Angles Of View

The theme which loosely connects the articles presented by this series in 1998 is Information. That subject, of course, is enormous and encompasses many disciplines and fields of study. The purpose here is considerably more modest and seeks only to nibble off a small corner of the larger issue by presenting data relating to the effective display of information as it is likely to appear on a projection screen. Since the basic metrics for assessing the qualities of such images depend greatly on familiarity with their quantitative underpinnings, it may be useful to look anew at

Light Reading - Suggestions and Observations

The amount of human ingenuity expended by our industry in the continuous improvement of what we have come to call Large Screen Displays is phenomenal. Just recall, if you will, how recently it was that a 1,000 lumen projector was beyond the reach of all but the most lavish budgets. And remember when SVGA was a really big deal?

This year a modest budget can acquire a projector with XGA resolution and an output of 2,000 lumens. And next year? Can 1024 x 768 become 1280 x 1024? Absolutely. Can 2,000 lumens become 3,000 lumens? Inevitably.

Because of these shining achievements, there is a question being posed in our industry today which many of us who have been around for a while thought we'd never hear. It goes like this: "I have the Hi-Lite projector model 15C (1,500 ANSI lumens) and I'm pointing it at a Da-Lite 100-inch diagonal screen which has a gain of x, and Is that going to be too bright?"

For the first ten of the last fifteen years people in the display business worried almost exclusively about doing everything they could to maximize brightness. For the last five of those years, we've come to try to optimize brightness. And now, suddenly, should we concern ourselves with reducing it? Amazingly enough, the answer may actually be yes.

The thing to remember about "brightness," of course, is that it is an entirely subjective term. And while that doesn't make the word by any means meaningless, it does emphasize that it has no objective, scientific validity. Even then though, we'll go on including the word in our question, we'll need to revert to other terms if we're to identify a way to calculate whether any particular projector and screen combination can (will) be too bright.

Let's start with the projector and recall that, while its ANSI lumen rating may help compare it with other projectors, it isn't by itself enough to answer our question. Obviously, we'll also have to know over how large an area this collection of lumens is going to be distributed. That brings us to consider the size of our screen which in turn brings us to contemplate its surface properties which are dependent on the distribution of the audience. On top of all that is the nature, quantity, and effect of whatever ambient light that will be competing with the image.

Breaking all that down into measurable units requires serially thinking in terms of lumens, foot candles, and foot Lamberts. Since, one way or another, these are all interdependent, let's briefly review their construction.

The most important thing to remember about lumens is that they quantify the amount of power useful to the human eye emanating from a projection device. If we use a photometer to measure that power, we will easily satisfy ourselves that an 800 lumen projector is twice as bright as a 400 lumen device. Now if we take two 800 lumen projectors and "double stack" them so that they are each filling the same screen, what will our photometer read? 1,600 lumens every time.

A misconception prevalent in our industry and erroneously promulgated by the author of these articles has been that double stacking projectors produces less than twice as much light. This assumption, we are now pleased to understand, is not true. 1 + 1 does = 2; every time.

In an important sense, however, this lineally additive quality of lumens is true only if the device measuring them is other than the human eye. Our eyes, you see, do not respond to changes in brightness in a straight, linear fashion. Instead, they react logarithmically to changes in their input and thus, for instance, we are able to find our way out of a darkened movie theater, yet continue to see when we emerge onto the brightly lit street outside it.

If we array a human audience before some screen and first illuminate it with a 1,000 lumen projector and then turn on a second 1,000 lumen projector, will the audience see twice as bright an image? No, it will not. The perceived brightness will increase only by about 50%. It will not double.

All this being true, it's awfully hard to predict whether some large number of lumens will (or will not) be too bright or bright enough. To find that answer reliably, we'll need to factor in some other units.

Foot candles are a unit of illuminance. They measure the luminous flux (lumens) per square foot at any point on a surface exposed to incident light. Lux are exactly the same, but per square meter (10.7639 ft²).

Foot Lamberts, conversely, are a unit not of illuminance but of luminance. As such, they measure the intensity of light per unit area leaving the screen. Foot candles are dependent only on the total amount of screen area and projector output. Calculating foot Lamberts requires both of those variables and screen gain.

A projector which outputs 1,000 lumens will cause 30 foot candles to fall on a 100-inch diagonal screen. But if that screen has a gain of, say, 1.8, 30 foot candles in will become 54 foot Lamberts out. (Please recall that, as these articles have been at pains to point out, this is only true because convention calls for the photometric measurements to be made always on-axis to both the projector and screen.)

Even though we now know how to determine luminance, we have still to be careful not to confuse it with brightness. In screen terms, luminance is the measurement of light from a surface while brightness is the subjective appearance of that surface. To drive home this point just one more time, 60 foot Lamberts of luminance will be twice as great as 30, but it won't be twice as bright.

Finally, we need to look at the last of these ponderables, ambient light. The difficulty here might at first be thought to depend on brightness issues. Actually, the real damage caused by ambient light occurs first in loss of contrast. Since the purpose of regarding information projected at a display screen is the assimilation of its content, we will need absolutely to distinguish the dark portions of the image from the light ones. Much as on this page, what we read is not the white paper, but the black lines of text strung across it.

Still, for reasons which are both reasonable and sound, only entertainment venues should have the luxury of zero ambient light. All commercial display systems have to be able to function under at least some of it. And the difference between the two viewing environments is about to become clear.

Years ago organizations like the Society of Motion Picture and Television Engineers (SMPTE) determined that a reliable luminance standard for showing motion pictures in a dark theater and onto a unity gain matte white screen is about 16 foot Lamberts. As a practical matter, the bright scenes we look at are often as much as 40% less than that but, for our purposes here, let's stick with 16 as an upper brightness threshold.

Now let's go to the TV part of SMPTE's work and see what has been determined as the acceptable upper limit of that technology's luminance. Since we don't typically watch television in a completely darkened room, it turns out that the new number is much larger than 16. It is, in fact, 50 foot Lamberts.

If that number is reliable for your TV, could it not be transferred usefully to demarcate when a projected display may become too bright? This article asserts that the answer is yes. Are there and will there be exceptions? Of course, but they will depend on ambient light levels that are unusual. Otherwise 50 foot Lamberts should be thought of as being, at last, enough.

Having so declaimed, let's see what consequences follow for choosing projection screens. In these articles and elsewhere Da-Lite Screen Company has for some years now been promoting the virtues of low gain screens versus the previously fashionable high gain ones. Although that thesis need not be repeated here, a new argument in its favor can now be introduced.

If we take 50 foot Lamberts to be our brightness benchmark and then choose (arbitrarily) a 100" diagonal screen with gains ranging from 1 to 3 and four projectors with outputs ranging from 750 up to 2,000 ANSI lumens, we get Figure 1.



Figure 1

As you can see, 3-gain screens are too bright in every case and 2-gain screens fare only a little better. Yet there are screens for sale out there still boasting gains as high as five. Go figure.