What is a millimeter grid on a 360 degree protractor used for in the lens centration process?

In the lens centration process, the millimeter grid on a 360-degree protractor is primarily utilized to accurately position and align the lenses within the optical frames. This grid allows for precise measurements that ensure that the optical centers of the lenses are aligned with the fitting line of the frames and the wearer’s pupils.

When fitting lenses, it is crucial that they are centered correctly to provide optimal vision. Misalignment can lead to optical distortion and discomfort for the wearer. The grid provides a clear visual reference that helps in making precise adjustments, ensuring that both lenses are aligned symmetrically and correctly positioned in relation to the frames.

While the other options touch on important aspects of lens preparation and fitting, they do not directly relate to the immediate function of the millimeter grid in the context of lens centration. The use of this grid is integral for achieving accurate fitting and positioning during the lens mount process.