Chapter 6
Calibration
IMAQ Vision for LabWindows/CVI User Manual
6-2
ni.com
Defining a Calibration Template
You can define a calibration template by supplying an image of a grid or
providing a list of pixel coordinates and their corresponding real-world
coordinates. This section discusses the grid method in detail.
A calibration template is a user-defined grid of circular dots. As shown in
Figure 6-1, the grid has constant spacings in the
x
and
y
directions. You can
use any grid, but follow these guidelines for best results:
•
The displacement in the
x
and
y
directions should be equal (
dx
=
dy
).
•
The dots should cover the entire desired working area.
•
The radius of the dots should be 6–10 pixels.
•
The center-to-center distance between dots should range from
18 to 32 pixels, as shown in Figure 6-1.
•
The minimum distance between the edges of the dots should be
6 pixels, as shown in Figure 6-1.
Figure 6-1.
Defining a Calibration Grid
Note
You can use the calibration grid installed with IMAQ Vision at
Start»Programs»National Instruments»Vision»Documentation»Calibration Grid
.
The dots have radii of 2 mm and center-to-center distances of 1 cm. Depending on your
printer, these measurements may change by a fraction of a millimeter. You can purchase
highly accurate calibration grids from optics suppliers, such as Edmund Industrial Optics.
Defining a Reference Coordinate System
To express measurements in real-world units, you need to define a
coordinate system in the image of the grid. Use the
CoordinateSystem
structure to define a coordinate system by its origin, angle, and axis
direction.
1
Center-to-Center Distance
2
Center of Grid Dots
3
Distance Between Dot Edges
dy
dx
1
3
2
