Extended calibration

To enable a larger measurement volume QTM uses a method called Extended calibration. The Extended calibration is used automatically as soon as a camera cannot see the L-frame, but has an overlap with other cameras.

In Extended calibration there is only some of the cameras that can see the L-frame. The other cameras are calibrated by the overlap with the reference cameras field of view. The result is a larger volume but with reduced 3D data accuracy compared with when all cameras can see the L-frame.

To perform an Extended calibration, make sure that at least two cameras can see all markers of the L-frame. Then to extend the measurement volume, the following three conditions must be fulfilled:

1. All parts of the measurement volume must be seen by at least two of the cameras. Since it is impossible to calculate 3D coordinates in parts of the volume where only one camera can see the markers.

2. The whole measurement volume covered by the cameras must be connected. This means that a marker must be able to move through the whole volume without passing any volume that less than two cameras cover.

3. In volumes where only two cameras overlap, the size of the volume must be large enough to fit the calibration wand, i.e. the diameter must be larger than the wand length.

When performing the calibration in an Extended calibration, it is very important to consider how the calibration wand is moved.

To achieve the highest accuracy, and indeed to make a successful calibration at all, it is vital that the calibration wand is moved considerably in the volume of the cameras that can see the reference object. It is even more important to move the wand in the volumes of overlap between different cameras. To ensure that you have time to do this increase the calibration time and also increase the Maximum number of frames used as calibration input setting on the Calibration page in the Project options dialog.

For example, if two cameras can see the reference object and three are positioned to extend the volume, it is very important to move the wand in the volumes of overlap between the two base volume cameras and the three extended volume cameras. The points collected in these volumes are the points used to calibrate the volumes relative to each other. If the wand is not moved enough in the volume of overlap, the extended volume of the three cameras will be very poorly calibrated relative to the base volume.

In addition to these volumes of higher importance, it is still important that all cameras can see the wand in as many orientations and positions as possible, just as in a normal calibration.