National Instruments NI myRIO, Essentials Manual

Page: 36 / 163

30 CHAPTER 4. CAMERA SETUP
7
Measure the actual image spatial resolution and compare it to your target
value with a percent error calculation. How well does the actual value match
your intended value?
8
Adjust the ruler position so that the 0 cm mark appears at the left edge
of the image; the right edge of the image now displays the horizontal FOV.
Record this value, numerically compare to your target value, and comment on
the degree of agreement.
4.8 Real-World Units
Up to this point you have manually translated measurements in pixel units to
physical units based on your recorded spatial resolution. You can easily calibrate
the camera so that measurements appear directly in any desired physical or
real-world
units. NI Vision Assistant provides a number of camera calibration
tools of increasing sophistication to meet the needs of your particular machine
vision application, and you will investigate two of them in this section.
Point distance calibration
The point distance calibration is the simplest technique and requires negligible
computational effort. This calibration assumes that the image sensor plane is
perfectly aligned with the object plane (zero tangential distortion ) and that the
lens is likewise perfectly free of distortion.
I
Study the video Step 2: Calibrate the Camera
3
to learn how to select two
points on a ruler to calibrate the camera.
9
Use the same method described in the video to calibrate your camera based
on the image of a horizontal ruler with the 0 mm mark visible at the left-hand
side of the image. Estimate the maximum error between the calibrated ruler
overlay and the ruler itself, and screen capture the ruler image at this location.
Grid calibration
Grid calibration uses an array of uniformly-spaced dots to characterize the lens
distortion over the entire field of view. Tangential distortion may optionally be
characterized, as well. Grid calibration fits a 2-D polynomial to the measured
3
http://youtu.be/2ycFJJcKbMs
(3:04)