# Watt ? Candle Power... ?

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• 03-25-2011, 04:29 PM
Ray Rogers
Watt ? Candle Power... ?
Anyone know or can easily measure the c.p. of a "typical" 60W light bulb?

I have some old contact printing data for a paper that says that typically, an exposure of X seconds at a distance of 50 cm, to a 50 c.p. light source should be just about right.

So, what would this be in terms of todays (?) tungsten light bulbs?

What are typical c.p. values for 60 light bulbs of today?

I have found confusing and conflicting information,
but so far, the values I have come up with are:

60W=16 c.p.
or
60W=19.35 c.p.
or
60W= 64 - 72 cp

Anyone know if these figures are anywhere near correct?

I have some understanding of the many issues involved here, but as given, it was thought that the rating in c.p. as given at the time the paper was made, would be useful to people with differet lamps... I think it must have at least some "ballpark" accuracy,

so a crude approximation is fine....
• 03-25-2011, 05:41 PM
DWThomas
No doubt this can vary a lot, as by running a filament hotter, one can trade off lifetime for intensity. A quick Google search yielded an article that says 1 lumen = 1 foot-candle, and that to an approximation, dividing the lumens by 0.07958 will give an approximation of candlepower.

Some 60 watt soft white "long life" bulbs here claim 780 lumens with a 1500 hour life. That would work out to about 9800 candlepower.

I suspect there are some assumptions made that might be good to know about. But that exhausts my knowledge!
• 03-25-2011, 06:20 PM
Photo Engineer
US 60 watt bulbs burn dimmer in Japan Ray, if you are on 50 CPS as is most of Japan.

PE
• 03-25-2011, 10:53 PM
gmikol
Not just 50Hz, but 100V nominal in Japan, compared to 120V in the US.

--Greg
• 03-25-2011, 11:12 PM
Ray Rogers
The most likely guesstimate so far is that 50 candlepower equates to about 34.5W current tungstens...
So I am thinking just calling it 40W...

BTW
What watt lights are most people using for AZO and Lodi Ma?
• 03-25-2011, 11:22 PM
tkamiya
Actually... half of Japan is on 60Hz and the other half is 50Hz. I really don't think frequency will affect brightness of incandescent light bulb... average power and RMS of sine wave AC is always the same regardless of frequency.
• 03-25-2011, 11:25 PM
Photo Engineer
We saw a difference when I was there. They used 100V at 50HZ.

This made many things 5/6ths dimmer and you could see the fluctuations as a flicker.

PE
• 03-25-2011, 11:38 PM
Worker 11811
It isn't possible to generalize the wattage of any lamp to its light output. Every different manufacturer's product could be different and they often are. One way for manufacturers to compete is to say that their lamps are more efficient than some other brand. (e.g. "G.E. lamps produce more candle power per watt than Sylvania.")

It isn't possible to generalize the light output of any lamp in actual use based on its specs. Like others said, burning a lamp in one place (country) could produce different amounts of light due to different voltage, current and frequency.

It isn't really the brightness of a lamp in an enlarger that determines how photographic paper is exposed. It is the amount of light per unit of square measure. If you take a lamp of a certain brightness and shine its light through a lens so that its light is spread over a piece of 8x10 paper you will get a different exposure than if you spread that light onto a 4x5 piece of paper.
(For purpose of discussion, this assumes that ALL of the light goes onto the paper and nowhere else.)

1) Candle power is an archaic measurement that isn't used anymore.
2) Light intensity is usually measured in lumens but candle power and lumens don't measure the same thing.

Candle power measures the perceived brightness of a light source from a certain direction but lumens measure the total light output in all directions.

Take a candle of a certain size which is made of a certain kind of wax. Light it and make it burn at a certain rate then look at it from a certain distance. That is one candle power.

Take that same candle and put it inside an opaque sphere which is perfectly mirrored on the inside. Open a small portal in the sphere and use a photocell to measure the intensity of the light coming from that portal. You would be measuring how many lumens the candle produced.

You can not say that a lamp of a certain wattage will burn with a certain brightness and you can not directly compare candle power to lumens but you can say that there are APPROXIMATELY 12.57 lumens per candle power.

Your average 60 watt household light bulb produces 650 to 700 lumens when burned under standardized conditions.

650÷12.57=51.7

700÷12.57=55.6

Therefore you can estimate that a 60 watt household light bulb, burned under average household conditions might produce somewhere between 50 and 55 candle power, give or take...

If that don't do what you need, try a 50 watt or a 75 watt bulb.
• 03-26-2011, 03:59 AM
Ray Rogers
Thanks... So, rather than "convert", perhaps I should just try to get one measured. I wonder, is there anyplace on earth that can still make measurments in candle power?

Does anyone know of a "Light Museum" ?
Maybe Edison has a museum or something?

One thing I omitted, the specfic light was described as: