Diffusers get better at handling LEDs

This modern light fixture uses light diffusion technology developed by Bayer MaterialScience LLC, which blends and minimizes LED hot spots – hiding the point light sources, yet allowing more light to pass than traditional light diffusion technologies.

It's no secret that Light Emitting Diodes (LEDs) have taken over such segments of the lighting market as electronic displays and traffic signals. Now, LEDs are moving into general illumination. In the process, they have opened the flood gates for designers and original equipment manufacturers (OEMs) working in architectural, commercial and industrial lighting. Though initially confined to the roles of adding color, accents, or decorative wall washing effects, recent improvements in light output and efficiency let LEDs compete head-to-head with traditional illumination sources.

LEDs come in a wide variety of colors. But warm and cool whites dominate in general illumination, and this reality can present a few aesthetic challenges for bright LEDs.

High-brightness LEDs are chip-based devices. They are intense point sources of light that are often less than aesthetically pleasing, especially in architectural spaces. Bright spots of concentrated light - an LED hot spot - can be harsh to human eyes and can be a distraction in lighting a larger space. Lighting manufacturers often need to use multiple LED point sources to make an effective luminaire. Yet lighting designers often seek to create a soft glow, making it necessary to employ diffusing, or light blending, technology.

The usual way of diffusing a point source is with a ground glass diffuser, patterned panel, or frosted cover that is positioned in front of the source. Problem is, the diffusing media also blocks some of the LED light. It has always been a fundamental lighting challenge to diffuse light sufficiently while simultaneously passing as many lumens as possible.

Generally speaking, the more pure the base resin, the better the overall light transmittance of the diffuser package. A line of polycarbonate resins from Bayer MaterialScience LLC, for example, has been specifically designed for the rigors of LED applications. When used as base resins, these “LED” grades transmit the most light of any polycarbonate - up to five percent more than general-purpose grades - which makes for more efficient diffusion.

The amount of light Makrolon diffusion-grade polycarbonate transmits depends on the thickness and diffuser loading. In this scenario, the term loading means the amount of diffusion additive - the more the diffuser, the more hiding power/diffusion. In the chart, low/medium/high refers to the amount of the diffusion additive introduced into the polycarbonate resin. The amounts are relative — so each level is double the previous amount.
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Polycarbonate is useful as a diffuser because it withstands high heat and impacts and exhibits high clarity. Clarity readings come via the American Society for Testing and Materials “light transmittance” test - ASTM method D1003. This is a relatively generic test where a sample that is 1/8-in thick is exposed to a d65 light source, which approximates mid-day sunlight. Clear materials will have a normal transmittance of between 87 to 93%, but this is an average value measured over the entire visible spectra using the d65 source. And, various materials do absorb differently over the visible spectra.

Because LED intensities peak at specific wavelengths, the way in which a material absorbs light at specific wavelengths is critical - thus LED optics are normally measured in integrated spectrophotometers to give accurate overall light transmittance efficiencies. In such tests, Bayer MaterialScience LLC LED polycarbonate materials typically have performed anywhere from 1 to 6 percent better than competitive materials.

As another example, the Bayer polycarbonate generally tests at between 87% and 90% light transmittance when using the ASTM D1003 method, but we have seen results anywhere from 83% to 93% using the integrated sphere for the same materials.

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