Paul K. Buah-Bassuah

A portable fibre-probe ultraviolet light emitting diode (LED)-induced fluorescence detection system

A portable fibre-probe fluorescence detection system comprising a continuous-wave high-power ultraviolet light emitting diode (UV LED) emitting at 365 nm as excitation source, a bifurcated fibre probe with a six-around-one fibre configuration to illuminate and read from a large target area (~3.6 mm2) and an integrated PC-coupled spectrometer has been developed. The construction, calibration and operation of the fluorescence detection system are described. Demonstrative test measurements with the system for possible inspection of different ripening stages on some batches of horticultural and agricultural products (lemon, mandarin, banana leaf and ivy leaf) have been performed and results presented. The system is portable, comparatively low cost, easily operated and relative immune to ambient light, thus being suitable for field measurements.

An Experimental Investigation of the Break-up of a Liquid Drop Falling in a Miscible Fluid

A liquid drop falling in a lighter miscible fluid either undergoes a cascade of fragmentations, each appearing as a dynamic instability, or it mixes by diffusion, depending on the value of a fragmentation number F. F is the ratio of the diffusion time to the time required for the fluid to convectively mix. We assign an accurate experimental value to the critical fragmentation number Fc, which appears to be universal. When the fragmentation occurs, the interfacial area increases via successive splittings which display striking symmetries. We present experimental evidence and a qualitative explanation of such a phenomenon.

Using violet laser-induced chlorophyll fluorescence emission spectra for crop yield assessment of cowpea (Vigna unguiculata (L) Walp) varieties

The use of violet laser-induced chlorophyll fluorescence (LICF) emission spectra to monitor the growth of five varieties of cowpea in the University of Cape Coast Botanical Garden is presented. Radiation from a continuous-wave violet laser diode emitting at 396 nm through a fibre is closely incident on in vivo leaves of cowpea to excite chlorophyll fluorescence, which is detected by an integrated spectrometer with CCD readout. The chlorophyll fluorescence spectra with peaks at 683 and 731 nm were used for growth monitoring of the cowpea plants over three weeks and analysed using Gaussian spectral functions with curve fitted parameters to determine the peak positions, area under the spectral curve and the intensity ratio F683/F731. The variation in the intensity ratio of the chlorophyll bands showed sensitive changes indicating the photosynthetic activity of the cowpea varieties. A discussion of the fluorescence result as compared to conventional assessment is presented with regard to discrimination between the cowpea varieties in terms of crop yield performance.

Fragment Formation in the Break-up of a Drop Falling in a Miscible Liquid.

When falling in a lighter miscible solvent, a drop of liquid deforms to a torus which then breaks up into several fragments or just disappears by diffusion. By using liquids of different compositions we show the universal behaviour of the phenomenon, and its dependence on two nondimensional numbers, the fragmentation number F, and the Schmidt number S. While F marks the transition from diffusion to splitting, here we show the role of S in controlling the number of horizontal fragments after the first break-up. The process is explained in terms of competitions of different time scales.

Track analysis of laser-illuminated etched track detectors using an opto-digital imaging system

An opto-digital imaging system for counting and analysing tracks on a LR-115 detector is described. One batch of LR-115 track detectors was irradiated with Am-241 for a determined period and distance for linearity test and another batch was exposed to radon gas. The laser-illuminated etched track detector area was imaged, digitized and analysed by the system. The tracks that were counted on the opto-digital system with the aid of media cybernetics software as well as spark gap counter showed comparable track density results ranging between 1500 and 2750 tracks cm?2 and 65 tracks cm?2 in the two different batch detector samples with 0.5% and 1% track counts, respectively. Track sizes of the incident alpha particles from the radon gas on the LR-115 detector demonstrating different track energies are statistically and graphically represented. The opto-digital imaging system counts and measures other track parameters at an average process time of 3?5 s.

A hybrid quantum eraser scheme for characterization of free-space and fiber communication channels

Abstract We demonstrate a simple projective measurement based on the quantum eraser concept that can be used to characterize the disturbances of any communication channel. Quantum erasers are commonly implemented as spatially separated path interferometric schemes. Here we exploit the advantages of redefining the which-path information in terms of spatial modes, replacing physical paths with abstract paths of orbital angular momentum (OAM). Remarkably, vector modes (natural modes of free-space and fiber) have a non-separable feature of spin–orbit coupled states, equivalent to the description of two independently marked paths. We explore the effects of fiber perturbations by probing a step-index optical fiber channel with a vector mode, relevant to high-order spatial mode encoding of information for ultra-fast fiber communications.

Violet diode laser-induced chlorophyll fluorescence: a tool for assessing mosaic disease severity in cassava (Manihot esculenta Crantz) cultivars

Violet diode laser-induced chlorophyll fluorescence was used in agronomical assessment (disease severity and average yield per plant). Because cassava (Manihot esculenta Crantz) is of economic importance, improved cultivars with various levels of affinity for cassava mosaic disease were investigated. Fluorescence data correlated with cassava mosaic disease severity levels and with the average yield per plant.

Imaging and phase measurements of 3D objects at 10.6 microns by digital holography

Digital holography in the mid infrared range is shown to be a feasible technique for optical metrological applications. The technique allows to reconstruct both amplitude and phase of wavefronts scattered by a 3D object. Experimental results of the method applied to the reconstruction of digitally holograms recorded at CO2 laser wavelength of 10.6 micron are reported. It is show that good reconstructions can be obtained even with the lower spatial resolution of IR recording detectors compared to visible CCD array. The results show that new prospective can be exploited by using high power CO2 laser sources in optical metrological applications.

Soil textural classification by a photosedimentation method.

A photosedimentation technique is used to analyze the size composition of soil samples. The number and size of the particles are determined, respectively, by the Stokes formula and the Beer-Lambert law, measuring time-of-flight and laser light attenuation simultaneously and hence evaluating solution turbidity. A simple software procedure has been developed to obtain fractional volume size distribution, taking into account the particles optical properties depending mainly on its size and refractive index. Laboratory measurements on calibrated particulates, showing their reproducibility and validation as well as a classification of ground samples, are presented. Size distribution data can then be utilized to obtain a textural classification of the soil samples for agricultural applications.

Making a microscope with readily available materials

The making of microscope devices using inexpensive or recovered materials is demonstrated. Examples of images illustrating the performance of such devices are presented. As a project at the undergraduate level, the task is effective in acquiring familiarity with optical imaging concepts and their practical implementation in the laboratory.