Radiometric analysis of LEDs and the use of rapidly pulsed infrared LEDs for portable sensing of gases

This thesis focuses on the radiometric analysis of commercial light emitting diodes (LEDs) and investigation and development of an infrared (IR) LED based optical sensing system for continuous monitoring of methane gas with wireless, on-the-fly and flexible data processing capabilities in indoor and outdoor environments. The areas of analytical use of LEDs as well as of gas monitoring with IR LEDs covered in the scientific literature are reviewed in Chapter 1. The first experimental section of this thesis (Chapter 2) focuses on radiometric analysis of LEDs. LEDs have been established as light sources in countless areas where they offer a better alternative to traditional light sources in terms of low-cost, small size and robustness supporting portability, performance parameters such as low noise and wide applicability. Lack of rapid, facile and accurate radiometric analysis of LEDs is a major limiting factor which constrains their purposeful use in analytical chemistry. A holistic radiometric analysis of LEDs in terms of absolute emission spectra analyses, radiometric power output, irradiances, radiant efficiencies as well as uncertainties are invaluable to analytical chemists as such analysis has the capability to justify the purposes of LEDs as light sources in analytical chemistry. We demonstrate and cross-validate a rapid, facile, accurate and low-cost radiometric analysis of LEDs with directional light output using a large active area silicon photodiode in a simple optical design with the LED light source in proximity, without the need for a calibrated light source. The obtained radiometric data for a wide range of 21 commercial LEDs in UV, visible and near-infrared (NIR) spectral range (255-950 nm) agree very well with two completely independent approaches: chemical actinometric and spectrophotometric methods. At first, an excellent agreement was achieved with accuracy within 5% for radiometric power output (mW) measured using chemical actinometric methods and further, the accuracy of irradiance (mW/cm2) measurement was within 2% when compared with a spectrophotometric method based on a radiometrically calibrated spectrophotometer. The measurement uncertainty at 95% confidence level for the values of radiometric power output were reduced 3-folds compared to the existing techniques. It is also demonstrated that this facile, accurate and low-cost radiometric analysis can be further extended to accurately measure quantum yield of photochemical reactions and fluence values in actinometric systems. The second experimental section of this thesis (Chapter 3) explores design of a portable platform for rapid pulsed signal generation and on-the-fly data processing using an open source micro controller (˜í¬¿C) with built-in field programmable gate array (FPGA). IR LEDs operated in a rapid pulsed mode are suitable for portable low-cost optical sensing of gases with the transmitted light detected by a IR sensitive photodiode. We design a ˜í¬¿C-FPGA based flexible and portable system, programmed with custom software, for rapid current pulse generation (ca. 2 ˜í¬¿s short pulses with a typical repetition rate of 1 kHz) to drive the sensing IR LED as well as for the optical sensing data acquisition and processing. Instrumental signal to noise ratio values (SNR) are investigated as the crucial performance characteristics of the system governing the limit of detection values. Digital data filtering is accomplished first by repetitive smoothing (averaging a number of raw data pulses usually 10 - 10,000), followed by boxcar averaging and Savitzky-Golay (2nd degree polynomial regression) based smoothing. Repetitive smoothing resulted in SNR improvement by a factor of ‚Äöv†v¿n (n is the number of repetitive pulses averaged). Then to determine the detected pulsed signal attenuation (measuring the pulse height), three different statistical methods applied to the corresponding data points at the baseline and at the pulse top were compared: simple averaging, linear regression, and 2nd degree polynomial regression. Finally, each of the digitally processed signal pulses resulted into one data point in time as a quasi-continuous data stream produced at a rate between 1000 and 0.1 Hz (1 point every 1 ms to 10 s, depending on the level of repetitive smoothing). All the in-house developed pulse generation and data processing algorithm were saved in a secure digital (SD) card and data processing was carried out on-the-fly and wirelessly transmitted via network connection. The minimum measurable absorbance corresponding to the highest SNR for n=1000 resulting in quasicontinuous data points at 1 Hz was found 10\\(^{-4}\\) a.u. This low cost portable system offers ultimate custom-defined software flexibility of on-the-fly data processing that can be applicable to a number of pulsed data acquisition and sensing scenarios including real-time indoor and outdoor monitoring of gases. In the third and last experimental section (Chapter 4), we investigate the design of a nondispersive infrared (NDIR) spectroscopy based sensor for continuous monitoring of gases, considering CH\\(_4\\) as a model gas, with rapidly pulsed near-infrared (NIR) LED. Continuous sensing of fugitive emission of gases in portable and remote conditions in indoor and outdoor environments are challenging due to the technical requirements for small size and low weight and the need of on-the-fly processing of large data streams. In this work, we design a facile, low-cost and weight nondispersive infrared (NDIR) spectroscopy based system for continuous sensing of atmospheric methane (CH\\(_4\\)) with rapidly pulsed nearinfrared light emitting diode (NIR LED) at 1.65 ˜í¬¿m. It uses a microcontroller with field programmable gate array (˜í¬¿C-FPGA) enabling on-the-fly and wireless streaming and processing of large data streams (~2Gbit/s). The investigated NIR LED based sensor offered favourable limit of detection (LOD) of 300 ppm (0.03%) CH\\(_4\\) and precision of ¬¨¬±5% (RSD). All the generated raw data were processed automatically on-the-fly in the ˜í¬¿C-FPGA and transferred wirelessly via network connection. The sensing device was then deployed in portable sensing of atmospheric CH\\(_4\\) at a local landfill, resulting in quantified concentrations within the sampling area (ca 400 m\\(^2\\)) in the range from 0.5% to 3.35% CH\\(_4\\) and was crossvalidated with GC-MS (2.1%). This NIR LED based sensor system offers a facile low-cost solution for continuous real-time, quantitative and direct measurement of CH\\(_4\\) concentrations in indoor and outdoor environments, and possesses future potential for remote monitoring of gases directly from mobile platforms such as, smartphones and unmanned aerial vehicles (UAV).