High speed, on-line 4D microscopy using continuously scanning white light interferometry with a high speed camera and real-time FPGA image processing

Achieving real time 3D measurement of microscopic surfaces is difficult, mainly because of the high bandwidths required for the data acquisition by the probe or camera, the transfer to the processor and the processing. In this paper we present the results of our second prototype on-line measurement system that we have developed using continuous scanning white light interference microscopy, a high speed CMOS camera and parallel processing with an FPGA that enables data processing rates of up to 160 Mb/s. Two fringe detection algorithms have been implemented, one based on the detection of the maximum fringe intensity and the other on the maximum of the fringe modulation function. The practical performance is demonstrated on the measurement of laterally translating samples, with 3D image rates of up to 20 i/s being achieved for an image size of 256 x 320 pixels, and 2.96 i/s for an image size of 640 x 1024 pixels over a depth of 5 mu m. Depths of up to 20 mu m can be measured. On-line 4D microscopy opens up new applications for characterizing surfaces that are moving or changing in a non-periodic way with time, such as in MEMS, soft materials, layer growth or chemical reactions.