Method
The method used for this experiment is outlined in the following steps:
1. Calibration of the experimental setup. Calibration is necessary to ensure that the images obtained are focussed and the scale factor necessary for processing of the images is obtained. Calibration can be done in a number of ways. The two ways considered during this experiment are: use of a calibration target and real-time analysis of the correlation vectors in the flow field analysed by the computer.
The first way makes use of a calibration target, given in Figure 2.
Figure 2 – Captured image of the calibration target
This calibration target is placed parallel to the light sheet and approximately in the middle. The calibration target consists or a grid of dots with a bigger central dot surrounded by four smaller dots. The distance between two dots on this calibration target is given as 5 mm. Images are captured of the calibration target when placed in the light sheet. These images are analysis by the computer and the scale factor is deduced by comparing the apparent distance between the dots provided by the perspective of the CCD camera and the actual distance of 5 mm. Before the images are captured, the camera is focussed such that all the dots appear sharp. This is confirmed by analysing the histogram of the greyscale image and ensuring that the signal peaks present are only black and white.
The second calibration method that can be employed is real-time analysis of the correlation vectors in the direction flow field processed by the computer. Typically noise signals are represented by chaotic and large vectors in this direction field. By focussing the camera, the volume of these vectors in the direction field can be minimised. When perfectly focussed theoretically there should not be any of these noise correlation vectors present. This however in practice is difficult to ensure when working under tight time constraints so a level of tolerance is adopted.