Hardware manual

Impact Reference Guide Camera
2-27 Datalogic Automation Inc.
x' = a*x*x + b*y*y + c*x*y + d*x + e*y + f
y' = g*x*x + h*y*y + i*x*y + j*x + k*y + l
Because there are six coefficients to be computed in each equation, a minimum of six dots must be present in
each quadrant of the image to successfully compute the mapping. If more than six dots are present, they are
all used to compute a best fit mapping. Larger dots provide a more accurate calibration because the dot cen-
ters can be more accurately located.
It is known that a circular dot center in an image can be found with a three sigma repeatability of 0.6/sqrt
(diameter) pixels. (Haralick and Shapiro, Computer and Robot Vision, 1992) It is also important to fill the
field of view with the dot pattern. Areas within the field of view that are outside the dot pattern are handled
by extrapolating the calibration found within the dot pattern, so any calibration errors become magnified in
those areas.
Once the calibration is done, any feature found in an image, such as a edge point or centroid, is passed
through the polynomial mappings to find its location in the "perfect" image represented by the known dot
target pattern.
A color camera has half the measurement accuracy (twice the error) of a greyscale camera with the same
number of pixels. The resolution rules-of-thumb for greyscale and color cameras are as follows:
The Bayer filter mosaic halves the effective resolution of the color cameras because each pixel detects either
red, green, or blue. The smallest detectable defect corresponds to the smallest size defect that always fills at
least one pixel. This is a theoretical lower limit. In most applications the defect will need to be larger. Also
see “Color Image Sub-Properties” on page 3-35.
Note that some tools provide sub-pixel options, which can improve the resolution and measurement accu-
racy of the image.
The four options for camera calibration are: From Target, Manually, From Points, and Pixel Size Only.
Snap
Click the Snap button to acquire and display an image in the image window. You can use this to see how
camera settings affect the image, or to position objects within the camera’s field of view. The acquired image
is also displayed in the image display area of the Design and Display tabs.
• NOTE: Clicking the Trigger Once button in the Summary Table buttons toolbar will not acquire a
new image for this image display.
Live
Click the Live button to continuously acquire images and display images in the image window until you
click the Snap or Live buttons. You can use this to see how camera settings affect the image, or to position
objects within the camera’s field of view. The acquired image is also displayed in the image display area of
the Design and Display tabs.
• NOTE: Clicking the Trigger Continuously button in the Summary Table buttons toolbar will not
acquire a new image for this image display.
Image Source
Greyscale Color
Smallest Detectable Defect 2x2 pixels 3x3 pixels
Gauging resolution 1/4 pixel 1/2 pixel