Master Thesis - Fachbereich Informatik

3.3. ILLUMINATION 43
f V
12mm 129mm
16mm 100mm
25mm 64mm
35mm 75mm
50mm 115mm
Table 3.2: Field of view of different fix-focal length lenses at the specified minimum object
distance.
V f=12mm 16mm 25mm 35mm 50mm
40 •77 •99 •156 •212 •312
60 •116 •149 •234 •318 •469
100 •193 248 390 530 •782
200 381 497 780 1006 1563
Table 3.3: Working distances to yield a certain field of view for different focal length lenses.
Distances that fall significantly below the minimum working distance are marked with a •.
same resolution. Thus, smaller tubes can be measured theoretically at higher precision.
The minimum object distance of the compared lenses, however, represents a certain limit
in precision. Tubes below 30mm can not be measured with higher, but with the same
precision as 30mm tubes. Reminding the tolerances introduced in Section 1.3, smaller
tubes have a smaller tolerance than larger tubes, and 20 − 30mm tubes have the same
tolerance.
At the upper bound, larger tubes need a wider field of view of the camera. Hence, a
larger region is mapped on the same image sensor, so one pixel represents more. For a
200mm measuring area the pixel representation is about 0.25mm. The field of view can
be achieved by placing the camera further away from the object. The distance increases
with the focal length of the lens. Table 3.3 shows the approximated working distance for
the compared lenses that are needed to result in a certain field of view. Distances that
fall below the minimum object distance are marked with a ‘•’.
It turns out that a 16mm focal length lens is best choice for tube lengths between 50
and 100mm, since this lens maps the required measuring areas onto the image plane at the
smallest working distance. However, tubes below 50mm can not be inspected with higher
precision with this lens. In this case, a 25mm focal length lens has to be selected. This
lens is the best compromise for small and large tube sizes. It has the drawback of a large
working distance of up to 780mm for 100mm tubes. Both a 16mm (PENTAX C1614-M)
and a 25mm (PENTAX C2514-M) focal lens have been used in the experiments.
3.3. Illumination
As introduced in Section 2.2, the right choice of illumination is substantial in machine
vision applications. Accurate length measuring of heat shrink tubes requires a sharp contrast
at the tube’s outline, especially at the boundaries that are considered as measuring
points. Any shadows that would increase the tube’s dimension in the 2D image projection