High Resolution LED-Spectroscopy for Sensor Application in Harsh Environment A sensor system based on LED-light sources and standard photodiode receiver is shown as an example of this sensor concept for in-situ gas measurements down to the ppb range. M. Degner, N. Damaschke, H. Ewald Dept. of General Electrical Engineering, University of Rostock Rostock, Germany martin.degner@uni-rostock.de E. Lewis Dept. of Electronic and Computer Engineering University of Limerick Limerick, Ireland Abstract— The usage of novel LED-light sources for spectrometric application is shown and described in this paper. The utilization of fiber optical linkage to a simple open path absorption cell and a proper control of the light sources is the key to a robust and high resolved measurement system. A novel optical sensor system based on this approach is realized to measure the concentration of nitrogen dioxide, sulfur dioxide with a resolution below 1 ppm and ozone down to 30 ppb at a 4 cm single reflection cell in a fraction of a second measurement time. In this setup the emitted light from Light Emitting Diodes in the ultraviolet to the visible wavelength range was used. The optoelectronics and the control electronics are separated from the optical sensor head where the pure optical sensor effect takes place. Therefore the sensor can be used in harsh environment for instance in an exhaust tailpipe system or close to discharge plasma in strong electromagnetic fields or at high temperature. Furthermore the sensor design is potentially low cost, quite small, long life and well suited for a large number of applications - from small battery powered hand held devices to industrial process control implementation. An LED-based sensor does not compete to laboratory chemical analytical devices but in many cases it is well suited for high resolved and fast online concentration measurements. Spectroscopy; LED; Sensor; I. INTRODUCTION In environmental and industrial application there is a strong need for cost effective and robust monitoring of chemical components. Optical spectroscopy is a valuable technique for such analytical applications. Precise laboratory devices usually are quite complex and expensive. Often they are based on specialized lasers (for one substance) or broad band light sources combined with complex spectrometric receivers. For instance the mainly used light sources in the UV-VIS-range are gas discharge lamps such as Deuterium or Xenon lamps. They have a quite limited life time, show spectral intensity fluctuations, are not low cost and not very robust. Thus high resolution spectroscopic analytics are not well suited for in-situ and low cost sensor application. The remarkable progress in semiconductor technology in the past few years has led to a wide spectral range (from 210 nm to some µm) that is covered by LED light sources. Due to the comparable high spectral power density on a tiny space, the ability of electronic modulation and their long lifetime, light emitting diodes are well suited for optical spectroscopy. The high efficiency, small size and its low cost enable the design of small sensors e.g. for hand held devices. The challenge of using LEDs is their limited total emission power and especially the difficulties in a precise stabilization of their output characteristics. Although we’ve realized a modular sensor system that can work with different sensor heads to detect gases with very high resolution. The sensor concept is based on the wavelength specific light absorption of gases or fluids. Molecules have spatial separated charges in its structure they act as forced or stimulated oscillators while an electromagnetic wave passes and reducing its energy. Most molecules show resonant properties in different wavelength regions. There are rotation bands in the far infrared, rotation vibration bands with its fundamentals and overtones in the mid and near infrared and there are resonances almost in the ultraviolet – visible range (UV-VIS) due to valence electron excitations. While in the infrared almost specific line spectra appear (a part is called “finger print region”) - the UV-VIS absorption is often dominated by overlapping of a number of broadened electron resonances. The spectral absorption characteristic follows directly from the setup of the chemical linkage of the molecule. Thus the spectral composition is unique for each substance. Specie concentrations and changes in their molecular linkage can be detected and monitored. Especially the broadband absorptions in the UV-VIS range and the spectral broadened absorptions of fluids are well suited for LED-based spectroscopy. The spectral emission characteristic of a light emitting diode is quasi narrow compared to these absorption characteristics. Different LEDs can be used as spectral selective light sources for sampling characteristic points of an absorption band. Further more LEDs are due to their limited spectral emission width also well suited for correlation spectroscopy because thus the relative intensity change in this method is quite high. Therewith LED-spectroscopy is also suitable for selective measurements of species that show spectral line absorptions. 978-1-4244-2833-5/10/$25.00 ©2010 IEEE