On February 4th, 2013, Lumentra CTO Dr. Venkat Venkataramanan presented an invited paper on near field photometry for OLED light sources at the SPIE Photonics West conference taking place in San Francisco. Performed jointly with Philips OLED group Germany and Lumentra Inc at the Institute for Optical Sciences at the University of Toronto, this work shows that near field photometry measurement can provide detailed specifications and quantitative comparison between OLED products for performance improvement.
OLED technology is rapidly maturing to be ready for next generation of light source for general lighting. The current standard test methods for solid state lighting have evolved for semiconductor sources, with point-like emission characteristics. However, OLED devices are extended surface emitters, where spatial uniformity and angular variation of brightness and colour are important. This necessitates advanced test methods to obtain meaningful data for fundamental understanding, lighting product development and deployment. A project on near field photometry led by Dr. Venkat Venkataramanan from the Institute for Optical Sciences at the University of Toronto shows that near field photometry measurement can provide detailed specifications and quantitative comparison between OLED products for performance improvement. In this work, a near field imaging goniophotometer was used to characterize lighting-class white OLED devices, where luminance and colour information of the pixels on the light sources were measured at a near field distance for various angles. Analysis was performed to obtain angle dependent luminous intensity, CIE chromaticity coordinates and CCT in the far field. Furthermore, a complete ray set with chromaticity information was generated, so that illuminance at any distance and angle from the light source can be determined. The generated ray set is needed for optical modeling and design of OLED luminaires. The results show that luminance at two positions of the OLED surface can affect the aesthetics of the luminaire and CCT can vary with angle by more than 2000K. This leads to the same source being perceived as warm or cool depending on the viewing angle. As OLEDs are becoming commercially available, this could be a major challenge for lighting designers.