Someone will correct me if I'm wrong, (they usually do) but I beleive that two models were produced. One with a fluorescent light, which needs external clip-on filters, and a second more expensive model fitted with a sodium lamp that does not. The sodium model is not safe until it has warmed up for about 5 minutes whereas the fluorescent one can be used straight away.
The second lever set a limit for maximum brightness, the first dimming the light by progressively introducing a mechanical shroud around the lamp.
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