2010 CIE Expert Symposium on Spectral and Imaging Methods for Photometry and Radiometry CIE x036:2010 47 MEASUREMENT OF WEIGHTED LED RADIANCE RELATED TO PHOTOBIOLOGICAL SAFETY Tongsheng Mou 1 , Yuqin Zong 2 , and Yoshi Ohno 2 1 Department of Optical Engineering, Zhejiang University, Hangzhou, Zhejiang, China 2 National Institute of Standards and Technology, Gaithersburg, Maryland, USA ABSTRACT Optical radiation safety of light-emitting diode (LED) products is being addressed in the International Electrotechnical Commission (IEC) standard document 62471-2006. Measurement of weighted radiance of LEDs, used to assess the maximal exposure related to the photobiological safety , is very different from that of traditional luminance/radiance, and is not traceable yet to any national standard laboratories. We developed two measurement systems using a spectroradiometer and a CCD camera to obtain the weighted radiance of LED sources. Keywords: LED, photobiological safety; measurement; blue light hazard; weighted radiance 1 INTRODUCTION Optical radiation with biological significance covers the spectral range from 180 nm to 3000 nm. Most important human tissues related to the photobiological effect are skin and eyes. Over exposure of optical radiation in the spectral range from 300 nm to 1400 nm can cause photochemical and thermal injury to eye’s retina. Optical radiation safety of LEDs was first addressed in the IEC standard document 60825-1 in 1993 for laser safety. However, LEDs were taken out from the scope of the laser safety standard IEC 60825-1 in Edition 2 in 2007 to avoid any confusion [1]. Currently, optical radiation safety of LEDs is addressed in the standard document S009/E:2002 for photobiological safety of lamps and lamp systems. This standard document was developed by the International Commission on Illumination (CIE) in 2002, and was adopted by the IEC as an international industrial standard IEC 62471/Ed.1 in 2006 [2]. Optical radiation safety of LEDs is also addressed in the standard documents for photobiological safety of lamps and lamp systems, EN 62471-2008 in European and GB/T 20145-2006 in China. High-power white LEDs with attached optics have complex beam profiles. Typically retina damage is the dominant hazard for these LED products. Due to the nature of human eyes, measurement of weighted radiance of LEDs, used to assess the maximal exposure related to the photobiological safety, is very different from that of traditional luminance/radiance, and is not yet traceable to any national standard laboratories. 2. PHOTOBIOLOGICAL RADIANCE RELATED TO EYE’S SAFETY The differences of photobiological radiance in comparison to traditional radiance are that a.) it is weighted by a photobiological response, e. g., blue light hazard (BLH) spectral function, b.) field of view is related to exposure time, c.) incident aperture stop of the instrument has the same size of an eye’s pupil, d.) there is accessible maximal exposure, e.) and the instrument is focused on the apparent source, not the true source. The retinal hazard caused by optical radiation includes thermal burn and photochemically induced retinal injury. For a white LED based on a blue LED chip and yellow phosphor, the main hazard is the photochemical retinal injury caused by the high color temperature, high luminance small spot. The hazard functions for both thermal and photochemical hazard assessments and the photopic function, V(λ), are entirely different. To assess LED hazard, it is required to measure spectral radiance in the spectral range from 300 nm to 1400 nm, which is wider than that of most of commercial spectroradiometers. It is also critical that the measurement geometry of the spectroradiometer mimics that of a human eye.