We build up an active triangulative metrology system with a high dynamic range for measuring the three-dimensional shape of microstructures and for reverse engineering. The triangulation is done using a stereomicroscope to project dot shaped measurement labels through one port. The second port is used to locate their positions. In a calibrated system, three-dimensional coordinates are allocated to the measured positions. Using a digital mirror device (DMD) the generation of user definable patterns is very quick and flexible. Furthermore, the DMD features high brightness and contrasts values. This is important to recognize and distinguish projected measurement labels. The illumination is done by a white high power light-emitting diode, which is collimated and coupled to the DMD with a total internal reflection prism. The acquisition of measurement data from bright and dark surface areas results in a high change in brightness of measurement labels. To measure bright and dark surfaces in one measurement, a high dynamic range imaging technique is required. To identify measurement labels a temporal coding is used. Identification enables the adaption of the brightness of each projected measurement label. This results in a homogeneous brightness distribution of measurement labels, which enables their measurement in a second step. As a result, the dynamic of the measurement system is expanded by the dynamic of the projection device.
"Active triangulation metrology system with high dynamic range," Journal of Micro/Nanolithography, MEMS, and MOEMS 10(3), 033007 (1 July 2011). https://doi.org/10.1117/1.3610168