The mechanical angle is the rotation angle of the deflection mirror in the scan system, i.e. the angle to which the mirror has actually been rotated.
The optical angle is the deflection angle of the laser beam exiting the scan system relative to the neutral position of the mirrors. The optical angle is twice the mechanical angle – rotating the mirror to a particular angle causes deflection of the emerging laser beam at twice that angle.
(Typical) Angular Deflection
A galvanometer scanner's typical angular deflection is the positive and negative maximum controllable optical angle. Scanners can be calibrated to meet various image field requirements.
The calibration angle is 96% of the max. deflection angle (mechanical) in one direction, from center to edge.
Full scale is the entire optical angle from one extreme to the other.
A defined aperture’s vignetting-free angle is the maximum optical scan angle for which no significant shadowing occurs during traversal through the optical system. In practice, it’s the usable scan angle taking all components into account.