The glass of the HOYA HRT filter also has UV absorbing properties making the HRT a combination UV/circular polarizing filter. The most common use for a circular polarizer filter is to darken bright blue skies in outdoor photography, but they also can reduce or eliminate reflections from non-metallic surfaces such as glass and water. By rotating the outer ring of the filter the change of effect can be seen by looking though the filter or through the viewfinder if it is mounted on a camera.