An apparatus for measuring parameters, such as moisture content or basis weight, of a fibrous web, e. ; and its object is to provide a backlight unit not involving the problem that the emitted light leaks out of the optical waveguide, even when the space around the cold-cathode tubes in the light source unit for it is filled with a liquid of which the refractive index is nearly the same as that of the glass material that forms the outer wall of the cold-cathode tubes. A user transaction may comprise the exchange, purchase, storage, or consumption of media such as, for example, audio, still images, video, and data.