Master Thesis - Fachbereich Informatik

46 CHAPTER 3. HARDWARE CONFIGURATION
(a) (b) (c)
(d) (e)
Figure 3.9: Polarized back lighting. (a) Image of diffuse back light through polarized glasses
used for viewing 3D stereo projections with no filter in front of the camera. (b) Setup as in (a)
with an opposing polarization filter in front of the camera. Almost no light enters the camera
at the polarized area. (c) Transparent heat shrink tube at polarized back light. There is a
strong contrast at the polarized area, while it is impossible to locate the tube’s boundaries at
the unpolarized area (bottom right). (d) Polarized back light through a conveyor belt. The
polarization is changed both by the belt and the tube, thus, leading to a poor contrast. (e)
For comparison: Back light setup without polarization.
best results both in background texture and transmittance. As can be seen, there are no
shadows at the tube boundaries.
Since the black tubes do not let pass any light rays, the contrast between background and
tube is excellent with all kinds of belt types tested. One advantage of black tubes follows
from this property: The printing on the tube’s surface is not visible in the image. On
the other hand, the transparent tubes do transmit the light coming from below. Positions
covered by the printing show a minor transmittance, hence, the printing is visible in terms
of darker intensity values in the image.
As introduced in Section 2.2.3, polarized back lighting can be used to emphasize transparent,
translucent objects. In an experiment, shown in Figure 3.9, the integration of
polarization filters has been tested. Two polarized glasses originally used for viewing 3D
stereo projections have been employed to polarize the light coming from the area back
light. First, the principle is tested without a conveyor belt. Two opposite polarization filters
are placed between light source and camera. As can be seen in Figure 3.9(b), the area
covered by the two polarization filters at right angle appears black in the image while the
areas without polarization filters are ideally white. A transparent tube between the two
filters changes the polarization, and hence, making it possible that light enters the camera
at locations that have been black before. There is an almost binary contrast between
object and background (see Figure 3.9(c)). At regions that are not affected by the filters,
there is no contrast at all making the tube invisible. Unfortunately these good results
have no practical relevance, since in the real application the light has to pass the conveyor
belt, too. If the belt is placed between the first polarization filter and the object, it also
changes the polarization at regions that do belong to the background (see Figure 3.9(d)).
The binary segmentation is lost and the structure of the conveyor belt is visible again.
While it is not possible to install the first polarization filter between conveyor and tube,
the polarized back light approach has no advantages compared to the unpolarized in this