Bill Schweber, Technical Editor
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Electroluminescent lamps: sharp answers to flat lighting
Complex chemistry and specialized drive circuitry give electroluminescent lamps their cool and eerie glow.
By Bill Schweber, Technical Editor -- EDN, 2/18/1999
Designers use electroluminescent lamps as backlighting for LCDs in pagers, cell phones, watches, and control panels and as lighting sources for safety strips and highlighting. Why would you want to use a light source that is less efficient than an incandescent lamp, requires several hundred volts of drive, and has a limited lifetime? The answer is simple: An electroluminescent lamp is the most practical way to evenly illuminate a flat area, whether that area measures 1 sq cm or several square meters. Although you could use point sources of light, such as LEDs, the requisite light diffuser is mechanically complex and costly, and it still doesn't provide an evenly spread glow without annoying illumination hot spots. Illuminating an irregularly shaped area or a flexible panel with this backlight glow is even trickier. These areas are where the electroluminescent lamp shines: It gives you the illumination you need, tailored to the shape of your display or panel. Electroluminescent lamps are based on the research of Professor G Destriau in France in 1936. Destriau found that when you suspend a zinc-sulfide phosphor powder along an insulator (he used oil on glass ceramics) and apply an ac voltage, the resulting electric field causes the phosphor to glow (see sidebar "Dust off those periodic charts"). By modifying the phosphor, you can change the color of the glow from blue-green to bluish, yellowish, and even dull violet. (Do not confuse electroluminescent lamps with thin-film electroluminescence (TFEL), a technology used for image formation. The underlying principles, construction, and drive-circuitry architecture for TFELs differ greatly from those parameters for electroluminescent lamps.) The brightness, L, of the Destriau-type electroluminescent lamp increases with applied voltage, V, according to the equation:
where A and B are, as they say in the textbooks, "constants to be determined." A and B depend on the phosphor mix that the electroluminescent lamp uses. You can first-order approximate the brightness as roughly proportional to the applied voltage, which typically ranges from 30 to 200V ac p-p. But voltage is not the only factor that affects electroluminescent brightness (see sidebar "Who'll do the driving?"). The applied ac frequency is another factor. Again, a first-order approximation is that brightness is proportional to the frequency for a fixed applied voltage. Most electroluminescent lamps use frequencies of 50 to 400 Hz, although some lamps use frequencies as high as several kilohertz. As the frequency increases to several kilohertz, the increase in brightness tapers off. Efficiency of electroluminescent lamps is relatively low. A good electroluminescent lamp yields about 3 to 5 lm/W of input power; a typical incandescent lamp yields output of 12 to 18 lm/W (corresponding to about 10% efficiency); a fluorescent lamp yields 50 to 80 lm/W. The LED is the champion at more than 90% efficiency. But, as with most visual and brightness subjects, these raw numbers are not the whole story. Designers most often use electroluminescent lamps in relatively low-intensity situations, when absolute output magnitude is not the primary factor. In these situations, obtaining an even glow over the entire area is more important. Electroluminescent illumination evenly distributes and sources where you need it, rather than shining on the display from a distance. Therefore, it is not wasted or diffused by distance. (Recall that a point source's intensity falls off as the square of the distance, so the flatness of the electroluminescent-lamp illumination and its location at the display panel make the electroluminescent lamp a much better choice than simple efficiency numbers indicate.) The heat that the electroluminescent lamp generates is not usually a problem, because the power levels are relatively low and the dissipation is spread over the surface area of the lamp, unlike the point source of an LED or incandescent source. Reference
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