Clear Aperture vs Application – Lithography / Inspection / LIDAR / Laser Cleaning

Many optics, including polygon mirrors, have a spec on surface figure within a defined area known as the Clear Aperture. Many optic manufacturing processes result in a roll-off area at the edge of the optic. The roll-off edge of a polygon mirror facet is illustrated with the yellow border in this photo.

The Clear Aperture spec on our polygon mirrors tells you how close to the facet edge you can expect to get the rated flatness of the mirror. The roll-off allowance at the polygon mirror facet edges is typically 0.5 mm.

ACTUALLY, THE USEFUL APERTURE IS DEPENDENT ON APPLICATION!

The original market driver for polygon laser scanners was lithography including copy machines and laser printers in 1975. These applications require extreme flatness for the required lithographic accuracy. Therefore, the roll-off allowance in polygon scanner specs results in a very conservative (small) Clear Aperture spec.

Today, uses for polygon scanners is way beyond lithography. Applications including agricultural product inspection, LIDAR, laser cleaning and surface treatment do not require lithographic accuracy. In these situations, 100% of the polygon facet may provide useful accuracy!