F-Theta lenses – also called scan objectives or flat field objectives – are lens systems often used in scan applications. Located in the beam path after the scan head, they perform various functions.
For instance, the objective focuses the laser beam at the focal point. When scanning, it also ensures that this focal point is always positioned in the working plane, perpendicular to the objective's optical axis. Additionally, the position in the working plane approximates the F-Theta condition, meaning the scan length (image height) is approximately proportional to the set scan angle. Deviations from this proportionality can be compensated by controlling the scan mirrors.
Key quality indicators of good F-Theta lenses are low field-curvature, a large image field and good, homogenous focus quality throughout the entire image field. Here, a crucial factor is whether the focus is diffraction limited. Additional requirements for F-Theta lenses can apply, depending on the laser's parameters. Examples here include avoidance of back reflections from the objective's lens surfaces, and/or thermal-lens effects.