Materials For LED Optics
Several articles can be found regarding the lifetime of Light Emitting Diode (LED) luminesces, although the primary focus has always been on the lifetime of the LED diode itself. Given that the LED luminesces is a system, it is important to diagnosticate all aspects of the system that can affect or limit lifetime. The LED lighting revolution plastic optics have been made primarily from acrylic (PMMA), polycarbonate or a modified version of either material (for example PMMI). More recently, the market share of silicon-based optical materials have also grown quite well. These silicone resins are suitable for precise molding and can be used for refractive lenses, TIR lenses, reflectors, diffusers and other optical components. Polycarbonate materials are utilized for LED optic and lens applications, especially when heat, flame, or impact resistance is required. Conventional transparent polycarbonate can show signs of hazing and yellowing when they are exposed to intense LED radiation in the 450-550-nm range and long-term direct outdoor sunlight exposure. In order to mitigate the absorption of this intense LED light, LED polycarbonate grades had been developed to extend the optical performance. A UV infusion process has also been described to produce parts that exhibit minor changes in yellowness similar to acrylic after UV exposure.
Most luminesce designers are prosaic with the need to optimize thermal performance evidenced by the large number of articles written on the subject. LED die lifetime has been shown to be driven primarily by junction temperature and to a lesser extent by current density, although the effects of current density and current spreading have been largely overlooked. LED luminaire designers are much less aware, however, of the importance of selecting the proper optical materials for the primary and secondary optics that comprise the luminaire with respect to lifetime degradation due to aging of the optics.
Different parts of a LED lamp require different properties of materials. There are four different product types for the various requirements.
● LED grades - With transparent polycarbonate, it’s possible to efficiently design and manufacture LED parts with complex geometries that would not be producible with glass. OFH LED grades are specially developed for use in lenses and secondary optics. They have an extremely high purity and thus a much better stability against LED light in comparison to standard or optical grades. Yellowness index and haze after long-term illumination with LED light are highly reduced, transmission remains on high levels.
● Diffusion colors - For ambient lighting, materials with high diffusion are smitted that have at the same time a high transmission value. Those two basically opposing properties can be balanced by color formulation experts according to customer needs with special diffusion color recipes. Close collaboration with customer in terms of desired transmission values, wall thickness of the diffusor, half power angle and lighting setup leads to perfect match for almost every customer requirement. Grades with and without UV protection and different viscosities are available in diffusion colors as well as diffuser sheets.
● Highly reflecting materials - Diffuse reflectors can be used for indirect lighting. OFH has developed special grades called RW (reflective white) that have high efficiency in reflecting visible light. Compared to the standard white grades, almost a reflection value up to 95% increased lead to very high brightness. For specular lighting metallization of standard grades is possible.
● Thermally conductive materials - Heat management in LED lamps is of utmost importance. It requires thermally conductive materials. Traditionally, metals are used which limit design possibilities. A thermal conductivity of around 20W/Km is an extremely good value for a thermoplastic material allowing replacing alumina in LED heat sinks.
● Polycarbonates - These are another strong contender that addresses the specific needs of LEDs. The LED-grade materials are designed to tolerate long-term heat exposure, transmit light effectively, and have good clarity. A diffusion additive can also be added to polycarbonates to mitigate glare.
As Optics For Hire continues to explore the opportunities unique to LEDs, the push for higher efficacies will keep introducing new heat-resistant materials and more customized optical solutions. Although the latter has already begun; 3D printing technology is speeding up prototyping and even printing optics to order. Such developments might emphasize solid-state lighting’s importance as a source of not only efficiency but also beautiful and controlled light in future.











