Calibration process

Vicon system performance depends on the accuracy with which the cameras and the capture volume are calibrated during a two-stage system calibration process:

Both stages of the system calibration process depend on the Vicon application software knowing the dimensions and relative marker positions of a specified calibration device. This definition of a calibration device is contained in a Vicon Labeling Skeleton (VSK) file. When you configure the calibration process in your Vicon application software, you load its VSK file.

The following sections provide a general overview of the calibration process. For details on calibrating your Vicon cameras and the capture volume, see the Vicon Vantage Quick Start Guide and the documentation for your Vicon application software.

Camera calibration

During camera calibration, the Vicon application software calculates the physical position and orientation of each Vicon camera in the capture volume based on the movement of a calibration wand. This stage calculates internal (focal length and image distortion) and external (position and orientation) camera parameters in two simultaneous phases:

• Linearization phase
  The optical distortion of the camera lenses and any other non-linearity in the system are measured, and a correction matrix is calculated. Corrections are applied to every frame of data subsequently acquired by each camera.

• Photogrammetric calibration phase
  The physical locations and orientations of the cameras are calculated, referring to a user-defined set of coordinates.

The objective of this stage is to describe the capture volume to the Vicon Vantage system.

To calibrate the Vicon Vantage cameras:

1. Turn on the Active Wand.
2. If necessary, move the LED Mode switch to Strobe mode.
   The LEDs begin to flash.
3. Place the Active Wand in the capture volume where it is visible to the Vicon cameras.
   When the LEDs appear to stop flashing, the Active Wand is synchronized to the cameras.
4. Wave the calibration wand throughout the empty capture volume, ensuring that the LEDs on the wand are visible to the cameras, until sufficient wand frames are spread evenly across the field of view for each camera.
   The Vantage camera display and your Vicon application software provide feedback to help you determine when enough wand data has been acquired to calibrate each camera. For details, see the documentation for your Vicon software.

Setting the volume origin

In setting the volume origin, the Vicon application software measures the position of a calibration device in the capture volume and sets the global coordinate system.

The objective of setting the volume origin is to set the global coordinate system so that subjects are displayed the right way up and so that you can change the way data is visualized in the Vicon application software workspace. Place the calibration device flat on the floor to identify the coordinates of the global origin (0,0,0), which represents the center of the capture volume, and the global axes (X,Y,Z), which represent the horizontal and vertical axes of the capture volume.

If you are using force plates, choose one of the force plate corners to be the position of the origin of the global coordinate system. This enables your force plates and the Vicon capture volume to share a common coordinate system.To do this, attach the supplied force plate alignment guides to the Vicon Vantage calibration device. The alignment guides ensure that you place the Vantage calibration device on the same position on top of one of the force plate corners each time you perform a calibration.

If you are not using force plates, adjust the feet of the Vantage calibration device to ensure that they are all flat on the floor. Start with the handle and adjust the foot between the fixed foot and the hand grip, and then adjust the other foot until the bubble in the spirit level is in the center. Next, on the top edge of the wand, adjust the foot on the left end until the bubble in the spirit level is in the center. To adjust the feet, first loosen the locking nut next to the metal frame and then turn the screw left to lower the calibration device or right to raise it. Then tighten the locking nuts to fix the feet in position.

Next place the Vantage calibration device either on top of the force plates or flat on the floor in the center of the capture volume in the field of view of at least three cameras. Position the left corner of the calibration device where you would like the volume origin to be and in the orientation that you would like the horizontal X and Y axes to be (for details of the Vicon Vantage calibration device, see About the Vicon Vantage calibration device). The calibration process will calculate the vertical Z axis from the positions of the X and Y axes, as shown in the following image.