Wand calibration method

The Wand calibration method uses a calibration kit that consists of two parts: an L-frame and a calibration wand. For a description of the available calibration kits, see chapter Qualisys calibration kits.

Place the L-frame so that the desired coordinate system for the motion capture volume is obtained. By default, the origin of the coordinate system is set to the corner of the L-frame, the primary axis (X) corresponds to the long arm, and the primary plane (XY) is determined by the short arm. This default behavior can be changed by using a different Coordinate system orientation in the calibration settings, or by using a transformation.

It is best if all cameras in the system can see all markers on the L-frame. If some cameras cannot see the L-frame, the Extended calibration method is used automatically, see chapter Extended calibration.

The calibration wand must be moved through the whole capture volume in X, Y and Z orientation to ensure that all axes are properly scaled. The calibration algorithm calculates the positions and orientations of all cameras from their view of the wand during the calibration. For more information on how to move the wand, see chapter Calibration tips and How to move the wand.

It is possible to refine the calibration without the L-frame. This method can be used in situations where it is hard to place the L-frame. For more information see chapter Refine calibration.

The Advanced calibration is a wand calibration method that includes optimization of the linearization files of the cameras. For more information, see chapter Advanced calibration.

Outline of how to calibrate (Wand calibration)

The steps below are just an outline of what should be done to calibrate the camera system with Wand calibration method.

Follow these steps to calibrate the camera system:

  1. Start a preview by clicking the New file icon .

  2. Place the L-frame in the measurement volume.

  3. Set the settings on the Calibration page in the Project options dialog, see chapter Calibration.

  4. Click the Calibration icon  and set the settings in the Calibration dialog, see chapter Calibration dialog.

  5. Click OK to start the calibration.

  6. Move the calibration wand in the measurement volume.

  7. Check the Calibration results and click OK.

If you have force plates, there will be a warning reminding you of measuring the force plate position again. Redefining the positions of the force plates is recommended, unless you are sure that the L-frame was in the exact same position as during the calibration that was used for the current force plate position definition.

If any problems with the calibration process occur, check the settings on the Calibration page in the Project options. If that does not help, check the troubleshooting list in chapter Troubleshooting calibration.

Calibration tips

During the calibration, the wand is preferably moved in the measurement volume in a way that allows all cameras to see the wand in as many orientations as possible. This way the cameras will be properly calibrated in all directions.

Make sure that the wand is visible to the cameras during the calibration, and try to avoid blocking cameras with your body for a longer time period, for example by moving around.

If you are using a directional wand, for example the wand from the 120 mm kit with markers pointing up on one side, or the active 500 mm kit, make sure that the markers are pointed towards the cameras.

How to move the wand

One suggestion on how to move the wand, is to move it in one direction at a time. Start by holding the wand positioned in the Z direction, i.e. the straight line between the two wand markers should be parallel to the Z axis. Move the wand in the entire measurement volume. It is important to fill the entire measurement volume with calibration points. Make sure that both the lower and upper parts of the volume are covered. Repeat the same procedure with the wand positioned in the X and Y direction. It is particularly important to collect points where there will be many markers during the motion capture.

In the pictures above the L-frame is not indicated to make the picture more distinct. The L-frame must of course always be present during the calibration. The box in the figure represents the measurement volume.

It is not necessary to hold the wand in distinct directions as described above, but the wand can be moved more freely as well. The most important is that the measurement volume is well covered and that the wand orientation is varied. The best moving method may vary depending on the application. It is recommended that the moving method is systematic and easy to repeat for consistent results.

Keep the movements of the wand smooth and controlled. Do NOT spin the wand at a high speed.