Diffusers get better at handling LEDs

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.

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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During processing, the Bayer polycarbonate material has a higher viscosity than ordinary grades (13 gm/10 miv vs. 20 to 35 for others), which lets parts solidify quickly. This property in turn helps molders produce consistent parts faster and with fewer defects. Diffusers made with this material generally range between 1 and 6 mm thick, but thickness really depends on manufacturers and their specifications. Certain grades of polycarbonate also offer high flame retardance and are designed to resist ultraviolet light. (If polycarbonate is not UV-stabilized, it will discolor [(yellow)] faster when exposed to UV light. The stabilizers found in the resin help slow this process.)

It is useful to compare LED-grade polycarbonate with ordinary diffusion materials. Standard light diffusion materials scatter light in most directions. They also reflect some light back toward the source, as well as absorb a portion. This chaotic scattering of light is inefficient. It can ultimately result in the loss of up to 60% of usable light output. Consequently, engineers sometimes must start with an LED package that generates 1,000 lumens to end up with 500 lumens transmitted through the diffuser. Clearly such performance is undesirable on a variety of levels.

In contrast, recently developed materials from Bayer MaterialScience LLC's Color Competence and Design Center in Newark, Ohio, can soften LED light with minimal reflection. Until now, light from translucent white colors could be diffused, but never at this light transmission level or with as little reflectance. With this technology, between 60% and 90% of the original light source output passes through the diffuser - a significant improvement over traditional technologies.

An example of this technology can be found in a bathroom ventilation fan with a built-in light produced by a leading supplier. The end product is a contemporary fan/lighting fixture that can clear steamy mirrors, damp walls and fogged windows, accompanied by a soft - yet bright - light to efficiently illuminate the space. Use of the Bayer light diffusing technology made it possible to field a light source that consumed about 40% less energy than originally planned, making the product more efficient while reducing its cost.

Another common technology that is utilized for diffusion is a diffusion film. This generally takes the form of a thin, high-haze layer added to the lens or cover of the luminaire. One fundamental problem with using diffuser films is that light is lost every time the light wave passes through another surface. Adding a layer can result in anywhere from two to six percent of additional light loss. Thus, where a typical 4 to 12 percent loss of light comes from natural scattering, a diffusion film makes this slightly worse.

Behind the technology

The technology from Bayer MaterialScience LLC uses a refractive rather than a blocking approach. More specifically, a fine particle mixture is added to the polycarbonate resin used to create the diffuser package. The size and shape of these particles, which is proprietary, makes this efficient technology possible. Light passing through the diffuser gently bends around these particles, dispersing in varied and mostly forward directions, while maintaining a high overall light transmission. More or less, most of the light waves bend a little - some more widely and others inward. The net effect is an overall wider distribution of the light. Very little light is blocked, reflected or absorbed.

To comprehend the phenomenon it is helpful to understand that when light - or any energy wave - strikes a different surface, there are scattering losses. When light enters a film, for example, it encounters a new media. When it leaves the film surface, it also encounters a new media. This is why a clear material never has 100% transmittance. Most of the losses are surface-related - thus the theoretical maximum transmittance is about 94%.

An additional benefit from the Bayer polycarbonate technology is that it is totally customizable. Unlike fixed patterns or frosted lenses, the special polycarbonate can be tailored to any specific application. This technology can even be added to flame-rated grades of polycarbonate - which may result in use of fewer components to meet UL certification.

Several factors in the design of the diffuser play a role in determining the amount of diffusion that is possible. For example, thicker lenses - as well as those that have higher diffusion levels - transmit less light but have better blocking power. The distance between the light source and lens also factors into the amount of light diffusion. The further the LED source from the diffuser, the more dramatic the diffusion effect.

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In addition to eliminating obvious light source points, light diffusion materials can be used to tint the color of the transmitted light. LEDs commonly go into numerous applications such as displays, exit signs, automotive lighting and ambient lighting in which color options are advantageous. Use of the diffusion technology gives product designers a broad palette of colors to customize applications.

Experts at the Bayer MaterialScience Color Competence Center can put colorants into the diffuser package so transmitted light appears as different colors. It is advantageous to incorporate the color into the plastic resin which is then molded, rather than use a traditional post-mold painting process. The molded-in color will not chip or scratch off, and the process itself is more efficient and economical than secondary operations.

One application where such innovation can help set progressive OEMs apart from the competition is automotive interiors. LEDs can provide a tailored design that is distinctive while consuming less energy. The finishing touches of a stylish and luxurious cockpit include an illuminated odometer and HVAC and radio dials, made possible via customized diffusion.