Fig. 1. Figures used in the patent “Centering Detection System for an Apparatus for Playing Optically Readable Record Carriers” (US Patent 4.057.833). (a) illustrates the experimental setup. (b) shows the zeroth order (35) and the first orders (36-39) of the beam diffracted by the information structure on the optical disc when reading with a focused spot. The centering error can be estimated using the phase change in the overlapped areas and, hence, allows precise tracking when reading the information. A similar approach was used later for overlay metrology in the lithography scanners.

Fig. 2. An example of the 0.38 NA lithography projection lens designed by Joseph Braat published in “Quality of microlithographic projection lenses,” SPIE Proc. Vol. 811, pp. 22-30 (1987). Figure courtesy of Joseph Braat.

Fig. 3. Joseph Braat in front of the Optics Research Group at TU Delft, where he worked as a professor from 1988 to 2008 (photo courtesy of Roland Horsten).

Fig. 4. Cover of the book Imaging Optics authored by Joseph Braat and Peter Török, published by Cambridge University Press.