Lens configuration for a thermally compensated chromatic confocal point sensor

A chromatic confocal point sensor optical pen operable to provide a signal usable to measure a distance to a surface and compensated for thermal sensitivity, the chromatic confocal point sensor 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 chromatic confocal point sensor 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 comprising at least two lens elements,
• wherein:
• each lens element is characterized by an optical power φ1, a coefficient of thermal expansion αTi, an index of refraction η and a coefficient of thermal defocus χi over a range of a temperature T, where
• χ
• i
• =
• α
• Ti
• -
• 1
• n
• i
• -
• 1
• ⁢
• ⅆ
• n
• i
• ⅆ
• T
• =
• -
• 1
• ϕ
• i
• ⁢
• ⅆ
• ϕ
• i
• ⅆ
• T
• ;
• at least two of the lens elements of the positive power lens portion are configured such that they at least partially compensate for an overall thermal sensitivity of the chromatic confocal point sensor optical pen including thermal sensitivity arising from thermal expansion of the housing;
• and
• the average of the coefficients of thermal defocus χi for at least two of the lens elements of the positive power lens portion is an average coefficient of thermal defocus that falls in a range that is at lowest 10 ppm per ° C.