Scanners
Galvo Scanner lenses
Most galvo scanner lenses are F-theta flat field lenses because they're
used for scanning flat objects and F-theta distortion makes it easier for the galvo software to make accurate shapes. An "F-theta lens" is one with barrel distortion such that the distance of a focused spot from its axis is nearly proportional to the angle of the incoming beam. A "flat field lens" is simply one that is designed to have an acceptably low field curvature for the task that it is performing.
The size of the lens depends on the field. They're usually pretty big, like 100 mm in diameter. They're even bigger if they're telecentric, meaning that the beam is always normal to the surface. Obviously, a telecentric lens must be at least as large as its field size.