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2 billion candlepower from 90 watts ?

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picbits

Well-Known Member
I was in a war museum in Malta last week and they had a searchlight from WWII. It stated it could produce 2 billion candlepower with a beam range of 5 miles powered by a 90 Watt Lister generator at 120 Amperes.

This seems like a whole lot of candlepower from a relatively small amount of input power.

Something with those figures just didn't feel right.
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
I would imagine it's simply because the entire power is in a tightly focused beam.

I'd also strongly suspect that it's a mis-print, and it's supposed to say 90V not 90W.
 

picbits

Well-Known Member
From reading the specs of other searchlights I also suspect they meant 90V @ 120A, some of the searchlights ran at up to 300A @ 90V
 

dr pepper

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That would require a 1.3v bulb to be 90w @ 120 amps, while possible that would be most unlikely, the switching, copper cables & contacts would all need to be huge, 90V is more likely, that gives 10Kw which with a 5 mile beam sounds more like it.
Looking at the size of the lister gen sets 10Kw makes sense, a 90w gen would be small enough to carry.
Be nice with a pair of those on yer car, maybe not for other drivers though.
 

Les Jones

Well-Known Member
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I suspect the light source was a carbon arc rather than a normal bulb. 90 volts is about the right voltage for that.

Les.
 

MrAl

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Hi,

Yes, 90w with 120 amps means the light source would have to run at 0.75 volts which cant be right.

A 90watt florescent could produce around 500cp in all directions. if we could 'fold' it's light pattern once we'd get 1000cp. Fold again and we are up to 2000cp, again and 4000cp, again and then 8000cp, again and tehn 16000cp, and although we are getting higher and higher the emission solid angle is getting smaller and smaller. It would have to be very small to be able to do something like 2 billion cp.

Another possibility would be 9kw, or even 90kw i guess.
 

dr pepper

Well-Known Member
Most Helpful Member
Oops your right picbits 0.75, no idea where I got 1.3v.
Certainly the can of the spotlight looks much like those used in filming back in the 50s & 60s which were carbon arc.
They must have been quite something to operate, but I spose the operators would have been more concerned with bombers blowing the place up.
 
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