User manual

Small Vision System User Manual 48
4.1.2 Calibration Target
The standard calibration target, check.pdf, can be printed out on a single piece of 8.5 x 11 inch
paper. In some cases, this image is too small, e.g., when using wide baseline cameras. There is another
target,
check2.pdf, that has squares twice as large as the standard one (54 mm instead of 27 mm). This
target must be assembled from four printed pieces of 8.5 x 11 inch paper. It is recommended for large-
baseline systems, where the target must be placed at a distance from the cameras in order to be seen in both
of them.
4.1.3 Imager Characteristics
The calibration process must be related to the geometry of the camera imagers. There are three
important parameters:
1. Pixel size in mm (width and height)
2. The width of the imager in pixels
3. The width of the image output by the framegrabber
The stereo rigs from Videre Design (MEGA-D, STH-V2, and STH-V3) have preset values; just use the
appropriate button in the Calibrate dialog. Otherwise, select the
Custom value, and choose values
according to the instructions below.
The pixel size can be found from the specifications of the imager. If you cannot find these, just use
defaults of 0.010 mm. The calibration will determine the pixel aspect ratio (width / height). The calibrated
lens focal length will not be correct because the pixel scale will be off, but that will not change the validity
of the calibration.
The imager width is the number of sels (sensor elements) in each line of the imager. Again, this can be
found in the imager specifications. If these are not available, just use the image width as it comes from the
framegrabber, e.g., for NTSC video it is 640.
The framegrabber width is the width of the image output by the framegrabber, in pixels. Typically it
will be an analog NTSC signal, which is 640 pixels. For digital imagers, such as the MEGA-D, the image
size in sels and the framegrabber width are the same.