A simple microcontroller based digital lock-in amplifier for the detection of low level optical signals

Traditionally digital lock-in amplifiers sample the input signal at a rate much higher than the lock-in reference frequency and perform the lock-in algorithm with high-speed processors. We present a small and simple digital lock-in amplifier that uses a 20 bit current integrating analogue-to-digital converter interfaced to a microcontroller. The sample rate is set to twice the reference frequency placing the sampled lock-in signal at the Niquest frequency allowing the lock-in procedure to be performed with one simple algorithm. This algorithm consists of a spectral inversion technique integrated into a highly optimised low-pass filter. We demonstrate a system with a dynamic range of 103dB recovering signals up to 85dB below the interference

Evaluating the mineral nutrient status of fresh pasture herbage using laser-induced breakdown spectroscopy

Comprehensive determination of the mineral nutrient status of pasture or horticultural crops currently requires leaf or herbage samples to be harvested and taken to a laboratory for analysis, which is both labour-intensive and time-consuming. This study examined the potential of laser-induced breakdown spectroscopy (LIBS) to provide immediate, in-field assessment of the mineral nutrient status of standing plants. Success would offer the prospects of spatially and temporally improved knowledge of plant status and of real-time remediation of nutrient deficiency. The mineral nutrient status of fresh pasture herbage has been evaluated using an Applied Photonics Ltd？ LIBS-6 system, comprising a Qswitched Nd:YAG laser, a SpectroModule-6 spectrometer (covering wavelengths in the range ~185 – 1064 nm) and an ICE 450 water-cooled Laser power supply. LIBS spectra obtained from the fresh pasture samples were collected with the aim of determining the nutrient status of the herbage. An enclosed Modular Sample Chamber housed the Qswitched Nd:YAG laser, generating energy in the near infrared region at λ = 1064nm. The laser power was set to 100mW per pulse. Samples of approximately 50g of fresh ryegrass and ryegrass/clover mixed pasture were loaded into a sample holder which allowed a flat ‗carpet‘ of individual leaves to be presented at a relatively uniform distance from the laser. The optimum vertical distance between sample and laser was determined by maximising spectra amplitudes. Automated control of the translation stage supporting the sample holder was used to ensure each shot of the laser was acquired from a fresh location. Spectrum sets comprising both 100 individual spectra and 100 shot accumulated spectra were obtained from each sample under both air and under the inert gas, argon. Following the spectral analysis of the fresh pasture, each sample was sent to a commercial laboratory for standard nutrient analysis, providing elementary composition on the most common plant elements (N, P, K, S, Ca, Mg, Na, Fe, Mn, Zn, Cu and B). Once standard nutrient analysis was completed a second set of spectra was taken from tablets formed from compressed dried ground powder remaining after laboratory analysis. To date, initial results from simple chemometric analysis have shown limited success in predicting the nutrient content of fresh pasture with slightly improved results in estimating tablet composition. While initial analyses have concentrated on trends and correlations between actual spectra and laboratory analysis, known emission spectra, have for the meantime, been ignored. Further in-depth analysis using more robust chemometric analysis, utilizing known emission spectra and adjusting for variations in plasma intensity are currently underway and are expected to improve the accuracy of composition measurement

Workshop on Smart Sensors - Instrumentation and Measurement: Program

On 18-19 February, the School of Engineering successfully ran a two-day workshop on Smart Sensors - Instrumentation and Measurement. Associate Professor Rainer Künnemeyer organised the event on behalf of the IEEE Instrumentation and Measurement Society, New Zealand Chapter. Over 60 delegates attended and appreciated the 34 presentations which covered a wide range of topics related to sensors, sensor networks and instrumentation. There was substantial interest and support from local industry and crown research institutes

Accelerating Monte Carlo simulations with an NVIDIA® graphics processor

Modern graphics cards, commonly used in desktop computers, have evolved beyond a simple interface between processor and display to incorporate sophisticated calculation engines that can be applied to general purpose computing. The Monte Carlo algorithm for modelling photon transport in turbid media has been implemented on an NVIDIA® 8800gt graphics card using the CUDA toolkit. The Monte Carlo method relies on following the trajectory of millions of photons through the sample, often taking hours or days to complete. The graphics-processor implementation, processing roughly 110 million scattering events per second, was found to run more than 70 times faster than a similar, single-threaded implementation on a 2.67 GHz desktop computer

Polarization tunable selective polariton generator

A selective polariton generator (SPG) design, based on surface plasmon antennae principles, is demonstrated to provide a selective light transmission peak. The polarization-sensitive structure selectively generates and transports polaritons of a desired wavelength through a circular subwavelength aperture. By varying the SPG structure around a central nanohole, we are able to control the peak optical transmission wavelengths via the polarization state of the incident photons. We find good agreement between simulations and experimental results

A combined optical, thermal and electrical performance model of a Building Integrated Photovoltaic/Thermal Concentrator (BIPVTC)

The electrical output of concentrating photovoltaic devices is significantly affected by the temperature of the photovoltaic cells. The ability to actively cool photovoltaic cells under concentrated radiation allows their electrical efficiency to be maintained particularly during periods of high solar radiation when concentration offers the maximum benefit. In this study, the design of a novel photovoltaic/thermal solar concentrator for building integration (BIPVTC) is discussed. The optical, thermal and electrical performance of the collector was theoretically modelled and validated with experimental data. The results show that BIPVTC offers improved electrical yields from both concentrating radiation onto the photovoltaic cells and also by actively cooling them

Polarisation and wavelength selective transmission through nanohole structures with multiple grating geometry

Excitation and localization of surface plasmon polariton modes in metal-dielectric structures can be utilized to construct nanophotonic materials and devices with tuneable optical dispersion. We present a selective polariton generator (SPG) device that demonstrates switching of light transmission based on surface plasmon antennae principles. This polarization-sensitive structure selectively generates and transports polaritons of a desired wavelength through subwavelength apertures. Two of these SPGs have been combined around a nanohole into a new, single device that allows polarization and wavelength selective switching of transmission. The multi-state operation is confirmed by experiment results

Proof of concept of diffuse optical tomography using time-of-flight range imaging cameras

Diffuse optical tomography is an optical technique to create 3-dimensional images of the inside of highly scattering material. Research groups around the world have been developing imaging systems using various source-detector arrangements to determine optical properties of biological tissue with a focus on medical applications. In this paper we investigate whether a range imaging camera can be used as a detector array. We used time-of-flight range imaging cameras instead of the conventional source-detector array used by others. The results provided in this paper show reconstructed images of absorption and reduced scattering of an object submerged in a tissue simulating phantom. Using the ranging camera XZ422 Demonstrator and the NIRFAST software package, we reconstructed 2D images of a 6 mm metal rod submerged in the centre of a 5 cm deep tank filled with 1% IntralipidTM. We have shown for the first time that range imaging cameras can replace the traditional detectors in diffuse optical tomography

Optical full Hadamard matrix multiplexing and noise effects

Hadamard multiplexing provides a considerable SNR boost over additive random noise but Poisson noise such as photon noise reduces the boost. We develop the theory for full H-matrix Hadamard transform imaging under additive and Poisson noise effects. We show that H-matrix encoding results in no effect on average on the noise level due to Poisson noise sources while preferentially reducing additive noise. We use this result to explain the wavelength-dependent varying SNR boost in a Hadamard hyperspectral imager and argue that such a preferential boost is useful when the main noise source is indeterminant or varying

Range imager performance comparison in homodyne and heterodyne operating modes

Range imaging cameras measure depth simultaneously for every pixel in a given field of view. In most implementations the basic operating principles are the same. A scene is illuminated with an intensity modulated light source and the reflected signal is sampled using a gain-modulated imager. Previously we presented a unique heterodyne range imaging system that employed a bulky and power hungry image intensifier as the high speed gain-modulation mechanism. In this paper we present a new range imager using an internally modulated image sensor that is designed to operate in heterodyne mode, but can also operate in homodyne mode. We discuss homodyne and heterodyne range imaging, and the merits of the various types of hardware used to implement these systems. Following this we describe in detail the hardware and firmware components of our new ranger. We experimentally compare the two operating modes and demonstrate that heterodyne operation is less sensitive to some of the limitations suffered in homodyne mode, resulting in better linearity and ranging precision characteristics. We conclude by showing various qualitative examples that demonstrate the system’s three-dimensional measurement performance