It is therefore anticipated that they will see hefty development in the next few years. In addition, these phosphors are more and more applied in safety and emergency marking. More recently in 2014, a test road strip was inaugurated in the Netherlands with marking achieved by means of persistent phosphors. The persistent phosphor levels off the flickering due to AC excitation and the result is a 20% improvement in electricity-to-light conversion. In 2012, patents were accepted which describe the use of persistent phosphors in LEDs working with alternative current (AC) and with the main advantage of eliminating the AC–DC transformer. Representative phosphors are Sr 2MgSi 20 7:Eu II,Dy III (blue, afterglow ~25 h), SrAl0 4:Eu II,Dy III (green, ~30 h), and Y 20 2S:Eu III,Mg lI,Ti IV (red, ~5 h). Since 1996, wealth of persistent phosphors have been discovered and new applications followed suit. Presently exact details about charge carriers and traps are still not well understood ( Van den Eckhout et al., 2010). Several models were proposed to explain light emission in this material but experiments did not always confirm them. With respect to lanthanides, SrAl0 4:Eu II has been described in the 1970s as having long decay time, its luminescence being seen 100 min after shutting down excitation, but the breakthrough came in 1996 when this phosphor was codoped with Dy III increasing considerably its luminance and lengthening its afterglow to about 10 h. Zinc sulfide doped with copper has long been the only persistent phosphor in use in watch dials, luminous paints, night-vision materials, and glow-in-the-dark toys. Observation of the phenomenon dates back to ancient China where luminescent paintings were known and, closer to us, to the discovery of the Bologna stone in 1602. Persistence luminescence (also called afterglow or long-lasting phosphorescence) is defined as a property of some materials to emit light long after the excitation has stopped. Bünzli, in Reference Module in Materials Science and Materials Engineering, 2016 4.2 Persistent Luminescence

Some of these worker and patient cohorts continue to be studied in order to gain a better understanding of the long-term effects of radium exposure. In post–World War I Germany, injections were given to treat tuberculosis, ankylosing spondylitis, and other diseases. Other patients sought relief by exposing themselves to natural water sources containing significant amounts of radium. Over-the-counter solutions containing radium (e.g., Radithor) were ingested by thousands of people seeking cures for these illnesses. Also around this time the American Medical Association accepted radium therapy as a treatment for rheumatism, mental instability, and a variety of other disorders. Many developed cancer (bone sarcoma) due to ingesting significant amounts of radium if they used unsafe work practices, for example, moistening and shaping the thin tips of their delicate brushes by insertion into the mouth. Workers painted ‘glow in the dark’ watch dials, clocks, compasses, and other instruments starting in the early part of the century through the 1960s. In the early twentieth century, luminous paint was developed that contained radium. Many people experienced significant exposures to radium before its harmful health effects were known. It was isolated from pitchblende by the Curies, Marie and Pierre, in 1898, who in 1903 jointly won the Nobel Prize in physics for their studies of radiation. Radium has a particularly interesting history. Gad, in Encyclopedia of Toxicology (Third Edition), 2014 Background Information

Radium dial painters