Transparent subject copy was employed by early workers and, as a result, relatively simple optical systems were required..
The Present-day art around 1937, employes reflected light from the subject copy to exite the phototube.
The following dokuments cover the general trend.
Thilo (U. S. Patent 1,924,700, H. G. Thilo).
Ranger (U. S. Patent 1,828,000 R. H. Ranger).
Scheppmann (U. S. Patent 1,763,482 W. Scheppmann).
Philpott (U. S. Patent 1,927,900, L. R. Philpott).
Potter (U. S. Patent 1,777,016, R. K. Potter).
Zworykin (I.R.E. Proc., Vol. 17, No. 3, March, 1929, V. Zwoykin).
Wright (U. S. Patent 1,809,617, G. M. Wright).
Gardner (U. S. Patent 1,950,171, P. C. Gardner).
Schmook (U. S. Patent 1,857,745, A. Schmook).
Wilson & Zworykin (Photocells and their Applications by E. D. Wilson and V. Zworykin, published by John Willey & Sons).
Elsey & Philpott (U. S. Patent 1,877,566, H. M. Elsey and L. R. Philpott).
Schroter (Hanbuch der Bildtelegraphie und das Fernsehens by F. Schroter, published by Julius Springer, Berlin).
Whitaker & Artzt (RCA Institutes Technical Press, Radio Facsimile, pp. 229-247, J. N. Whitaker and M. Artzt).
(C) Marius Rensen