Automated optical distance measurements for counting at a defined solid angle.

Two optical distance measurement devices have been compared for accurately defining the source-detector geometry of alpha-particle counters. The first consists of a travelling microscope with unifocal lens, to which a linear distance gauge is attached. The focusing is done by human eye. The second is a modern 3D coordinate measurement machine, equipped with a CMOS colour camera as the sensor. Focusing is performed automatically by software. With both devices, distance measurements of various drop-deposited sources on glass and stainless steel disks have been performed. There is a good mutual agreement of results. Even though the new device can sample more reference points, the accuracy of the measurements has not improved drastically due to the imperfect flatness of the sources. The main advantages of the new system are its ease of use and the lower health risk involved due to the larger distance between the performer and the source.