From the 1960s the use of radium paint was discontinued. In many cases luminous dials were implemented with non-radioactive fluorescent materials excited by light; such devices glow in the dark after exposure to light, but the glow fades. Where long-lasting self-luminosity in darkness was required, safer radioactive promethium-147 (half-life 2.6 years) or tritium (half-life 12 years) paint was used; both continue to be used today. These had the added advantage of not degrading the phosphor over time, unlike radium. Tritium emits very low-energy beta radiation (even lower-energy than the beta radiation emitted by promethium) which cannot penetrate the skin, rather than the penetrating gamma radiation of radium and is regarded as safer.