Master Thesis - Fachbereich Informatik

3.3. ILLUMINATION 45
(a) (b) (c)
Figure 3.8: Back lighting through different types of conveyor belts. The structure of the
belt determines the amount of light entering the camera, thus, influencing the image quality
significantly.
must be avoided. In addition, the illumination setup should cover both black and transparent
tubes, whereas the transparent tubes are translucent while the black ones are not.
The surface of both materials appears mat under diffuse illumination, but shows specular
reflections if illuminated directly with point light sources.
In a first experiment with standard desktop halogen lamps a front lighting setup was
tested. Two light sources have been placed at low angle to illuminate the tube boundaries
from two sides at the measuring area inside the guide bars. The results are shown in
Figure 3.7(a) and 3.7(b). This setup yielded good results with black heat shrink tubes,
but it turned out to produce unacceptable reflections just at the measuring points with
the transparent ones. Such reflections could be reduced by changing the angle of light
incidence, but still left strongly non-uniform results. Although the halogen lamps are
operated at DC power, the AC/DC conversion of off-the-shelf desktop lamps if often not
stabilized, thus, leading to temporal and spatial variances in image intensities and color.
This effect has been observed throughout the experiments with the desktop lamps at video
frame rates of 50fps.
Using a professional, flicker free, front lighting system with two fiber optical line lights
illuminating the tube ends (see Figure 3.7(c)), the image quality could be increased as can
be seen in Figure 3.7(d). However, there are still a few shadows left.
Experiments with a back light setup have been accomplished, too. A calibrated fiber
optical area light is placed at a certain distance (about 1-2cm) below the conveyor belt.
The light has to shine through the belt, thus, it is important to use a material that is
translucent. A typical belt core consist of a canvas (e.g. cotton) and a rubber coating,
whereas thickness, structure and density of the canvas as well as the color of the rubber
determine how much light can enter the camera. In the optimal case, no light at all would
be absorbed by the belt what is technically hardly possible.
Five different belt types have been tested. Some of the results can be seen in Figure 3.8.
Each sample in this experiment consists of a transparent rubber coating and a white canvas
as base. The structure of the belt canvas is visible in each image as background pattern.
Obviously, the background should not influence the detection of the tube’s boundary.
Thus, the goal is to find a belt type that allows for back lighting without adding to much
unwanted information to the image that could complicate the measurements.
In Figure 3.8(a), the coarse texture of the background significantly affects the tube ends
of the transparent tube at the bottom. A sharp boundary is missing, making accurate
and reliable measurements impossible. The belt type in Figure 3.8(b) has a finer texture,
but transmits only a little amount of light. Figure 3.8(c) shows the belt type that yielded