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Old 1st Feb 2019, 12:10 pm   #12
Lucien Nunes
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Join Date: Apr 2008
Location: London, UK.
Posts: 2,508
Default Re: Mercury Arc Rectifier - How...

We should conduct a proper study on the UV emission but as far as I am aware, occasional direct viewing of a working MAR is harmless. My understanding is as follows:

MARs operate at very low pressure in order to minimise voltage drop. The sole function of the bulb is to maintain the low pressure by providing a condensing surface for vapour that is emitted from the cathode pool. At low pressure, UV emission from the discharge column is mainly UV-C at 184.45 and 253.7nm. These short wavelengths are harmful, but almost completely filtered out by the glass envelope.

By contrast, mercury lamps designed to emit UV are typically medium-pressure types in which significant radiation occurs at 365.4 nm (UV-A). They use quartz envelopes that are transparent to UV.

I am not aware of special precautions being taken in industry to prevent casual exposure to light emitted by MARs. Their cabinets usually surround the bulb itself, but grilles without louvres or light-traps are standard. Although most MAR installations date from the days of lower levels of worker safety than today, photokeratitis (arc-eye) was well understood and recognised in industry when MARs were commonplace.

It would be good to get real photometric data at all wavelengths of concern from a functioning bulb though.
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