Another important quality criterion is the homogeneity regarding the color temperature, as the human eye notices a deviation of just a few Kelvin as a difference in color. The goal of the manufacturers is therefore to achieve a consistent color temperature in each individual component across each batch.
The optical characteristics of the finished LED module are ultimately determined by the phosphor proportions and the characteristics of the individual blue dies. Deviations in both parameters inevitably lead to visible differences. Many manufacturers accept a certain fluctuation margin in LED production and subsequently select the LEDs in accordance with defined deviations from the original color. In this so-called binning process, the exact color temperature is ultimately defined not only by the nominal value of the color temperature, but also by the binning rank of the LED / modules. The production of LEDs with only one die requires sorting in up to 25 bins to guarantee uniform optical characteristics within each bin. The deviations are significantly lower in arrays. That is because the optical deviations of individual blue dies balance each other out — the bigger the number of individual dies, the smaller the difference between the basis arrays. That leaves mainly the phosphor blend and its application onto the matrix surface as a source of deviation. Sharp has optimized the manufacturing process of LED arrays to such a degree that the difference in the color temperature between individual LED modules is practically no longer visible to the human eye. As one of the few manufacturers of LED arrays, Sharp is able to offer all new LED modules in a color selection of only two bins according to "Mac Adam Step 5." The goal is to optimize the manufacturing process to such a degree that all produced arrays achieve the tightly construed criteria of "Mac Adam 3" in a bin, with chromaticity coordinates that are so close to the defined values that the human eye can no longer detect any difference in color. Sharp has already reached that goal in the modules of the 50W MegaZeni series.
Since LED arrays closed the gap to single-chip LEDs in terms of light efficiency, they are increasingly establishing themselves as a design principal for LED-based lighting fixtures, as the advantages are obvious: the good scalability leads to a wide spectrum of single-point light sources in various performance classes. The compact construction simplifies the design of cooling elements and secondary optics. This means the formation of multiple shadows is avoided, which is always present when multiple SMD LEDs must be used. The arrays also are superior in quality to single-chip modules: The matrix wiring creates a very high reliability and improves heat conductivity, which increases efficiency and lifespan. In addition, the arrays are much more homogeneous in their optical characteristics — Sharp has produced LED modules with nearly completely consistent characteristics, which are almost indistinguishable from each other optically. With the Zenigata line, Sharp offers a portfolio of high-quality LED modules with CRI values of at least 80, and with a broad spectrum of color temperatures, with light currents of 200 up to nearly 7,000 lumen.
About the author: Uwe Hock is manager Lighting Business at Sharp Microelectronics Europe (Hamburg, Germany).
For technical details, including specifications of the different Zenigata series, click here.
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