The apparatus has a housing, a treatment window (11), and a guide unit (20) that is placed adjacent to the treatment widow. A roller has a main area and a secondary area that are adjacent to each other and are visibly distinct from each other. A body is arranged with a through-opening in relation to the roller. The opening is dimensioned so as to display in an alternating manner the main area indicating to the user that the apparatus has reached a desired treatment position, and a portion of the secondary area indicating that the device is beyond the treatment position. An independent claim is also included for a method for using a treatment apparatus.
