Stephen O'DRiscoll

A novel imaging technique based on the combination of differential optical absorption spectroscopy (DOAS) and tomographic reconstruction is presented. This approach, called Tomographic-DOAS, makes it possible to image the spatial distribution of multiple pollutants in an arbitrary region of the atmosphere. Tomographic-DOAS relies on a mathematical model relating measurements of differential optical density to an underlying spatial distribution of pollutants. This model requires the development of an optical system capable of generating multibeam paths or sensing geometries. The optical instrumentation used to construct such sensing geometries is described and a novel scanning beam arrangement that allows a single beam DOAS system to be adapted for this purpose is proposed. To assess the performance that can be expected from such an optical system, a computational fluid dynamic (CFD) model of pollutant behaviour in a typical street canyon is used as a phantom distribution. The imaging process is then modelled and the performance of a number of common algebraic reconstruction algorithms assessed by comparison to this phantom. This assessment is conducted using a new performance metric that combines measures of reconstruction accuracy and speed. The results of this analysis indicate that algorithms such as ART and SART are well- suited to this reconstruction process. However, a new algorithm called WART exhibits improved performance in a number of situations. As part of this assessment, the effect of sensing geometry beam loss and differential optical density noise is simulated. These results indicate that catastrophic degradation of reconstruction quality can occur in some cases of beam path loss and that, in general, noise contamination introduces false high-frequency features in reconstructions and degrades their overall quality.

We describe a novel camera phone-based optical sensing platform capable of real-time quantitative fluorescence-based measurements. The platform employs the phone's camera as a photodetector in a non-imaging optical configuration that simultaneously detects fluorescence from a number of sensor spots on a disposable optical sensor chip. The platform combines the functionality of the camera with the phone's data-processing capabilities to facilitate on-phone analysis of detected fluorescence intensity. It is envisaged that such a platform will have significant potential in application areas such as environmental monitoring and healthcare where there is a growing demand for decentralized diagnostics using low-cost, portable devices, particularly in remote or low-resource environments.

We demonstrate the use of a camera phone as a low-cost optical detector for quantitative analysis of a high-sensitivity C-reactive protein (hs-CRP) enzyme-linked immunosorbent assay (ELISA). The camera phone was used to acquire images of the ELISA carried out in a conventional 96 well plate. Colorimetric analysis of the images was used to determine a standard curve that exhibited excellent agreement with a fitted 4-parameter logistic model (R ² =0.998). The limit of detection (LOD) for this approach was determined to be 0.026±0.002 μg/ml (1.035±0.079 μM) CRP. Furthermore, these results were found to be in very close agreement with measurements obtained for the same assay using a laboratory-based instrument. These findings indicate the basic technology to enable low-cost quantitative home-based monitoring of an important clinical biomarker of inflammatory disease may already be present in the patient's home.

In this letter, we report a simple optical strategy designed to significantly enhance the fluorescence collection efficiency of planar substrate-based optical sensors. The strategy involves the use of refractive and reflective optical elements that permit supercritical angle fluorescence to be transmitted through the lower surface of the planar substrate and redirected toward an optical detector. Importantly, this approach facilitates simple integration with existing multianalyte sensor systems and microfluidic platforms by leaving the sensor's upper planar surface unstructured. In comparison to a conventional planar substrate configuration, the proposed approach can provide better than an order of magnitude (factor of 20) enhancement in the fluorescence intensity detected. It is envisaged that this approach will have significant potential in a range of application areas where there is need for low-cost optical sensor technologies.

In this letter, we report on the development of a novel camera phone-based UV-dosimeter for monitoring the solar disinfection (SODIS) of water. The dosimeter consists of a UV indicator, methylene blue, dispersed in an ethylcellulose-based polymer matrix. To provide quantitative measurement of UV dose, we demonstrate the use of a camera phone to analyze dosimeter color change in response to UV exposure. The dosimeter response exhibits excellent agreement with a polynomial model (R²) >; 0.99) over the UV exposure range tested. A notable advantage of the dosimeter described here is that it can be deposited on a variety of substrates with the potential to be incorporated into water containers. It is envisaged that use of such a dosimeter in conjunction with mobile phone technology will enhance the use of SODIS thereby impacting significantly on the challenge of providing clean drinking water in developing regions of the world.

Electron Multiplying Charge Coupled Devices (EMCCDs) are CCD cameras with potentially single-photon detection ability. Signal amplification is achieved by way of a unique electron-multiplying structure built into the silicon, and the gain can be varied in order to overcome the read-noise floor, which is the usual limiting factor in reading out a conventional CCD at high frame rates. In combination with its high quantum efficiencies, the EMCCD holds great promise for time-resolved photometry. We report here results from two observing campaigns aimed at assessing the suitability of EMCCD technology for detecting short-timescale, low-amplitude variability in blazars. Data were taken on the 2.2m telescope at Calar Alto using both front-illuminated and back-illuminated EMCCD cameras from Andor Technology’s iXon range. Approximately 410,000 science frames were recorded over 10 nights. The results presented here illustrate the photometric stability achieved with the cameras, under typical observing conditions. In general, photometric precision down to the level of a few millimagnitudes is found to be possible. We argue that reliable photometry is best achieved with high data collection rates (typically 4 frames per second) coupled to ultra-low-noise detectors such as the EMCCD.

Developments in imaging technology over the past decade have provided impetus toward the realization of automated data reduction systems within the astronomical community. These developments, in particular advances in CCD technology, have meant that the data envelope associated with even modest observing programmes can reach gigabyte volumes. We describe the development of an automated data reduction system for differential photometry called PhotMate. For issues of reuse and interoperability the system was developed entirely within the IRAF environment and now forms the backbone of our data analysis procedure. We discuss the methodologies behind its implementation and the use of IRAF scripts for the realization of an automated process. Finally, we place the effectiveness of such a system in context by reference to two recent observing runs at the Calar Alto observatory where we tested a new low light level (L3) CCD. It is our belief that this observing campaign is an important indicator of future trends in observational optical astronomy. As the cost of such devices decreases, their usage will increase and with it the volume of data collectively generated, toppling large astronomical projects as the primary data generators.

Potential multi-path spectroscopic differential absorption systems for the measurement of trace gases in the atmosphere are presented. Recommendations are made for system designs to be used for tomographic differential optical absorption spectroscopy (DOAS) measurements. The difficulties in producing multiple simultaneous beams from a single artificial light source are discussed, while the need for further tomographic resolution information is highlighted. An innovative prototype scanning-DOAS system is presented. The system is designed to generate multiple sequential paths, without the need for bulky and expensive mechanical systems. The scanning DOAS system can easily be placed in an industrial production plant or in a street canyon for multiple direction monitoring. The scanning DOAS system is currently being tested at the Cork Institute of Technology (CIT).© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.