Calibrating the Measurement Tool

Pixelink Capture provides the ability to measure in pixels, or in manually calibrated units. Before measuring in units, the Measurement tool must be calibrated. Measurements are made based on the relative location of camera pixels. Calibrating the tool allows measurements to be displayed in real world units (i.e. millimetres, microns, inches, mils, etc). 

The Spatial Calibration window of Pixelink Capture can be used to calibrate the Measurement tool, by following the steps below:

1) To create a new calibration click the button. A calibration line will appear in the preview window (see image below). Drag and reposition the calibration line to cover the span of a known distance in the preview window. To resize the calibration click and drag the ends of the line. To move the line, click and drag on the middle.

In this example the ticks on a ruler are used as the known distance (from 1 cm to 2 cm). Commonly used objects of a known distance are rulers (as in this example), components of a known and standard size (such as a 1.5 mm wide resistor) or a custom printed pattern of a known size.

2) After adjusting the line to match your calibration target, select the appropriate unit of measurement (from the example above, cm or mm):

3) Enter the appropriate value in the Units in Line field :

4) Finally, click Save to Save and begin using the Measurement tool. 

If you have multiple calibrations saved, you can switch between saved calibrations by clicking on them: 

Calibration Accuracy

The accuracy of the measurement calibration is largely dependant on the camera and lens setup of your system. Two of the larger variables impacting accuracy are:

  • The accuracy of the physical measurement of the calibration target. For example, if the size of the calibration target is known to be 9.782 mm wide, that is a much more accurate measurement than if the calibration target was only known to be 9.8 mm. A more technical way to say this is that 9.782 mm has two more significant digits than 9.8 mm.

  • The sharper the focus of the calibration target is, the more accurate the calibration will be. If the image is blurry, the edge of the calibration target may appear to spread across multiple pixels, lowering the accuracy. On a sharper image, the edge of the calibration target will spread across fewer pixels, increasing the accuracy.

When discussing accuracy, it is important to note the difference between accuracy and precision. Accuracy is how close the measurements are to the true dimensions of their real life size; Precision is how repeatable the measurements are. The accuracy of the measurements is based on the two factors listed above. The precision of the tool is more well defined; It is precise to the size of an individual pixel on the sensor.

Tips for Calibration

  • Properly light the calibration target, to ensure that it's easy to see (and not over saturated).

  • Bring the target into focus. If the image is blurry, the edges of the calibration target may appear to spread across multiple pixels, making it more difficult to accurately place the line.

  • Use the zoom feature to zoom in on the target. To do so, select a zoom percentage (other than Fill) from the drop down menu:

And then use the scroll wheel of your mouse, or the plus and minus magnifying glasses, to adjust the zoom to make it easier to clearly see your target, as demonstrated by the images below: