Luminous dials first appeared during the Great War when soldiers needed to tell the time in the dark. Early forms used Zinc Sulphide compound agitated by a radioactive salt. It was painted on hands and was potentially dangerous to those applying it. Its use was banned in the 50's, since Tritium, a substance with a low radio activity, replaced it. Other methods have been devised. Timex's 'Indiglo' uses electronic luminescence; a button on the side of the case causes a tiny current from the battery to the electrodes and gives off energy in the form of light. Seiko uses fluorescent material on the dial, activated by any exposure to light.