Injection moulded lens array for high power LED modules

Abstract

Light emitting diodes (LED) are increasingly replacing traditional light sources due to their better energy efficiency and potential for reliable operation with long lifetime. The price of LED-based luminaires is still fairly high. One way to decrease the manufacturing cost is to use single lens optics for many LEDs. Optics should provide high efficiency together with low cost and excellent reliability. This paper introduces injection moulded lens array designed for high power LED modules. The lenses were employed with a series of modules containing 4 × 4 white LEDs whose total luminous flux varied from 660 to 2280 lm. The lens had dimensions of 73.4 mm × 73.4 mm × 13.9 mm and it included alignment pins and sides that could be used for sealing. The lens optics gave elliptical 30° × 90° lighting pattern with the uniformity of the illuminance on plane surface better than 3∶2. The lens was optimized for Philips Lumileds Luxeon Rebel, but it was tested also with Cree X-Lamp, Luxeon K2, OSRAM Golden Dragon Plus, OSRAM Diamond Dragon and Seoul Z-power LEDs. This paper presents goniometer measurements of the luminous intensity and compares them to the simulations. Two different lens materials, namely cyclo olefin polymer (COP) and polyamide (PA), was used. The paper lists experiences obtained using these lenses. Especially the reliability of the lens and its attachment is discussed. In total, 24 LED modules were stressed in environmental tests including thermal cycling, thermal shock, moisture and corrosion. Observations made on the lens materials as well as lens attachment materials are depicted. Moisture and corrosion was not a problem but high temperatures challenged both the lens and adhesive materials. 22 modules were subjected to a life test of 6000 h where they were driven with a constant current of 700 mA and monitored continuously. The life test revealed failures in both lens materials. The results challenge the reliability of the plastic lenses in high power LED modules operating in high temperature applications.