Image Acquisitionand Proces

Setting Up 31
Although this would be useful for a barcode (as the information you are attempting
to acquire is basically a one-dimensional line proÞle across the code), most
detector arrays are two-dimensional, so this equation needs to be applied across both
dimensions. Another example is measuring the size of an apple. Consider a particular
variety of apple, which has a maximum horizontal size of 8 cm, its vertical size will
not exceed 10 cm, and the required accuracy of the measurement needs to be 1 mm
(0.001 m). The minimum sensor resolution is determined as:
È m
Sensor Resolution H
=
Ê
ˆ
Í
pixels
2 008 . ù
Ë 0 001m
¯ú = 160
.
indicating a sensor with a resolution of at least 160 ¥ 200 is required.
2.3.2 DEPTH OF FIELD (DOF)
È m
Sensor Resolution V
=
Ê
ˆ
Í
pixels
2 010 . ù
Ë 0 001m
¯ú = 200
.
The DOF of a lens system is the lens-perpendicular spatial range of which objects
remain in focus; any objects outside of the DOF will not be in focus. The DOF is
directly related to the blur diameter, as shown in Figure 2.19. Consider a beam (a
group of adjacent rays) of light traveling from the object through the lens and
focusing on the CCD array of a camera. As the beam travels through the lens, it
undergoes geometric aberration, both spatially and temporally, so the beam defocuses.
If the beam is defocused enough to cover an area of the CCD that is larger
than the required resolution (the blur spot), then the image will be out of focus. This
phenomenon also can occur due to chromatic aberration, which is evident when
different frequencies of light (e.g., colors in the visible spectrum) are diffracted at
different angles.
Blur
Diameter
Object
Depth Of Field
Lens
FIGURE 2.19 Depth of Þeld.
One method of increasing DOF is to place a limiting iris in front of the lens
(Figure 2.20).Unfortunately, using this method causes less light to irradiate the lens,
so you will need to increase the amount of light falling on the object to achieve
similar contrast levels.