Unveiled by Pyser-SGI Ltd., the PS20 is designed for measurement calibration of microscopes and machine vision systems. The patterned slide includes concentric circles and squares, line gratings, grid and dot arrays, geometric root 2 progression of dots and square blocks, and coarse and variable fine linear line scales. Starting from a fixed datum point mark, each pattern or array can be located using X-Y coordinates. Each slide has a permanent and unique serial number and can be supplied with full or partial UKAS certification of accuracy. The soda lime glass substrate measures 76 × 25 × 1.5 mm and is packaged in a polished wooden case. Optical density is >2.5, and general overall accuracy is 0.5 μm.