The names Dia Optical Co., Ltd. and Dia Opt. Works both appear on the Dia Six, an inexpensive 6×6 camera supposedly made in Japan in the 1950s.
- Sugiyama, Kōichi (杉山浩一); Naoi, Hiroaki (直井浩明); Bullock, John R. The Collector's Guide to Japanese Cameras. 国産カメラ図鑑 (Kokusan kamera zukan). Tokyo: Asahi Sonorama, 1985. ISBN 4-257-03187-5. Item 4122.
- About us page of a company called Dia Optical Co. situated in Osaka Japan.