Microscopic height measurements on moving objects with digital holography

Using digital holography in camera-based interferometers, 3D surface topography can be measured extremely quickly and with sub-wavelength precision. Using spatial phase-shifting, a single camera image is sufficient to reconstruct complex valued wavefronts for multiple wavelengths. Recently, measurements on moving objects were demonstrated using setups with 1× magnification. Increasing the lateral resolution by implementing larger magnification in a microscopic setup would open up new application fields, but the larger numerical apertures (NA) of microscope objectives make the acquisition even more sensitive to motion. In this work, we show the first microscopic setup, measuring objects moving at several mm/s using two-wavelength holography. Despite the large NA of 0.42 of the 10×-objective in our setup, measurements can be acquired at 75 mm/s and beyond. Using two lasers emitting slightly different wavelengths (637.76 nm and 632.87 nm), a maximum height difference of 41.3 mm can be detected unambiguously. One single image covers a lineshaped measurement area of 3.7 mm × 0.2 mm with a lateral pixel pitch of 0.47 mm. In order to inspect larger objects, single frames can be stitched together, permitting an infinite measurement area in the direction of motion. Gap-free stitched measurements are limited to 75 mm/s due to the framerate of the camera. Measurements of the groove depth averaged over several pixels on a groove standard show a repeatability exceeding 10 nm at 35 mm/s and 20 nm at 75 mm/s.