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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
LEDs. Principles of obtaining a white glow. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / LEDs The invention of blue LEDs completed the "RGB circle" and made white LEDs possible. Exists several ways to create white LEDs with its own merits and demerits. First way - mixing LED radiation of three or more colors. On fig. 4.2 shows the production of white light by mixing in a certain proportion the emission of red, green and blue LEDs. In principle, this method should be the most efficient. For each of the LEDs - red, green or blue, you can choose the current values corresponding to the maximum of its external quantum yield of radiation. But with these J (LED current) and V (LED operating voltage), the intensities of each color will not match the values required for the resulting color coordinates in the white area. This can be achieved by changing the number of diodes of each color and making a source of many diodes. For practical applications, this method encounters inconveniences, since it is necessary to have several sources of different voltages, many contact inputs, and devices that mix and focus light from several or more LEDs. Even the highest quality RGB LEDs are characterized by the fact that the light spot obtained when they illuminate the surface, although it is white over a larger area without any shades, nevertheless, colored stripes in the form of arcs still stand out along its edges. This is due to the fact that the crystals that emit blue, red and green light are naturally somewhat spaced apart from each other in the LED.
The second way is mixing blue LED radiation with yellow-green phosphor radiation. On fig. 4.3 shows the production of white light using a blue LED crystal and a layer of yellow phosphor deposited on it. This method is the simplest and currently the most economical. The composition of a crystal with heterostructures based on InGaN/GaN is selected so that its emission spectrum corresponds to the excitation spectra of phosphors. The crystal is covered with a layer of gel with phosphor powder in such a way that part of the blue radiation excites the phosphor, and part passes without absorption. The shape of the holder, the thickness of the gel layer and the shape of the plastic dome are calculated and selected so that the spectrum has a white color in the desired solid angle. Currently, about a dozen different phosphors for white LEDs are being studied. On fig. Figure 4.4 shows the structure of a 5 mm white light emitting diode. The third way - mixing the radiation of three phosphors (red, green and blue)excited by an ultraviolet LED. On fig. Figure 4.5 shows the production of white light using an ultraviolet LED and an RGB phosphor.
This method uses principles and phosphors well developed over the years for fluorescent lamps. It requires only two contact inputs per emitter. But this method is associated with fundamental energy losses during the conversion of light from a diode in phosphors. In addition, the efficiency of the radiation source decreases, because. Different phosphors have different luminescence excitation spectra, which do not exactly correspond to the UV emission spectrum of the LED crystal. For white sources, not only the color coordinates of the total spectrum of different components of the emitter are important. Long-term studies of fluorescent lamps have shown that for color characteristics it is necessary to take into account the reflection of light from surfaces with different reflection spectra. This accounting can be quantitatively substantiated by empirically entering the color rendering index as the average value of the color rendering indices from 8 standard color surfaces.
Color rendering index, Ra - CRI (Color Rendering Index), characterizes how close to "true" the colors of objects will be seen when viewed in LED light. Under "true" means colors formed using the test source. Ra takes values from 1 to 100:
An index over 80 is good, over 90 is excellent. Summation of LED radiation more than three colors makes it possible to obtain white light with a color rendering index close to 100%. The color rendering index for the sum of blue LED emission with yellow-green phosphor emission is lower than for other methods, but it can be improved by using an additional orange-red phosphor.
For the mass use of LEDs in conventional lighting, psychophysiological studies of the visual perception of the color of LEDs are needed. The future will show in which applications it is advisable to use white LEDs of each type. Author: Koryakin-Chernyak S.L.
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