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Sci Tech
Light retention
Why does a television retain some light even after it is switched off?
V.P. SREENATH
Chidambaram, Tamil Nadu
Basically a television produces light by the process called cathodo-luminescence involving conversion of energy from fast-moving electrons when hitting the face-plate of the picture tube into light eventually leading to reproduction of pictures. The face plate of the TV picture tube is coated with a blend of blue-yellow emitting phosphors or a combination of blue, green, red, phosphors as tiny dots of triangles or stripes according as whether it is a black &white or color television system.
Phosphors used in cathodo-luminescence based applications such as conventional television picture tubes, computer monitors, radar screens etc., convert energy from fast moving electrons into visible light depending on chemical compounds involved. Usually zinc-sulphide based phosphor system with dopants such as copper, manganese are employed respectively yielding blue, green emissions.
The persistence duration of the glow from phosphors normally ranging from a few milli-seconds to minutes is determined by the chemical constituents and its applications. The prolonged glow from the luminescent materials, called phosphorescence or delayed luminescence can be attributed to several factors such as type of electronic transition, defect chemistry, temperature etc.
More interesting thing is if the picture tube is touched using bare-hands immediately after switching off the TV, we can see a weak-glow in the place touched by hand associated with a hissing a sound. This is due to the relaxation of static charges producing phosphorescence glow continuing for some time even after the TV is switched off.
R. JAGANNATHAN
Luminescence Group
CECR, Karaikudi, Tamil Nadu
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