Topic Lenses & imaging

A galvanometer scanner deflects a beam through an angle, but a conventional lens places the focused spot at a distance proportional to the tangent of that angle. An f-theta lens is designed so the displacement is proportional to the angle itself, which makes the scan linear and the control electronics simple.

An achromat cements two glasses with different dispersions so that two wavelengths share a focus. It removes most longitudinal chromatic aberration and, as a side effect, reduces spherical aberration. It does not make the lens perfect: a third wavelength still lands slightly elsewhere.