Abstract: A backing sheet or mat for use in the exposure of photosensitive materials under vacuum conditions, includes a sheet of porous, woven fabric having a nonreflective surface, a sheet of flexible, but substantially rigid, polymeric material beneath the fabric sheet, and a sheet of flexible rubber material beneath the polymeric sheet. Each of the sheets is attached together to form the backing sheet and a plurality of apertures extend through the thickness of the sheets of polymeric and rubber material and are arranged over substantially the entire area of the latter sheets.