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Design News, May 2013

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The best color sensors and soap bubbles have something in common… thin-film technology. Perhaps the most interesting portion of a soap bubble's life is when swirls of colors dance and move across its surface. applications, it may not be good enough for precision control of high-brightness LEDs in lighting applications. This is due to their lossy filter characteristics (resulting is less sensitivity), dye stability and aging issues, and less than optimal passband overlaps. Transmission Interference filter Absorption filter Passband > 95% 60% – 70% Out-of-band < 1% 10% - 20% Table 1 – Typical characteristics of interference and absorption filters For demanding applications, especially when used in a feedback loop to control lighting, we need miniature color sensors built with interference filters covering the photodiodes. Such a sensor could leverage thin-film manufacturing techniques to deposit sequential stacks of Figure 1 – One of Nature's interference filters filters directly on the photodiode surfaces. The end-product Recall that white light is composed of many colors, each would have high-sensitivity, precision passband control and corresponding to a particular wavelength. If you "absorb" edge-shaping, with superior long-term stability. certain wavelengths in the visible spectrum, the resultant reflected light is no longer white, as some colors will I recently started working with a company called be absent. One of the most precise tools for selectively MAZeT which is now introducing them. Their line of reflecting and passing colors is an interference filter. color sensors use interference filters deposited onto an array of PIN photodiodes. Particularly interesting in An interference filter is a sandwich of semi-reflective applications involving the human eye, some devices have surfaces separated by a "spacer" of given thickness. The interference filters tuned to mirror the eye's tri-stimulus internal reflection causes some wavelengths to interfere response, as defined in DIN 5033 Color Measurement (e.g., a wave crest meets a wave trough and cancels out) Standard. It "sees" light the way we do. A complete colorwhile allowing a narrow spectrum to pass through. In controlled (or white light color temperature controlled) practice there are often stacks of these sandwiches placed system involves a color sensor front-end, microcontroller, sequentially to achieve the desired passband edge shapes. PWM-driven color or bi-chromatic LEDs, and an optical Since the refractive index of air (n = 1) differs from that feedback path for the sensor to sample the instantaneous of soapy water (n ≈ 1.3), the film is semi-reflective on mixed color/CCT being emitted. An evaluation board its inner and outer surfaces. Thus the soap film itself includes the sensor in a LCC8 package and USB becomes our "spacer". Initially, this film might be interface. It's shown below on my business card. many hundreds of nanometers thick, causing the longer wavelengths (red end of the spectrum) to destructively interfere. Without red reflecting back, the bubble appears greenish. As water evaporates and the film begins to thin, yellow wavelengths destructively interfere, leaving blue. Eventually green then blue wavelengths will no longer be reflected, and the bubble will appear magenta… then yellow… then… pop! Generally, color sensors are built using photodiodes having broad-spectral response. So to distinguish the individual values of red, green and blue light, the photodiodes have red, green and blue filters placed above them. These are absorption filters, made from dyes or pigments. While this is often acceptable for some Cary Eskow is Global Director of the Solid State Lighting and Advanced LED business unit of Avnet Electronics Marketing. An ardent advocate of energy efficient LED-based illumination, he has worked closely with LED manufacturers, advanced analog IC and secondary optics vendors since his first patent using LEDs was issued two decades ago. Avnet works with customers through their national team of illumination-focused sales engineers who are experienced in thermal, drive stage and optics design. Prior to his LED lighting focus, Cary was Avnet's technical director and managed Avnet's North American FAE team. To submit questions or ideas, e-mail Cary at LightSpeed@Avnet.com Figure 2 – A MAZeT color sensor (with hexagonal window) mounted to a small PCB If you'd like more information on MAZeT, visit our website (www.em.avnet.com/LightSpeed) or send a note to LightSpeed@Avnet.com. Your questions and comments are always welcomed To learn more about designing an LED-based illumination system, go to: www.em.avnet.com/LightSpeed magenta cyan yellow black ES245350_DN1305_011_FP.pgs 05.02.2013 23:11 UBM

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