Microdensitometers have already solved the problem of maintaining high speed and high accuracy across large regions of space. Modifications are needed to operate with much higher speed and in the reflection mode. Experimental results confirm accuracies better than 0.1 um over areas over 40 cm.
H. J. Caulfield,
David L. Kryger,
"Use Of Microdensitometers As A Basis For Highly Accurate Metrology," Optical Engineering 18(5), 185483 (1 October 1979). https://doi.org/10.1117/12.7972416