Since the PrismMaster® is using a computerised autocollimator no precise, difficult and time consuming alignment procedures are necessary. The tilt table is simply rotated until the autocollimator is approximately square to the first surface of the prism or polygon. To achieve this, the operator is rotating the table until the reflected image of the autocollimator appears on the PC-monitor within a defined range area. The same procedure is repeated to obtain a reading from the second surface of the prism. The tiresome and difficult visual alignment becomes obsolete.
 
This is not only incomparably easier and comfortable for the operator, but very important for the overall accuracy of the instrument. In other goniometers the overall accuracy - even when using an accurate encoder - is decisively limited by the ability of the operator to visually and repeatedly align the autocollimator perfectly square to the sample surface. This ability is normally limited at +/- one crossline thickness. Tacking for example an autocollimator with focal length of 300 mm and a crossline thickness of 10µm, the angular error corresponding to 10µm is 3.5 arcsec. Certainly this error can be reduced repeating the measurement several times and calculating the main value, however, this is inefficient and dependent on the ability and the degree of tiredness of the operator.
 
Measurement in reflection mode
 
The measurement starts using the mouse or pressing a PC-key. The position of the autocollimator reticle and of the rotary encoder is automatically recorded. Following up the angle indication on the digital display, the table is rotated until the reflected image on the second surface of the prism appears on the monitor within the range area. This range area is designated by a red coloured rectangle on the PC-Monitor. A mouse click completes the measurement and the angle value is displayed on the monitor.