Angles Of View
This series has remarked previously on the increased expectation being placed upon audiences interacting with visual displays. Looking, it has further suggested, is no longer adequate to describe the generic visual task whereas Reading is. Everybody knows that the act of reading is one of the primary ways in which each of us assimilates formally prepared information. However, it still may be worthwhile to explore whether "reading" a projected presentation is the same as"reading" a book. Let's see, then, what may be said about
Reading Displays - Behind the LinesLet's look analytically at what's happening when we sit comfortably at our desk or in our favorite armchair and open a book. If the pages that enjoy our regard are hardbound, their dimensions are something like 8 or 9 inches tall and usually about 6 inches wide.
A random (and not at all scientific) selection of 30 hardback volumes yielded an average print density of 42, 70-character lines for a typical page. The smallest font height in the sample was 1/8". All of the pages in the sample were white and all of the ink used in the printing was black. Since the resolution of this printed display is essentially analog, it's difficult to quantify. But because some number will be useful to the discussion which follows, let's set its value conservatively at 600 dots per inch (dpi).
Next let's consider the brightness of the light sources which may illuminate our book. A not very systematic series of photometric measurements taken in several offices and in several armchairs indicates that 30 foot-candles of light may typically illuminate the texts we read. It should be added that even under that much light, the contrast ratio of the monochromatic page is observably excellent.
Lastly, we consider viewing distance and viewing angle. Since reading a book is a solitary activity, the least favored viewer is also the only viewer. Further, that person (that pair of eyes) is essentially on-axis to the page and is only about two page heights away (15 to 20 inches). From that vantage point, the worst viewing angle is only about 10o and would be measured to the corner of the block of printed text.
Those, then, are among the metrics of reading printed information. How different or how similar are the measured standards for reading projected information? Let's have a look.
But first a stipulation. Whatever else that's about to be said, we need to remember that the whole purpose of projecting information is so that it may simultaneously be delivered to an audience numbering greater than one. Furthermore, not only does a visual presentation get presented to a group, it gets presented to a group in a time limited way. This is significant because each "reader" of a projected display, therefore, tacitly relinquishes control of the time she will take to assimilate the data. That authority is transferred either to the presenter or, in the case of command-and-control applications, to events and circumstance.
For the comparison, let's use a 100-inch diagonal matte white screen and an LCD projector rated at 500 ANSI lumens and endowed with a native resolution of 768 x 1024. From guidelines well established elsewhere within our industry, we know that the audience field for such a display should not be deeper than 30 feet (six screen heights) and that, maximally, no viewer should be positioned at a larger angle to the screen than 45o.
A little trig discloses that the width of the permissible back row could be as large as 30 feet. A little more trig reveals that the Least Favored Viewer could, therefore, be as much as 42 diagonal feet away from one of the screen's corners.
With all that in view, let's turn to the page of our projected "book." Optically, the whiteness of the screen and the whiteness of the paper are effectively identical. The amount of light constructively illuminating it, however, is significantly reduced. Fifteen foot-candles is all our 500-lumen projector can deliver to a 60 by 80-inch screen. Destructively, of course, there's ambient light to worry about which, even if we control it carefully, will inescapably degrade image contrast by some amount.
What we mean by contrast here, of course, is the achievable black level of the system. And the bad news for the projected page is that there isn't one. The black level of a printed document is, obviously, just exactly as black as is dictated by the opacity, saturation, and gloss of the ink that makes it up. Once that ink has been, let us say, "deposited" onto that white page, it soaks its way right into the fibers of the paper and renders them permanently and indelibly black.
Printed black, therefore, absorbs light and fails, to all intents and purposes, to reflect it. Projected black, on the other hand, is an oxymoron. When we look at monochomatic text projected onto a white screen, the sad fact is that we can never see black.
The responsibility for this deficiency, incidentally, is only partially the projector's. That device usually is the component which establishes the system's maximum black level and which no front projected display can ever manage to maintain.
If our 500-lumen projector has a rated contrast of 100:1, this might reasonably suggest that when it dumps 15 foot-candles of light onto one of our screen's square feet, the black characters within that square foot will have a total luminosity of only .15 foot-candles. If true, that would be a most satisfactory black level.
The trouble is, even if that .15 foot-candle is the minimum amount of light which the projector can project, it's unfortunately a lot less than any matte white screen can re-radiate. If we think about that last sentence for a moment, we will see that the problem lies not so much in the projected lines of text but in the screen behind them. Fabulously efficient surface that it is, the one thing it can't (and shouldn't!) ever be is black.
The fibers of this "page" are not ever going to be dyed black. Not one of them will ever absorb light. Every one of them will scatter light from any and all sources with egalitarian abandon. So now if we consider just a single "black" character projected onto our screen, what we'll see is the paper beneath the type doing a very good job of radiating light across, into, and out from the very area the projector would like to convince us is black.
True, we can diminish this particular effect by choosing a screen surface which has gain. Since higher gain screens have narrower dispersion patterns than lower gain surfaces, the higher the gain, the better the contrast. However, as this series has been at pains to point out elsewhere, the qualitative price to be paid for high gain often far outweighs any benefit in contrast.
The point in going into such detail about black level here is that trying to read text that doesn't possess much is hard, often very hard.
Another attribute which makes reading projected text so much more demanding than a printed version is resolution. We said earlier that a page of well-printed text has a resolution of at least 600 dpi. Let's see how that compares to a good projected display.
The smallest "dot" an electronic projection device can work with is a pixel. If our projector is an XGA device, it has 786,432 "dots" with which to write not only the text characters on its page, but every square inch of the page itself. And, it's got to distribute those "dots" symmetrically, no extra concentration for the black zones allowed.
On the 100-inch diagonal screen we've been using as an example, there are 4800 square inches. In each of the those inches, the pixel density will be just less than 164. To raise that number to 360,000 (600 dpi) would require a projector with a native resolution exponentially higher than anything we can contemplate today. And, even if we did have one with resolution that high, we remain OK only as long as we don't increase the screen size.
Resolution is an extremely useful index of information content. Established as a limit, the resolution of a display defines, quite precisely, the maximum amount of information it can contain. It isn't that the words comprising this line of text increase in number or simplicity as their resolution is increased, it is that, as more and more pixels can be devoted to the tracing of each stroke and curve of each character, they all become unmistakably easier to read.
In a way, the very reason this issue is problematical is because our industry's presentation technology and the computers which drive it have just recently begun to approach the capacity of the printed page to display information. Exciting as this breakthrough is, it remains beset by a host of difficulties and challenges only some of which are being identified here.
Most of us in the A/V industry tend to think of visual displays according to engineering standards. Convergence, bandwidth, color temperature, screen size, half-angle, viewing angle, etc., etc. Although all of these are vital criteria, none accounts directly for what are called the human factors. These are the ergonomics of how efficiently human beings may be helped in their assimilation of projected data by the parameters of the display system itself.
These are questions about which most in our industry arguably know little and demonstrably not enough. As our customers' demand for information increases, however, they are questions which not only must rigorously be asked, they must scientifically be answered.