One of the problems of early tanks was the difficulty of viewing outside the tank whilst remaining safe in hostile environments. The earliest tanks had vision slits which gave a very limited view of the exterior and were a source of bullet fragments ("bullet splash") entering a tank. Periscopes were tried and, although successful, the field of view through a periscope is necessarily quite narrow. Ideally what was required was a device which would allow a tank commander to have an all round view without exposing the commander to projectiles and fragments. This problem remained at the end of WW1.
An elegant solution to this problem was devised in France and applied to tanks built by FCM (Société des Forges et Chantiers de la Méditerranée) after WW1. This relied on the stroboscopic effect. An early example of this effect is seen in the Victorian toy, the Zeotrope, which used a slotted cylinder with series of images on the inside of the cylinder. When the cylinder is rotated the images appear to fuse into a single moving image and the slots appear to vanish - this is caused by the phenomenon of the persistence of vision - human vision actually sees the world in 0.1sec slices, our brains supply the interpolations to give apparent continuous vision. (YouTube video of a Zeotrope).
A stroboscopic cupola was fitted to the FCM 1A prototype, possibly in 1919. A stroboscopic cupola was also fitted to the FCM Char de Bataille prototype. Two cupolas were fitted to the monstrous FCM 2C. No other French tanks, as far as is known, were fitted with stroboscopic cupolas.
The details of the cupolas fitted to the FCM 1A and Char de Bataille prototypes are unknown but this is not so for the cupolas fitted to the FCM 2C. The FCM 2C cupolas were composed of two main assemblies, an internal frame with 7 triplex glass vision blocks supporting an electric drive motor for the outer shell of the cupola. The outer shell was cylindrical and made of 30mm thick nickel-chrome steel. The slots for the stroboscopic effect were 2mm wide and slightly wedge shaped being longer on the outside compared to the inside. The slots were arranged as 9 groups of 5 slots with the spaces between the groups being about 20% smaller than the spacing within a group of slots. The outer shell of the cupola was rotated at about 250-300 rpm which gave a satisfactory stroboscopic effect. The whole cupola was hinged at the rear and could be tipped up with a screw jack to improve ventilation of the tank and allow direct vision when not in combat conditions. The base ring of the cupola had additional view ports fitted with glass blocks. It was asserted that the French stroboscopic cupolas were resistant to bullet splash and gave the tank commander a 360° view although the brightness of the view was somewhat reduced.
In a project spanning from 1920 to 1925 the US Army Ordnance Dept experimented with stroboscopic cupolas. The American stroboscopic cupola had only a single slotted cylinder, it was claimed the cupola was very vulnerable to bullet splash from 0.30 cal bullets. The project was abandoned in 1926 with the conclusion that periscopes were superior to the stroboscopic cupola. A single Mark VIII tank was fitted with a stroboscopic cupola for trials.
The German tank designer Edvard Grotte was employed by the Soviet Government to lead a design bureau to design new advanced tanks for the Red Army in the late 1920s. The medium tank produced by Grotte's design team was something of a "tour de force" of tank technology and included a stroboscopic cupola. Only a single prototype T-22 tank was produced in 1931, the Soviet authorities thought the tank to be too expensive and would be difficult to produce with the limited Russian production plants. No information on the characteristics or performance of the cupola has been found.
Roger Todd of the Landships forum for finding diagrams of the stroboscopic cupolas.
User "Steve Zaloga" of the Landships forum for his clarification of the design of the FCM 2C cupolas.