Differential pumping may be made more efficient by application of modified collimators of the grid type, first used for optical collimation only in spectrometers for the extreme uv spectral region. Compared with conventional types of differential pumping, either larger windows can be used, or the gas flow can be reduced. For optical purposes we had to build up a system with a rather large window, with an effective area of 15 mm2. The differential pumping properties of this system are compared to those of a conventional pumping system with an effective area of 3 mm2and the results clearly show the advantage of the new design.