The German government etched symbols onto the prisms of their Fresnel Lenses. This is the story of those symbols. The German government marked most of their lenses with a government certification symbol. Before 1933 and the Nazi Reich, the symbols used were the German Imperial Eagle and a symbol that looked like a castle, which represented the German Seamarks Service. These marks were etched onto the lens glass to certify that the lens met the requirements for government usage. The etching was performed by a government optical engineer who was called the "Geheimer Revisor," meaning "Inspecting Engineer." After the Nazi government came into power the symbol was changed to the Nazi Eagle holding the Swastika. Above that are the letters RVM, which stand for Reichsverkehrsministerium meaning "Reich Transport Ministry." Below are the letters SD, which stand for Seezeichendienst meaning "Seamarks Service."