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In the POH of the Cessna 310B, two "ultraviolet lights" mounted in the overhead panel and lighting the instrument panel are described. They have starter buttons, to turn them back off the dimming rheostat has to be momentarily set to zero intensity. But it is left unclear what kind of lights they are exactly - are they real UV lamps, or just normal filament / LED lamps producing blue-violet light through the blue-colored lenses ? If they are real UV lamps, wouldn't you risk getting burned from the UV rays ?

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    $\begingroup$ I suspect they were low powered black light (UV) lights under the glareshield (?) which are pointed directly at the cockpit instruments, whose faces, numbers, needles, pointers, etc. were coated in a phosphorescent, shining brightly when illuminated by UV light. Exactly why Cessna made such a choice back then, is unknown. One possibility is that it produces smooth, uniform, illumination of critical flight instruments at night, without additional visible light, which degrades night vision. $\endgroup$ Apr 8 at 15:51
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    $\begingroup$ They are mounted in the overhead panel, on the ceiling of the cabin. $\endgroup$
    – TheEagle
    Apr 8 at 15:53
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    $\begingroup$ @CarloFelicione I guess that phosphorescent has a simple white color, right ? $\endgroup$
    – TheEagle
    Apr 8 at 15:53
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    $\begingroup$ Posted as a comment not an answer because I don't have any documentation, but UV lights are also known as "Blacklights", and are used to make fluorescent materials glow. I suspect that there is fluorescent paint on the gauges, and the UV light just makes them stand out more at night. (Also, no, you're not in danger of "being burned by UV rays". Shortwave UV-C is dangerous, but the longwave UV-A produced by commercial blacklights is completely harmless at normal intensity.) $\endgroup$ Apr 8 at 15:54
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    $\begingroup$ Ahh, I have looked up the part number from the IPC for the UV lamp, and wbparts.com indeed classifies the emitted light as Ultraviolet light. So this makes sense ! Thanks for your comments. $\endgroup$
    – TheEagle
    Apr 8 at 15:59

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The UV lights were used because they made the white instrument and panel markings glow brightly without affecting your night vision. It was one workaround alternative, after radium paint was abandoned, to get a similar effect of lighting only the letters and markings. Nowadays the same effect is achieved with instrument post lights and internal backlighting of control panels shining through letters.

The lamps would be UV-A lamps, typical "blacklight" bulbs, and back in those days would be gas filled low pressure mercury vapour lamps, like fluorescent bulbs.

Like fluorescents, they require a ballast (a capacitor) to provide an electrical jolt to start them up, which is what the starter button is for.

If you have them shining at the instrument panel, you aren't going to get that much UV exposure, but probably after many hours if you had some of the UV shining on your forearms and hands, you might start to get a sunburn. It would depend on what sort of hood or shield is over the light to control where the light shines.

The issues of extended exposure to UV, and no doubt reliability issues with mercury vapour lamps and the fact they don't provide useful illumination beyond making white things glow, is probably why they faded from use over the years.

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  • $\begingroup$ "The issues of extended exposure to UV, and no doubt reliability issues with mercury vapour lamps and the fact they don't provide useful illumination beyond making white things glow, is probably why they faded from use over the years." - Also, mercury, which probably won't be good for the pilot if the lamp breaks in a crash. $\endgroup$
    – Vikki
    Apr 11 at 0:18

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