Arc lamps were used in some early motion-picture studios to illuminate interior shots. One problem was that they produce such a high level of ultra-violet light that many actors needed to wear sunglasses when off camera to relieve sore eyes resulting from the ultra-violet light. The problem was solved by adding a sheet of ordinary window glass in front of the lamp, blocking the ultra-violet. By the dawn of the "talkies", arc lamps had been replaced in film studios with other types of lights. In 1915, Elmer Ambrose Sperry began manufacturing his invention of a high-intensity carbon arc searchlight. These were used aboard warships of all navies during the 20th century for signaling and illuminating enemies. In the 1920s, carbon arc lamps were sold as family health products, a substitute for natural sunlight.