Systems and methods are disclosed for improving optical spectrum fidelity of an integrated computational element fabricated on a substrate. The integrated computational element is configured, upon completion, to process an optical spectrum representing a chemical constituent of a production fluid from a wellbore. The systems and methods measure in situ a thickness, a complex index of refraction, or both of a film formed during fabrication to generate a predicted optical spectrum. The predicted optical spectrum is compared to a target optical spectrum. Revisions to a design of the integrated computational element are conducted in situ to improve optical spectrum fidelity relative to the target optical spectrum. Other systems and methods are presented.