CD measurement of photomasks is currently mostly done with microscopes using white light. In the development phase of ion projection lithography, it is useful to evaluate the limitations ofthis approach. Here, the focus is on measurements with a Muetec2OlOfLeica LWM200 optical microscope which operates with white light. Stencil masks for ion projection lithography consist ofa 3im thick silicon membrane with a top carbonic protection layer of about O.5im. For a feature size of O.4tm, which corresponds to a wafer technology of O.lOj.tm (IPL uses a 4:1 reduction), an aspect ratio of about 10: 1 results. So, it is questionable if transmissive measurements are appropriate. Effects of reflections on the sidewalls and diffraction effects influence the repeatability. The absolute intensity of the transmitted light is reduced compared to thin mask layers. Results of transmissive measurements are presented and compared to reflective measurements on the same tools. But, as the stencil openings are retrograde, the CD which is critical for the use in the ion exposure tool is defmed on the top of the Si layer. This can be measured by a reflective method. For stencil openings it resulted a repeatability of less than l6nm 35 for a whole range of structure sizes down to O.5j.tm (smaller sizes were not evaluated for this purpose yet). This is comparable to the repeatability of chromium-on-glass masks, so a good perspective for future use results assuming an evolutive development of optical CD microscopes.