The X-ray CCD’s which form the primary component of the SXI will carry out observations while operating at a temperature of -120 degrees centigrade. Gases are present in the satellite environment (which originate from within the satellite) and there is a danger that these gases will freeze out onto the surface of a CCD. If this happens it will not be possible to make full use of the CCD, and so a filter has been designed to prevent such gases from reaching the SXI. This so-called “Contamination Blocking Filter” (CBF) is to be installed in front of the SXI. The thickness of the CBF must be large enough so that it can withstand the shocks encountered during launch yet no so large as to substantially block off the transmission of X-rays. It is therefore important to verify that the X-ray transmission efficiency of the CBF meets its design requirements. Such measurements were made during experiments using the “photon factory” at KEK, a high-energy particle accelerator facility. Here, the CBF transmission rate for X-rays and UV light as well were measured.