Signalers using flash lamp, near Bouzincourt, 10 July 1916. Lamps were another signaling option, but one that could only be used front-to-rear, or the Germans would have been able to intercept the messages. Semaphore flags were technically another option, and were standard issue equipment for batteries, but the number of men that volunteered to wig-wag flags on the Western Front must have been low. This flimsy dugout also shows how little protection was needed in a relatively quiet sector.
Source: IWM photo Q149.