Multicolor LED sensor

ABSTRACT We present a compact and cheap sensor device based on the combination of a standard RGB-LED with a luminescence material. The multicolor LED acts as the exciting light source, the luminescence light detector and simultaneously as an optical filter. As luminescence material various phosphor materials or crystals can be used, depending on the physical property to be sensed. Possible applications will be discussed. Keywords: LED, optical sensor, phosphor, luminescence 1. INTRODUCTION The use of phosphors as sensing material has various measurement applications in physics, chemistry, biology and medicine [1]. Experimental setups consist of the exciting source (laser or light emitting diode (LED)), the sensing material, the photodetector and an optical filter to suppre ss the excitation wavelength in the detection channel. They are very often realized with the help of optical fibers [2] for light transmission that have the advantage that measurements are possible in electromagnetic disturbed surroundings. On the other hand, the pn-junction of LED’s can be used as photodetector. Miyazaki et al [3] have investigated the emission and extinction spectra of LED’s in their work on wavelength selective photodetection and found out that the spectral responsibility of some LED’s behaves like a bandpass filter blue-shifted with respect to the emission profile. They explained this behaviour with the semiconductor band structure of the LED (light emission requires direct band gap recombination, absorption is also possible by indirect band to band transitions). Colorimetric measurements are a further application of LED’s as light sensing devices [4], where the discharge time of the LED-capacitance is determined by the light intensity. Sensing devices as mass products or the application in sensor networks require simplified setups. Here we show that the combination of a multicolor LED with a luminescence material can be used as a compact and low-cost optical sensor device. In our setup, the RGB-LED works simultaneously as the exciting light source, the luminescence light detector and the optical filter. Thus the LED forms together with the sensing material a small and flexible measurement device without being in need of modifying the LED in any way.