This report describes a dynamic monochromatic interferometer designed and built to look at the printing interface in a thermal printer during the image printing process. The apparatus was developed around a Kodak XLS 8600 printer, where the printhead was replaced by a transparent alumina rod, while the printer's paper and dye donor transport mechanisms were unaltered. The optical interference pattern that is observed within the printing nip provides a quantitative measure of the contact zone and spacings that exist within both, the donor lube to printhead and the donor dye to image receiver interfaces. The apparatus enables visual examination of the influence of donor and receiver surface characteristics on the contact within the printing nip. Further, it enables microscopic observation of particle transport through the nip and shows the impact of particle adherence to the printhead on interface separation.