The arrangement and method are for the steam pre-treatment of chips during the production of cellulose pulp to avoid the blow-through of gases in the steam pre-treatment vessel. The system’s clients can add imagery in the form of questions, comments, and graphics to a currently displayed image. A second amplifier input stage is adapted to operate when the mean of the two signals is near a lower rail voltage. The messages are encoded with a message encoder of the transmitter and transmitted to the receiver. The method includes projecting a spot along a first axis onto the irregular surface, focusing an image of the spot along a second axis onto a detector, wherein the second axis is non-coaxially arranged with respect to the first axis, processing signals from the detector, and calculating the position of the irregular surface based on at least one isolated desired frequency component of the signals.