How Bright is Right?

When we’re deciding on a projector and screen (or screen placement), the question that always comes up is, “Will it be bright enough?”

Lots of folks think that they can just get a brighter projector and that will solve their problems, but it ain’t always that simple. The more common problem encountered when we’re trying to project in a well-lit space is the contrast — or lack thereof. No matter how many lumens the projector outputs, if there is too much ambient light we will have a poor image.

When we look at a screen, we see a white surface. With respect to the video image, the ambient light level on the screen will be video black. That is the darkest level the image can possibly reach. So in a well-lit space, this can be a problem.

Digital Projection has a very useful screen brightness calculator on their website (http://www.digitalprojection.com/content/ view/149/114/) as well as a detailed explanation of how it works. It’s interesting to put some numbers in it and play around a bit. Take the example given (see Fig. 1).

The first problem we will encounter is that we are losing some light output by cutting off some of the pixels to create an image with a 16:9 aspect ratio. Next, we are using a very large screen. We are spreading the available light over a large surface area. Finally, we are front projecting, so the image is reflected light rather than direct light. That means that the reflectivity of the screen, or screen gain, also comes into play. Screen gain is a measure of how much of the incident light is reflected from the screen compared to a screen coated with titanium dioxide. Some screens reflect more light than a titanium dioxide screen, but no video screen is a perfect reflector; otherwise it would look like a mirror, and all you would see is the lens of the projector. This is part of the reason why a rear projected image works better; we are looking into the lamp rather than at an image that is bounced off the screen.

Back to our contrast issue. In a staging situation, be it concert, corporate AV or whatever, we can focus all of our stage light away from the screen. In these scenarios, house lights are very dim or off, and all the stage lighting is directed away from the screens, or kept off of them with shutter cuts or barn doors. This means we can have very low ambient light on the screen, which produces a higher contrast level.

For a permanent installation such as a board room or a church the ambient light levels are higher and so we begin to see the impact of ambient light. Look at the second example. The only change was to double the amount of ambient light on the screen from 10 to 20–foot lamberts. The screen still shows 47.4–foot lamberts but now our contrast ratio has fallen from 5.7 to 3.4 and leaves us with a washed out looking image.

|We can overcome this by adding a second projector and jumping to 20,000 lumens, but somehow I think it would be a lot easier to just turn the lights down a bit. The problem with adding another projector to the mix is that, in addition to the cost, if we drop the ambient light back down, the image can glare, or be seen as too bright.

Our other option in the install would be to do a light fall study and see if we can direct the light away from our screens or go to rear-screen projection. Another alternative would be to set up a lighting zone so that the lights around or over the screens can be switched off as needed. Yet another way to address this can be to go away from matte white screens. Screens with a gray finish will lose some of their reflectivity, typically with screen gain of about .8, but they give back some of the contrast that is needed in these situations.

All in all, when you’re selecting your projection system, consider the application. What kind of environment will the image have to overcome in order to be seen properly? What are the challenges involved? Work with your lighting designer to find a way to make it all happen. More and more I am seeing the line blur between video and lighting, and I think this can be a good thing. Video projectors are now coming to market with DMX512 control. Projectors can now have a moving mirror head mounted on them to re-position the image from a lighting console. The creative opportunities are only beginning to expand. This will make the issue of “how bright is right” more critical in every application.

Paul Duryee has been in professional AV starting in the mid- ‘80s as a stage tech specializing in production audio and video. He is currently the lead systems designer for Maxx Technology, a design/build AVL integrator based in Nashville. He currently resides just outside Nash-Vegas, TN with his wife, daughter and two dogs, Ricky and Lucy. He can be ridiculed/ reached at pduryee@plsn.com