Witness boxes

Witness plate can be exposed without removal from the box

Witness box opened

Witness boxes are used to bring the disks to the place where to check for deposition.   Arrived at the place where to measure the box is openend for exposure of the disk.  Upon completion of the measurement intervall the box is closed.

At a convenient moment in time the measurement can done.  The witness plate is a passive integrating measurment system.