The black on the bulb ( it covers the complete bulb, except for the 2 spots where the bayonet pins contacted) is carbon from the burning 11kV switch bay which this light was in. It was in a minisubthat is across the park where I live, and the one 11kV PILSWA gland decided to burn out, and set the minisub fibreglass on fire whilst doing so. In the left bottom you can see the picture of the metro busy loading the old unit on a truck, after they had worked all night removing the unit and putting in a replacement. It made quite a bang when it failed, and took out the entire circuit for around 2 hours until they had isolated the 2 feeder lines to this minisub as well as the balancing secondary feed. the fire brigade arrived after the fire was out, and used the all purpose fireman key to open the gate to the substation.
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Is this for the infrared radation?