http://www.helmut-singer.de/fix/she.html (forth listing from bottom) Optics UDSSR IR-FILTERSCHEIBE USSR Infrared Filter with diameter 170 mm. Blocks completely the visible part of light. Maximum translucence at about 870 nm. Made of hardened and heat-resisting glass. Glass thickness 3 mm. fully tested: 19,40 [](appr. US$ 19.-) Ref. #: 07 / 0924 / 7660 (19.02.02 )