Chromatic sensor lens configuration

A CPS optical pen operable to provide a signal usable to measure a distance to a surface, the CPS optical pen comprising:

• a housing;
• an aperture that outputs source radiation and receives reflected radiation;
• and
• a lens configuration arranged along an optical axis of the CPS optical pen to receive the source radiation from the aperture, focus the source radiation and output it toward the surface with axial chromatic dispersion, receive reflected radiation from the surface and focus the reflected radiation proximate to the aperture with axial chromatic dispersion, the lens configuration comprising:
• a doublet lens element comprising a first portion located nearer to the aperture and a second portion located farther from the aperture;
• and
• a positive power lens portion located farther from the aperture than the doublet lens element, the positive power lens portion including at least one lens element, wherein:
• the first portion of the doublet lens has a first Abbe number, the second portion of the doublet lens has a second Abbe number that is higher than the first Abbe number, and the doublet lens provides an optical power that is one of a) a negative optical power and b) a positive optical power that is less than 40% a total optical power of the lens configuration.