National Instruments IMAQTM User Manual

Refer to Chapter 5, 

, for more 

information about applying calibration information before making 
measurements.

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 the best results: 

•

The displacement in the x and y directions must equal (dx = dy).

•

The dots must cover the appropriate portion of the working area.

•

The radius of the dots must be 6–10 pixels.

•

The center-to-center distance between dots must range from 
18 to 32 pixels, as shown in Figure 6-1.

•

The minimum distance between the edges of the dots must be 6 pixels, 
as shown in Figure 6-1.

Figure 6-1.  Defining a Calibration Grid

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 the 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.

1

Center-to-Center Distance

2

Center of Grid Dots

3

Distance Between Dot Edges

dy

dx

1

3

2