Embodiments described herein provide apparatus and methods of etching a substrate using an ion etch chamber having a movable aperture. The thickness of the sheet passing through the nip is determined based on the amount of displacement of the displacement member. A lock lever rotatably supported by the first plate rotates about a second axis between a locked position and an unlocked position. Light irradiated from LED cell is, by action of respective lens of first lens array, separately collected and is thereafter, by action of respective lens of second lens array, separately magnified; and light exiting therefrom is combined in mutually superposed fashion at irradiation surface.