Marc Baumgardner, PhD

LED-Spectroscopy

Lasers have been used for many years as a means of combustion diagnostics because they are non-intrusive and offer high accuracy and precision measurements. By far, two of the most common and well understood techniques are laser-induced absorbance and fluorescence wherein the wavelength of the laser is tuned to a specific optical transition of a target species (typically with a precision of much less than a nano-meter). Such techniques offer a means of speciation and temperature measurements within a variety of combusting systems such as flames and engines. However, these tools are not without their disadvantages, the most obvious being cost. A typical laser diagnostic setup using a tunable diode laser can easily cost half a million dollars (likely more) and take up a significant portion of laboratory space. Furthermore, with the advent of increased desire to have on-board diagnostics of combusting systems in both industry and academic settings, the above drawbacks prohibit the use of lasers in many applications.

Light-emitting diodes (LEDs) are small, inexpensive, consume very little power, and have recently become available with high enough power ratings in narrow enough wavelength ranges that make it possible for them to be used in absorption and fluorescence techniques in much the same way that lasers are currently. LEDs have been used for a number of years in bio and atmospheric sciences in a similar manner as proposed herein, but at wavelengths in the infra-red and not in combusting systems. Alternatively, the current study uses recently available deep-UV and near-UV LEDs as a combustion diagnostic tool in simple flames with the future goal of developing small, portable systems that could be used on “real-world” combustion engines and burners. Fig 1 is a basic layout of the experiment:

Fig 1. Schematic of the experimental setup depicting an absorption measurement.