 16:9 screen which is readily available .

NOTE: If you’re displaying details that need to be read, (like a lot of text), you should divide by 3 instead of 4, If you’re displaying highly detailed content of small sizes, (like a detailed spreadsheet with many rows of small numbers), you should divide by 2 instead of 4 .

Light Level in Room

The room’s lighting, along with the screen size will determine the required brightness of the projector . It is important that you take the time to measure the lighting level in the room . Using your smartphone, get a light meter app from your app store . They’re free . I use one called “Light Meter .” Measure the room lighting with your light meter app from the viewing area of the room, as it will be lit when projecting . If there are windows, try to take the measurement when the sun is brightest . The number should be displayed in LUX . The formula to find the minimum projector brightness is:

Foot-Lamberts (fL) x screen diagonal x screen diagonal ÷ 337 x screen gain

To get Foot-Lamberts (fL) from LUX, divide LUX by 3.426.

Since most projection screens have a gain of 1, all we must do is multiply FootLamberts (fL) by the diagonal (in inches) and then by the diagonal (in inches) again, and then divide that number by 337. If you’re using a screen with gain, multiply 337 x gain and use that number instead of 337 in the formula .

Let’s say we measured the room lighting at 250 LUX .

Using the 110-inch diagonal screen we chose above, here’s the formula:

250 ÷ 3.426 = 73 fL 73 x 110 x 110 ÷ 337 = 2,621 Lumens (minimum)

Again, we won’t find any available projector with a brightness of 2,600 Lumens, so we round up to choose a 3,000 Lumens projector. You can always go a little brighter on the projector because most people perceive brightness as clarity. However, it is not good practice to go brighter than halfagain the required brightness . An image that is too bright can cause eye-fatigue for the viewers .

Distance from Projector to Screen

Finally, we know the screen size and we know the projector brightness . All we must do is find a projector of that brightness that will fully fill a screen of that size, at a selected distance. This part usually doesn’t require any formulas because we can use the manufacturer’s lens calculator online . You should know the formula, nonetheless.

The ideal place to mount the projector is about 15ft back from the screen

Throw Distance ÷ Image Width

In our above example, we are using a 110” diagonal, 16:9 screen, which is 8’ wide,

(multiply width x 0 .87) . The ideal place to mount the projector is about 15ft back from the screen . Here’s the calculation:

The projector lens we need is 1 .875:1 lens ratio . Many small projectors of this brightness have a zoom lens with a ratio of around 1.39 to 2.09:1. So, the 1.875:1 lens requirement falls right in the middle of the zoom range, which is perfect. Don’t select a lens and projector combination with a lens ratio that is too close to the extents of the zoom. If you do, and your measurements were off by a few inches, you might not be able to focus the image .

15 ÷ 8 = 1.875

Conclusion

We now understand how the answers to the three basic questions above will make buying the correct projector easy and foolproof . All we have to do next is brush up on our math .