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4.12 Test Images
Test images are generated in the camera FPGA, independent of the image sensor. They can be
used to check the transmission path from the camera to the frame grabber. Independent from
the configured grey level resolution, every possible grey level appears the same number of
times in a test image. Therefore, the histogram of the received image must be flat.
A test image is a useful tool to find data transmission errors that are caused most
often by a defective cable between camera and frame grabber.
The analysis of the test images with a histogram tool gives the correct result at a
resolution of 1024 x 1024 pixels only.
4.12.1 Ramp
Depending on the configured grey level resolution, the ramp test image outputs a constant
pattern with increasing grey level from the left to the right side (see Fig. 4.60).
Figure 4.60: Ramp test images: 8 bit output (left), 10 bit output (middle),12 (right)
4.12.2 LFSR
The LFSR (linear feedback shift register) test image outputs a constant pattern with a
pseudo-random grey level sequence containing every possible grey level that is repeated for
every row. The LFSR test pattern was chosen because it leads to a very high data toggling rate,
which stresses the interface electronic and the cable connection.
In the histogram you can see that the number of pixels of all grey values are the same.
Please refer to application note [AN026] for the calculation and the values of the LFSR test
image.
4.12.3 Troubleshooting using the LFSR
To control the quality of your complete imaging system enable the LFSR mode, set the camera
window to 1024 x 1024 pixels (x=0 and y=0) and check the histogram. If your frame grabber
application does not provide a real-time histogram, store the image and use a graphic software
tool to display the histogram.
In the LFSR (linear feedback shift register) mode the camera generates a constant
pseudo-random test pattern containing all grey levels. If the data transmission is error free, the
histogram of the received LFSR test pattern will be flat (Fig. 4.62). On the other hand, a
non-flat histogram (Fig. 4.63) indicates problems, that may be caused either by the cable, by
the connectors or by the frame grabber.
4.12 Test Images
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