Simon R. G. Hall

Characterization of deformable mirrors for spherical aberration correction in optical sectioning microscopy

In this paper we describe the wavefront aberrations that arise when imaging biological specimens using an optical sectioning microscope and generate simulated wavefronts for a planar refractive index mismatch. We then investigate the capability of two deformable mirrors for correcting spherical aberration at different focusing depths for three different microscope objective lenses. Along with measurement and analysis of the mirror influence functions we determine the optimum mirror pupil size and number of spatial modes included in the wavefront expansion and we present measurements of actuator linearity and hysteresis. We find that both mirrors are capable of correcting the wavefront aberration to improve imaging and greatly extend the depth at which diffraction limited imaging is possible.

LED Junction Temperature Measurement Using Generated Photocurrent

LED-based lamps that are currently on the market are expensive due to the complex packaging required to dissipate the heat generated. This also limits their performance and lifetime due to the degradation of the phosphor or individual LED chips, in the case of RGB sources. There is a strong commercial imperative to develop in situ technology to measure and ultimately compensate for the thermal environment of a luminaire.

Compressive Current Response Mapping of Photovoltaic Devices Using MEMS Mirror Arrays

Understanding the performance and aging mechanisms in photovoltaic devices requires a spatial assessment of the device properties. The current dominant technique, electroluminescence, has the disadvantage that it assesses radiative recombination only. A complementary method, laser beam-induced current (LBIC), is too slow for high-throughput measurements. This paper presents the description, design, and proof of concept of a new measurement method to significantly accelerate LBIC measurements. The method allows mapping of the current response map of solar cells and modules at drastically reduced acquisition times. This acceleration is achieved by projecting a number of mathematically derived patterns on the sample by using a digital micromirror device (DMD). The spatially resolved signal is then recovered using compressed sensing techniques. The system has fewer moving parts and is demonstrated to require fewer overall measurements. Compared with conventional LBIC imaging using galvanic mirror arrangements or xy scanners, the use of a DMD allows a significantly faster and more repeatable illumination of the device under test. In this proof-of-concept instrument, sampling patterns are drawn from Walsh-Hadamard matrices, which are one of the many operators that can be used to realize this technique. This has the advantage of the signal-to-noise ratio of the measurement being significantly increased and thus allows elimination of the standard lock-in techniques for signal detection, reducing measurement costs, and increasing measurement speed further. This new method has the potential to substantially decrease the time taken for measurement, which demonstrates a dramatic improvement in the utility of LBIC instrumentation.

Fast Current Mapping of Photovoltaic Devices Using Compressive Sampling

Light Beam Induced Current (LBIC) measurements are a useful tool in photovoltaic (PV) device characterisation for accessing the local electrical properties of PV devices. The main disadvantage of a typical LBIC system is measurement time, as a raster scan of a typical silicon solar cell can last several hours. The focus of this paper is the reduction of LBIC measurement time by means of compressed sensing (CS). The CS-LBIC system described in this paper can theoretically reduce measurement time to less than 25% of that required for a standard LBIC raster scan. Measurement simulations of a CS-LBIC system are presented as well as a practical demonstration using a digital micro-mirror array, which further reduces the measurement time by an order of magnitude. Instead of a raster scan, the PV device under measurement is sampled by a series of patterns and the current map is reconstructed using an optimization algorithm. Simulations of CS-LBIC measurements using the 2D spatially-resolved PV-Oriented Nodal Analysis (PVONA) model developed at CREST are used as a tool to explore the capabilities and verify the accuracy of this measurement technique as well as its ability to detect specific defects, such as cracks and shunts. Simulation results confirm that the CS sampling theory can be applied as an effective method for significantly reducing measurement time of current mapping of PV devices. An initial CS-LBIC system prototype has been built at the National Physical Laboratory (NPL) and measurements of small area devices (1cm x 0.8cm) using this system are given. The current maps are created using a Digital Micromirror Device (DMD) kit as a pattern generator. The response time of the micro mirror array is less than 20μs. This is another factor in the reduction of measurement time, as the movement time of an x-y translation stage is considerably slower. Initial measurement results show that current maps of PV cells can be acquired with 75% fewer measurements which, combined with the fast response of the pattern generator, can reduce LBIC measurement time by an order of magnitude.

Improvement and commissioning of a novel technology for the measurement of laser-beam profiles

Measurement of the laser beam propagation factor M2 is essential in many laser applications including materials processing, laser therapy, and lithography. In this paper we describe the characterisation of a prototype device using a cross-distorted diffraction grating known as an Image Multiplex (IMP(R)) grating, to measure the M2 value of laser beams. The advantage of the IMP(R) grating instrument lies in its ability to simultaneously image nine positions along the beam path. This enables beam propagation parameters to be calculated both for pulsed lasers and lasers with rapidly changing propagation characteristics. This is in contrast to the scanned technique recommended by the ISO, which is relatively slow and in practice can only be easily used with cw sources. The characterisation was accomplished by comparison of results from the IMP(R) grating device with those obtained using the accepted methodology described in the ISO 11146 series of standards through measurements conducted by the National Physical Laboratory. The scope of the work also included provision of a traceability route to international standards, and an uncertainty budget, to allow the intended user community to have confidence in measurements obtained when using the device, and to enable them to use it as part of their quality framework.

Compressed Sensing Current Mapping Spatial Characterization of Photovoltaic Devices

A new photovoltaic (PV) device current mapping method has been developed, combining the recently introduced compressed sensing (CS) sampling theory with light beam induced current (LBIC) measurements. Instead of a raster scan, compressive sampling is applied using a digital micromirror device. The aim is to significantly reduce the time required to produce a current map, compared to conventional LBIC measurements. This is achieved by acquiring fewer measurements than a full raster scan and by utilizing the fast response of the micromirror device to modulate measurement conditions. The method has been implemented on an optical current mapping setup built at the National Physical Laboratory, U.K. Measurements with two different PV cells are presented in this paper and an analytical description for realization of an optimized CS current mapping system is provided. The experimental results illustrate the feasibility of the method and its potential to significantly reduce measurement time of current mapping of PV devices.

Compressed sensing current mapping methods for PV characterisation

The Compressed Sensing (CS) sampling theory has been combined with the Light Beam Induced Current (LBIC) method, to produce an alternative current mapping technique for photovoltaic (PV) devices. Compressive sampling of photocurrent is experimentally implemented using a Digital Micro-mirror Device (DMD). The main advantage of this new method for current mapping is that measurement time can be significantly reduced compared to conventional LBIC measurement systems. This is achieved mainly by acquiring fewer measurements than a raster scan would need and by utilizing the fast response of the micro-mirror array. Two different experimental layouts are considered in this work. The first is a small area optical set-up based on a single wavelength laser source. The second layout utilizes a commercial Digital Light Processing (DlP) projector through which compressive sampling is applied. Experimental results with both experimental schemes demonstrate that current maps can be produced with less than 50% of the measurements a standard LBIC system would need. The ability to acquire current maps of individual cells in encapsulated modules is also highlighted. The advantages and drawbacks of the method are presented and its potential to significantly reduce measurement time of current mapping of PV cells and modules is indicated.

Optical technique for photovoltaic spatial current response measurements using compressive sensing and random binary projections

Abstract. Compressive sensing has been widely used in image compression and signal recovery techniques in recent years; however, it has received limited attention in the field of optical measurement. This paper describes the use of compressive sensing for measurements of photovoltaic (PV) solar cells, using fully random sensing matrices, rather than mapping an orthogonal basis set directly. Existing compressive sensing systems optically image the surface of the object under test, this contrasts with the method described, where illumination patterns defined by precalculated sensing matrices, probe PV devices. We discuss the use of spatially modulated light fields to probe a PV sample to produce a photocurrent map of the optical response. This allows for faster measurements than would be possible using traditional translational laser beam induced current techniques. Results produced to a 90% correlation to raster scanned measurements, which can be achieved with under 25% of the conventionally required number of data points. In addition, both crack and spot type defects are detected at resolutions comparable to electroluminescence techniques, with 50% of the number of measurements required for a conventional scan.