We have developed a multi-wavelength light source and apply it to micro welding inspection. We project light rays from multiple white light super bright LEDs on a reflection grating. These LEDS are mounted on the specific position to give the diffracted ray with the wavelength range from 480 to 680 nm. The diffracted ray is pass to an optical fibre. The orientation of fibre tube is fixed so diffracted light from each LED will give a specific wavelength. The bandwidths of lights from all LEDS are quite narrow (< 50 nm), which is possible to be used as a light source for multi-wavelength imaging. The diffracted light is transported via optical fibre to a microscope used to inspect the micro welding points on a microelectronics circuit. Due to its very small size, the welding defects are hard to detected, even with high intensity white light. We find that, for typical welding points, the light with wavelength 500 to 600 nm can significantly enhance the contrast of welding image. This agree well with our preliminary result that, with our RGB light source, the maximum contrast can be obtained with the green - yellow to orange - red light. We expect our results to be useful to the inspection of micro welding, especially when integrated to some automation, e.g. machine vision system.

We simulate the wave propagation through various mediums using a graph-theoretical path-finding algorithm. The mediums are discretized to the square lattices, where each node is connected up to its 4th nearest neighbours. The edge connecting any 2 nodes is weighted by the time of flight of the wave between the nodes, which is calculated from the Euclidean distance between the nodes divided by the average velocity at the positions of those nodes. According to Fermat’s principle of least time, wave propagation between 2 nodes will follow the path with minimal weight. We thus use the path-finding algorithm to find such a path. We apply our method to simulate wave propagation from a point source through a homogeneous medium. By defining a wavefront as a contour of nodes with the same time of flight, we obtain a spherical wave as expected. We next investigate the wave propagation through a boundary of 2 mediums with different wave velocities. The result shows wave refraction that exactly follows Snell’s law. Finally, we apply the algorithm to determine the velocity model in a wood sample, where the wave velocity is determined by the angle between the propagation direction and the radial direction from its pith. By comparing the time of flight from our simulation with the measurements, the parameters in the velocity model can be obtained. The advantage of our method is its simplicity and straightforwardness. In all the above simulations, the same simple path-finding code is used, regardless of the complexity of the wave velocity model of the mediums. We expect that our method can be useful in practice when an investigation of wave propagation in a complex medium is needed.