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Video Projector Brightness Levels

By Different Lights - Contrasting among Brightness Levels

Let us begin by considering the most ubiquitous light source of all: the light which everyone in our industry reflexively calls "ambient." All of us have seen it (in fact, you're reading by it right now), but fewer of us have ever looked at it carefully. In a way, the most common things that can be said about it have to do with what it isn't rather than what it is.

The first of those negative definitions is that ambient light is light that doesn't come from the projector. The second is that ambient light is not direct light. Thus, we would not call a floodlight pointed at a projection screen an "ambient" light any more that we would describe as "ambient" the light of the morning or afternoon sun shining straight through our window.

While those two exclusions from the definition are helpful in refining our sense of the phrase, the collection of light and lights which is left remains large and amorphous. To name just a few obvious ones, there are fluorescents recessed in our ceilings, incandescents on tables next to our chairs, or mercury vapor lights glaring above our trade shows. Fortunately, the exact nature of these light sources, their wavelengths, etc., is not nearly as important as are their location and their intensity (amplitude).

Lastly, we come to name the most dominant attribute of ambient light and this is its presence. To all intents and purposes there is one and only one venue in which ambient light need play no part and that, of course, is the entertainment venue. When we go to the movies or settle down in our home theater, the only thing we want to see is what's on the screen and so we watch it with all the other lights off.

In all commercial venues, however, at least some of those ambient lights must, should, and will be left on. None of our commercial customers should either be asked or ever be willing to interact with their visual displays in zero ambient light. If we recall that the business of those displays is the presentation of important information to groups of viewers arrayed before them, then it's not at all difficult to see that the process is most effectively accomplished with the lights on. Reasons supporting this truth are so well established that they are hardly worth repeating. Ambient light enables taking notes, facilitates watching presenters, and discourages falling asleep, to name just three.

The value, then, of ambient light to users of professional presentation systems is easy to establish qualitatively. The problem comes when systems designers attempt to establish the same value quantitatively. The specified light source, the projector, comes with its quantity of light output measured in convenient units and clearly marked. These days 1000 ANSI lumens can reliably be assumed to be exactly that.

But what about the ambient light sources? How can we know or how can we learn what their values are? Actually, this question is a little simpler than it might at first appear. The good news is that we really don't care how much ambient light is permeating through a room. We only care about how much of it is falling upon the surface of the screen upon which we will want to project our information.

It really doesn't matter how much light is falling upon our desktop, for instance, because when we lift our heads and direct our gaze toward the screen, which is in or against the wall across the room, the desktop is no longer within our field-of-view. Fortunately, it is not physically possible to look at both simultaneously.

Yet some amount of the ambient light filling our chamber will inescapably reach our screen where it will have a negative effect on image quality. And, as we shall see below, it doesn't take much.

Now, since ambient light is a fact, and since even if we design the lighting in our room very, very carefully there will still always be some amount of it incident to our screen. The only other light source available to our manipulation is the projector. How do we choose, now that we can choose, among the excellent variety of lumen outputs currently available to us?

In an earlier article [Vol. IV, No. 11], we observed that 50 was a kind of upper threshold number for brightness and that if a screen were, in fact, to exhibit a luminance level of 50 foot Lamberts, it might actually be too bright for all but unusual environments. While that guideline remains useful, it turns out that there is a considerably more refined methodology for balancing among competing light sources in a system design.

One of the very best seminars offered at Infocomm ‘99 was given by Dick Blaha (blaha@mitre.org) and was entitled Effective Application of Display Technology. During that part of his presentation which dealt with Contrast, Mr. Blaha maintained that the design goal for a display system's Contrast Ratio should be "at least 10 to 1, white/black in the operating ambient light conditions."

He then went on to enumerate the determinants that combine to create the observed ratio. They are:

1. Ambient Light at the display surface.
2. Display black level
3. Display light output (Lumens)
4. Display surface (gain and reflectivity)
5. Image design

The graph above illustrates the extreme sensitivity that Contrast has to incident ambient light. A contrast ratio that starts out in a darkened room at 100:1 will drop precipitously under the influence of as little as a single foot candle of ambient light incident to the screen's surface. And only a scant 2 foot candles can depress the ratio of a front projected display to below the minimum recommended ratio of 10:1. The equation which produces these curves is:

where LA is the Lumen output of the projector divided by the surface area of the screen multiplied by the screen gain (if any). p is the term which accounts for whether the display is front or rear projected. If front, let p = 1.0. If rear, let p = 0.2. LB is the display black level and Lamb is the ambient light incident to the screen's surface, measured in foot candles.

Other extremely useful equations involving Contrast Ratio (CR) which Mr. Blaha presented are these:

where Image Area is expressed in ft² and Lamb in incident foot candles. Here p is a coefficient for expressing the reflectivity of the screen surface and generally may be set to equal 1.0 for front projection and between 0.15 and 0.2 for rear.

If you do not know the precise black level, LB, of your system, you can get a good approximation for it by solving

which assumes, of course, that CRdark = 50.

Lastly, if you want to ensure that your system has a contrast ratio of at least 10:1, solve

In setting forth these equations (for which, again, Mr. Blaha must be thanked), we at Da-Lite Screen Company hope that we are providing to our customers, the dealers and designers of the A/V industry, some specifically useful new tools. If they are incorporated regularly into the processes of system design and specification then, we believe, visual displays of ever higher quality will inevitably result.