Metrology tools comprising aplanatic objective singlet

A metrology tool for determining a characteristic of a structure on a substrate, comprising:

• an illumination source configured to direct a radiation beam at the structure;
• and
• an optical detection system configured to detect radiation after interaction with the structure over a wavelength range, wherein the optical detection system comprises an aplanatic singlet lens configured to focus the radiation on to a detector, wherein the aplanatic singlet lens has:
• an aplanatic wavelength that is within the wavelength range;
• a front surface and a back surface having on-axis curvatures at which a spherochromatic aberration of the aplanatic singlet lens is within 20% of a minimum spherochromatic aberration;
• and
• a plurality of other lens characteristics comprising two or more of a material, a thickness, a numerical aperture, a focal length, and a magnification;
• and
• wherein the minimum spherochromatic aberration is the minimum spherochromatic aberration for the aplanatic singlet lenses having the same other of the plurality of lens characteristics.