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The idea of a GRATING condenser was conceived by THOMAS WILHEIN and ULRICH VOGT, Institute for X-
In brief, the aim was to design a condenser which would improve two unfortunate inherent properties of traditional Zone Plate condensers. Firstly, instead of a focal “spot” at the focal plane, the intention is to create a large focal field of flat illumination, into which one can position the sample under observation. This eliminates the need to place the object away from the focal plane of the condenser (where the diverging light has expanded to a desired size) and/or mechanically rotate the condenser in order to achieve a sufficiently large field to illuminate the object.
The second aim was to avoid (or at least reduce) unwanted diffraction effects which are caused by the illumination of traditional condenser ZPs by very coherent X-
The condenser consists of annuli of gratings each facing towards the centre of the optic. The grating constant in each annulus is chosen such that the diffraction from all gratings will coincide at the required focal plane. The size of the illumination field will obviously be approximately the same as the size of these gratings. In the first design, there were 256 gratings, in 5 annuli, each grating measuring ~50 x 50 µm.
In subsequent designs, we have fabricated 1 mm diameter condensers with ~1000 gratings ~12 x 12 µm each, with gold central stops directly on the condenser. The lines/grooves of the outermost annulus is usually the same as the objective micro ZP, down to 30 nm, so that the two optics have matching Numerical Apertures (NA)
The Gallery shows some images of one of our Grating Condensers