There's a blizzard of information on flat-panel displays. That's not so surprising, given the 30 varieties of displays available and the 85 or so manufacturers that make them. The technologies they employ run the gamut from fluorescent and plasma types, both almost century-old technologies, to the more youthful entries in the marketplace like organic light-emitting diodes (OLEDs), liquid crystal on silicon (LCoS), and inorganic electroluminescent types. These more recent arrivals are just beginning to capture the market's attention.
In such a climate, it would seem that steering a course to sound display decisions would be difficult. Fortunately for the user, display technology is evolutionary rather than revolutionary. Therefore, it's unlikely that some highly appealing display technology will suddenly spring out of the woodwork overnight, leaving the designer who already made a decision lamenting, "If I had only known about . . ."
Lawrence E. Tannas Jr. of Tannas Electronics, Orange, Calif., says, "It is not uncommon for display technology to take a generation to evolve. And even after 20 years, many display innovations that may eventually make it have yet to reach production. But their evolution continues on, at various rates of activity, depending on material advances and market demands." Tannas should know. He's a past president of the Society for Information Display (SID), author of several books on the subject, and a lecturer on display technology at the University of California at Los Angeles.
Still, despite the 30 varieties, the good news for the designer faced with selecting a display comes from the well-established display families. An example is active thin-film-transistor (TFT) displays, which are proven and will meet most needs in the 2- to 10-in. diagonal range. Furthermore, TFTs will most likely be the only choice in the 14- to 28-in. range.
As for the larger displays, when you start moving into screen sizes in the high teens and above, TFT prices begin accelerating and go ballistic, moving rapidly into four and five figures. Luckily, a proliferation of projection techniques are arriving that employ LCoS technology. They may prove to be an effective means of sidestepping the high costs of TFT. Also, not far off in the future lies the inorganic electroluminescent display.
If one is designing a small handheld unit where weight and low-power consumption are of paramount importance, he or she can find microdisplays beginning to pop up with diagonal dimensions of less than 1 in. and exhibiting astonishingly low power consumption. Can you find a display that consumes less than 6 mW? You bet.
Whatever the display, the eye-to-screen viewing distance is crucial. Just how far will the eye be from the display? It turns out that if you have 20/20 vision, you are endowed with a visual acuity that equates to one minute of arc. A valuable number, known as the "optimum viewing distance," is derived from this fact. It can assist a designer in establishing the pixel resolution that's necessary for a comfortable viewing of the display in an intended product (Fig. 1). This reduces to the angle subtended by adjacent pixels in the display of choice. The relationship is governed by:
α ≈ (d/L) 57.3 × 60
where:
α = visual angle in minutes
d = the center-to-center distance between pixels
L = the distance from the display to the viewer's eye.
"Unlike photographic images," Tannas says, "you pay for every pixel." So, you want to see all the pixels you have paid for. If you get too close, the angle subtended by two adjacent pixels exceeds one minute. You then see the details of the pixels, which is sometimes called pixelation or raster noise. In the same vein, if you get too far away, so one minute of arc spans more than one pixel, "then you don't see all the pixels you have paid for," he concludes.
The term "artifact" identifies anything you don't want the viewer to see. By making the distance between adjacent pixels equal to one minute of arc, the viewer won't see the lines, corners, electrodes, or the pixels, but will see the image. By choosing the optimal distance, the viewer is assured that he or she will see all the information, but none of the artifacts.
Just like every other technology, there's no shortage of terminology in the world of displays. A decent familiarity with some of the key terms can be useful (see the table).
Keep a few things in mind with regard to display selection for portable applications where both power consumption and weight issues are so critical. When totaling the power requirements, include all of the electronics needed to produce a final image. This includes analog drivers and analog-to-digital converters (ADCs), as well as DRAM structures—and the inverter, if required. If the displayed contents change frequently, the higher refresh rates will raise power consumption. Incidentally, where a TFT LCD may consume 4 to 5 W, a microdisplay will consume between 10 mW and 1 W.
As far as weight goes—as a portion of the total product weight—a color TFT LCD, the backlighting, and a battery on a portable computer or portable DVD device can amount to 25% to 60% of the overall product's weight. Obviously, a microdisplay that might weigh in at a gram gives the designer a good head start in realizing a truly lightweight design.
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