Brenda H. Timmerman

High-resolution digital holography utilized by the subpixel sampling method

A novel (to our knowledge) approach for resolution improvement in digital holography is presented in this paper. The proposed method is based on recording the incoming interference field on a complementary metal-oxide semiconductor (CMOS) camera with subpixel resolution. The method takes advantage of the small pixel size of the CMOS sensor, while overcoming the reduced fill factor. This paper describes the experimental and numerical procedures. The improvement of the obtainable optical resolution, image quality, and phase measurement accuracy are demonstrated within this paper.

Intelligent diagnostic optics for flow visualization

Abstract A review of several current optical diagnostics used for flow visualization is presented and the limitations and strengths of each technique are described. A new type of intelligent diagnostic optic designed for making three-dimensional measurement of velocity in a gas turbine combustor is proposed. The diagnostic uses an in-line tomographic approach combined with correlation theory to spatially locate structure within the flow. A discussion is then made as to why some optical diagnostics have been more successful than others in their general application. The potential advantages of evolving new technology and the implications for future instrumentation are also discussed.

Human corneal stromal lamellar organisation : a polarised light study in pseudophakic eyes

A novel qualitative method is presented for determining the macroscopic distribution of collagen lamellae within the central human cornea in vivo. A modified digital photo slit-lamp camera adapted for use with circular polarised light is used. 19 eyes of 10 pseudophakic subjects (with prosthetic intraocular lens implants) are involved in the experiment. Confocal elliptic/hyperbolic populations of collagen fibrils in central and intermediate corneal zones are identified in all eyes with a superotemporal-inferonasal preferred orientation. Approximate mirror symmetry is found to exist between pairs of eyes. The results demonstrate capability for detail not previously reported. The identified distribution of central collagen is consistent with the reported birefringent properties of the central cornea and is compatible with X-ray scatter findings.

Resolution improvement in digital holography: comparison between synthetic aperture method and spatial averaging method

In the last two decades digital holography emerged as one of the most promising techniques for obtaining the complex object-wave (amplitude and phase). However, due to the coherent nature of the light source used in digital holography, the reconstructed hologram is subjected to speckle noise. Moreover, the resolution and size of the sensor employed in digital holography are smaller compared to the formerly used holographic plates in optical holography. This results in a reduced resolution for the reconstructed hologram. This paper discusses two resolution improvement methods, which are both based on the same recording process. However, the recorded data is processed differently to obtain a resolution improved reconstruction. The two methods are compared in terms of corresponding optical resolution, phase accuracy and processing time.

Optical investigation of heat release and NOx production in combustion

Two passive optical techniques are described to investigate combustion. Optical Emission Tomography (OET) is used for non-intrusive study of heat release through the detection of chemiluminescence by the hydroxyl radical that is generated in the burning process. The OET technique described here is based on a passive fibre-optic detection system, which allows spatially resolved high-frequency detection of the flame front in a combustion flame, where all fibres detect the emission signals simultaneously. The system withstands the high pressures and temperatures typically encountered in the harsh environments of gas turbine combustors and IC engines. The sensor-array is non-intrusive, low-cost, compact, simple to configure and can be quickly set up around a combustion field. The maximum acquisition rate is 2 kHz. This allows spatially resolved study of the fast phenomena in combustion. Furthermore, the production of NOx is investigated through the emission of green light as a result of adding tri-methyl-borate to a flame. In combustion, the tri-methyl-borate produces green luminescence in locations where NOx would be produced. Combining the green luminescence visualisation with OET detection of the hydroxyl radical allows monitoring of heat release and of NOx production areas, thus giving a means of studying both the burning process and the resulting NOx pollution.

Application of flow visualization at MIT to measure the ejection density mixing rate of a high-speed combustor

A rugged optical method has been designed for making measurements in a hostile industrial environments such as the combustion exit flow ofa gas turbine engine. Typically the gas temperature in such a combustor is 1000 K and the speed of the flow 700m/s. A transient shock tube has been constructed at MET (Massachusetts Institute of Technology ). The objective being to simulate the mixing rate for different types of exit combustor exit nozzles. A direct particle visualisation approach has been developed combining a c/w Argon/ion laser and an electronically shuttered image intensifier. The combination has been used measure the density distribution average over the complete 3ms run time ofthe transient shock tube facility. The system also has the potential to measure the entrainment of the surrounding ambient air and make velocity measurements.

Digital image processing techniques for the analysis of fuel sprays global pattern

We studied the fuel atomization process of two fuel injectors to be fitted in a new small rotary engine design. The aim was to improve the efficiency of the engine by optimizing the fuel injection system. Fuel sprays were visualised by an optical diagnostic system. Images of fuel sprays were produced under various testing conditions, by changing the line pressure, nozzle size, injection frequency, etc. The atomisers were a high-frequency microfluidic dispensing system and a standard low flow-rate fuel injector. A series of image processing procedures were developed in order to acquire information from the laser-scattering images. This paper presents the macroscopic characterisation of Jet fuel (JP8) sprays. We observed the droplet density distribution, tip velocity, and spray-cone angle against line-pressure and nozzle-size. The analysis was performed for low line-pressure (up to 10 bar) and short injection period (1-2 ms). Local velocity components were measured by applying particle image velocimetry (PIV) on double-exposure images. The discharge velocity was lower in the micro dispensing nozzle sprays and the tip penetration slowed down at higher rates compared to the gasoline injector. The PIV test confirmed that the gasoline injector produced sprays with higher velocity elements at the centre and the tip regions.

Long range visualisation of 300nm diameter particles as a diagnostic for high speed combustion flows

The paper describes an automated Particle Image Velocimetry (PIV) system designed for synchronised combined 2- and 3-component velocity measurements. The system has been designed to make measurements of velocity in the exhaust region of a scaled version of a gas turbine engine operating at high subsonic velocities and high temperatures (typically 800°C) for various configurations. For the PIV camera light detection the flow was seeded with 0.3μm aluminium oxide and 0.2-0.3μm oil mist particles. Tests were performed to determine optimum camera positions based upon Mie scatter calculations for the particles. An evaluation is presented discussing the design and accuracy of the system.

Instantaneous Impact Measurements of an Anodised Aluminium Surface Using a Birefringent Phase Sensitive High Speed Camera

Photoeleasticity has a long history of application to material testing. However, it often requires a complex coating of a suitable material to reveal the stresses experienced. The alternative is to create a transparent birefringent model. The disadvantages being that the measurements can be masked by the effect of the coating applied or in the case of a model, fail to mimic the properties of the original object. Further the coating or modelling process severely limits the type of application which can be applied. In this case the aluminium has a been anodised with a 8 micron phase sensitive coating.

Digital Double Exposure Holography by Means of Polarization Optics

A suitable setup for deformation measurement based on polarization optics, ensuring high phase step accuracy, is demonstrated. Digital Double Exposure Holography has been applied for the investigation of the camera captured intensity images.