Shaping LED diffuser performance with polycarbonate materials

Shaping LED diffuser performance with polycarbonate materials

The light-emitting diode (LED) lighting market is growing steadily due to consumer acceptance of the energy-efficiency advantages of LED technology. To remain cost-competitive, many manufacturers choose plastic materials for their applications, due to greater design freedom and lower assembly cost.

Customers are accustomed to the soft white diffused light from incandescent bulbs. To this end, many plastic manufacturers incorporate precisely engineered diffusion additives into their plastic. The pellets that are subsequently created can be injection molded into parts such as lenses, or processed by extrusion into profiles, sheets and films. This “built-in” diffusion offers the advantage of eliminating a secondary operation that adds to system cost, and also allows for greater design flexibility in the molded shape.

Scientists at Covestro discovered that older diffusion additives, which tend to reflect light, did not meet LED light transmission requirements. In order to achieve a high lumen output while maintaining an acceptable level of diffusion, Covestro utilizes a light diffusion technology focusing on light refraction versus reflection, which leads to higher light transmission (Figure 1).

Figure 1. Transmission of diffusion colors as a function of thicknes

Transmission of diffusion colors as a function of thickness

Another advantage of built-in diffusion is that surface texture can be added to the tooling surface in order to provide a good balance of diffusion and transmission. For example, the combination of built-in diffusion and surface texture on Makrolon® Lumen XT polycarbonate sheet from Covestro improves the overall performance, resulting in higher light transmission for a given diffusion level (Figure 2).

Figure 2. Effect of surface texture applied to built-in diffusion on Makrolon® Lumen XT polycarbonate sheet from Covestro

Performance improvement due to texture

Makrolon Lumen XT sheet, textured

Sheet with built-in diffusion, untextured

Diffusion technology not only affects LED light transmission, but also the color of the LED light exiting the plastic. Older diffusion packages tend to lower the color temperature of the LED light passing through, thereby making it more yellow. To mitigate this effect, Covestro created new diffusion colors that do not significantly alter the color temperature of the LED light passing through (Table 1).

Table 1. Comparison of correlated color temperature of diffusion packages

Scans Color temerature (K) Color temperature difference (K)
LED ony 5971 0
LED + old diffusion 5806 165
LED + new diffusion 5910 61

Another aspect to consider with LED technology is meeting UL requirements. Since many LED-based replacements are considered electrical enclosures, meeting UL 94 flame-retardancy requirements are necessary. Covestro offers polycarbonate-based solutions that help meet these requirements.

Overall, judicious selection of plastic and a diffusion package helps designers attain the desired look for their application. To help achieve this goal, Covestro offers a wide variety of diffusion polycarbonate resins, sheets and films.

Read the full white paper on LED diffusion technology here.

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Makrolon® is a registered trademark of Covestro Group.