Japan paints a brighter future for glow-in-the-dark material

Japan paints a brighter future for glow-in-the-dark materialSteve Bush I have always thought of luminous paint as the sort of stuff that is only good for making toy skeletons glow in the dark. To me, the glow is minimal and fleeting, not useful for any serious application. But a few minutes with my head under the desk (normal EW behaviour) has changed my opinion – luminous technology has come a long way and now looks fit to enter the real world. There are two realistic ways to produce light-emitting paint. Combining a phosphor with a radio-isotope is one. This has the advantage of producing light continuously, and at a predetermined level of brightness. However, handling radioactive compounds in production needs special precautions and radioactivity is not exactly a marketing plus. The other route to glow-in-the-dark behaviour is to use a material that absorbs electromagnetic energy and re-emits it slowly at a visible wavelength. This is the approach taken in toys. It is non-radioactive, but luminosity with traditional ZnS:Cu materials decays quickly with time, and light output is low. The head-under-the-desk revelation came from a material called LumiNova from Nemoto, a Japanese chemical company. Nemoto set out, and succeeded, to produce a luminous material that is ten times brighter and glows ten times longer than ZnS:Cu. The company will not disclose the chemistry behind LumiNova, except to say that is based on metal oxides. It is possible that it uses rare-earth chemistry like that used in CRT phosphors, but this is only speculation. Although LumiNova is bright by luminous paint standards, it is not dazzling, producing a creditable 1cd/m2 one minute after exposure, decaying logarithmically to 1mcd/m 2 after 10 hours. Nemoto’s representative in the UK is display company Densitron. It sells LumiNova in sheets, doped into plastics, and as custom printing. “We can see a lot of applications for it, replacing LCD backlights in some circumstances and for legends on and around controls,” said Densitron spokesman Nick How. Experiments at Densitron show that a couple of hours in sunlight will fully saturate the material, even through the displays polarisers. As little as five minutes is required to charge the material in direct sun light. Signs, camping equipment and low-level safety lighting are other non-electronics applications that are foreseen. Luminous materials at a glance Parameter LumiNova ZnS:Cu Colour 520nm 530nm  (Yellow –    green) Brightness 300mcd/m 2 20-30mcd/m 2 Duration 2,000min 200min Charge time 30min 4min

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