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Figure 14: Misalignment of the line sensor
for angular measurement : 2. :
Figure 15: Transmissivity of color slides
on the flat surface, the whole light is reflected; if the beam is reflected on a channel area, half of the beam cross
section is reflected with a delay of A/2 and weakens the resulting reflected beam.
The line sensor detects the light signal in radial direction of the disk, 49 concentrical traces are analyzed. The angular
information is coded in a 15 bit gray code. From every word to its neighbour only one bit changes. Therefore errors
cannot lead to completely different detected angles. For error redundancy the 15 traces are repeated twice as a block.
Each bit of the result is calculated by bitwise median filtering.
For determination of the binarizing decision level the disk has two inner and two outer traces with alternating bits as
references for the line sensor (figure 12). Each trace has its own phototransistor, and all output currents of these
reference transistors are simply added. A quarter of this current is the reference current for the threshold of the
remaining 45 trace detectors. The binarisation of the signal is done immediately after the photosensors with current
comparators. The speed of the binarisation depends on the contrast between two serial bits. Smaller optical contrast of
the serial bits accelerates the quantization, as the phototransistors are approximately slew rate determined.
The block diagram of the angular sensor device is shown in figure 13. The upper two blocks decode the angular value
and generate a 15 bit dual number for the angular position of the disk relative to the sensor line. The lower two blocks
detect position errors caused by dirt or humidity on the disk surface or by misalignment of the line sensor.
In the upper part of figure 14 a code is shown where several bits change between two angular positions. In case of
misalignment of the phototransistors as shown the detected angle can be completely different from the actual angle.
We have shown, that with the realized principle of triple gray code in the lower part of figure 14 even rotational
misalignments of the line sensor of four radial sectors lead to a maximal error of one angular sector.
5. COLOR ANALYSIS WITH A LINE CAMERA
Besides the optical power density in quite a few applications the spectral composition as a function of wavelength or
the color metric measurement of three valencies as RGB components is important. Examples are the visual check of |
the colour rings varnished on resistors or discrete semiconductors and pyrometrical measurements. Another
application is the segmentation of objects in color images. In a monochrome analysis shades are usually detected as |
objects. In color analysis shaded areas are correctly recognized, as only the luminance and not the chrominance is
changed at the border of the shaded area.
We investigated two different alternatives of color filtering applicable for a small number of vision chips. First we |
produced different colour slides, using an electronic exposure device (PCR2 by Agfa) with 4096 by 2732 pixels on a
slide of 36 by 24 mm°. The resulting pixel area is 9 by 9 um? on the slide. In our experiments the smallest possible
white lines had a width of 27 um. The film development as a chemical process is diffusion like and generates brighter
areas, unexposed areas remain black. For this reason only black lines could be produced in the minimal structure width
of 9 um.
Figure 15 shows the transmission for a white slide, a black one and three colour slides of the base colors cyan, yellow
and magenta. All slides are transparent for infrared light, even the black color slide. No slide is transparent for
ultraviolet radiation. It is clearly seen that magenta has to be composed by red and blue light. Yellow is not transparent
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995
l
