The requirements on ultrasoft X-ray filter for ultrasoft X-ray emission spectroscopy in situ monitoring of sputtered films during growth are specified. Transmittance and absorptance spectra of 100 nm thick foils are calculated in the 100-1000 eV photon energy region for a large number of elements. A few materials of special interest for filter design are studied in the UV energy range. A simple fiter preparation method is described and has been used to fabricate Ag and In foils. Al and Ag have favourable low energy properties, i.e. low UV transmittance and high reflectance. In, Rh and Sn and a few other metals have large transmitting windows in the ultrasoft X-ray region where the important C Kalpha and N Kalpha emission lines appear. Be and C (graphite) are examples of materials having high transmittance in the X-ray region. Graphite is of particular interest since there are many techniques for preparation of thin foils described in the literature. Methods of how to combine the materials and their properties in order to design a filter with desired properties are discussed.