The photographic plate is placed on a glass platten in an XY table. The table has approx 22 cms of travel in each axis. However, as the table is flat, it is possible to scan selected areas of larger plates. As the light source is incoherent there are no problems with interference effects between the surfaces of the glass platten and the plate.
Each axis is driven by a dc motor connected to the table via a lead screw. A tacho provides velocity feedback to the drive electronics. The position of each axis is determined from a moiré fringe grating. These have a line spacing of 8 microns. In the current system electronics exist to subdivide these fringes by 8, giving a spatial resolution of 1 micron. Raster scans are performed by driving the table in X, at constant speed (25 mm/sec) for the required scan, incrementing the Y axis by a sample size, and then driving the table back to its original position in X.