We dope the commercial resist UVIII with the pH sensitive dye coumarin 6 (C6) to demonstrate a fluorescence microscopy technique for detecting latent photoacid images in exposed chemically amplified resist films. The spectroscopic properties of C6 in the resist matrix are investigated in order to select spectroscopic filters which optimize the fluorescence image contrast of exposed patterns. We apply this technique to focus calibration of a projection lithography system and show a significant correlation of the results to scanning electron microscope analysis of developed features. Hence, we demonstrate the utility of this technique as a fabrication line diagnostic for focus calibration and validation (i.e., proof of focus) prior to postexposure processing of the resist film.