 All are subject to damage from impact, so should be protected with a metallic shield.Made from strips of high-quality glass with evenly etched marks just like the marks of a ruler, but very small (typically 5 μm apart, but in some instances can be smaller, such as 1 μm for a lathes cross slide).Two optical sensors (phototransistors or photodiodes) are placed very close to each other to make a linear incremental encoder.