Candle power to lumens?

I'm sure this has probably been covered but I can't get the search function to work at all . First off I suck at math so please keep it simple. I would like to be able to figure how many lumens sat a 1 million CP spot light is.

Brian

I'm no photometrist (I don't even know if I spelled it right) but the short answer is you can't.

Lumens is a measure of the total light output of the light. IIRC, candlepower is a measure of the spot intensity. (I don't use CP very often.)

A few definitions... http://www.theledlight.com/lumens.html

And sorry, but this is technical... http://www.madsci.org/posts/archives/2001-04/987003940.Ph.r.html

CaptCarrot said:

A few definitions... http://www.theledlight.com/lumens.html

Click to expand...

Jeez, what an annoying article. To save anyone else the pain of reading it, the important bit seems to be:

[FONT=Arial, Helvetica, sans-serif]LUX is an abbreviation for Lumens per square meter.
Foot-candles equal the amount of Lumens per square feet of area.[/FONT]
[FONT=Arial, Helvetica, sans-serif]So, that one candlepower equivalent equals 12.57 lumens.[/FONT]

[FONT=Arial, Helvetica, sans-serif]Though I'm less than convinced it's correct. That would mean my 2 million candlepower spotlight puts out 25 million lumens. Even allowing for a bit of over-estimation, that seems just a tad too high.
[/FONT]
[FONT=Arial, Helvetica, sans-serif]Do they really mix American and SI units in that manner?
[/FONT]

Candlepower depends as much on how the light is focused as anything else. A light that puts out 30 lumens in a tight spot will have a higher candlepower than a light that puts out 50 lumens in a flood. That's why you can never tell how bright a light is by candlepower. It's really easy for a manufacturer to make a pathetically dim light that is well focused and then brag about their candlepower measurement. The brightest part of a beam doesn't tell much about the beam overall.

Very informative

Dodge said:

...[FONT=Arial, Helvetica, sans-serif] That would mean my 2 million candlepower spotlight puts out 25 million lumens. Even allowing for a bit of over-estimation, that seems just a tad too high...[/FONT][FONT=Arial, Helvetica, sans-serif][/FONT]

Click to expand...

If it was 2 million candlepower overall(at any angle) that it will have 2.5 Million lumen. And the ratings aren't even correct, they are way under 500,000 candle power at the most focused spot.

Short answer, candle power can not be converted to lumens unless it is the same at every angle of measurement. A usual 1 million candlepower light produces 300-600 lumen, I read that somewhere.

Dodge said:

So, that one candlepower equivalent equals 12.57 lumens.

Click to expand...

No no no. It only does if the light source radiates equally much in all directions (4 pi steradians), which no flashlight does. Without knowing the exact beam profile, or otherwise integrating up the entire amount of light, you are unable to calculate how much light it puts out.

According to the annoying article:

"[FONT=Arial, Helvetica, sans-serif]Candlepower, or CANDELA is a measure of how much light the bulb produces, measured at the bulb, rather than how much falls upon the thing you want to light up."

Are they just completely wrong here? I guess they are. I like the Wikipedia explanation better:

"[/FONT]The candela is the luminous intensity, in the perpendicular direction, of a surface of 1/600 000 square metre of a black body at the temperature of freezing platinum under a pressure of 101 325 newtons per square metre."

Dodge said:

According to the annoying article:

"[FONT=Arial, Helvetica, sans-serif]Candlepower, or CANDELA is a measure of how much light the bulb produces, measured at the bulb, rather than how much falls upon the thing you want to light up."
[/FONT]

Click to expand...

As far as I know, yes, that is completely wrong. As I understand, that would be a great definition for lumens, not candlepower.

Dodge said:

I guess they are. I like the Wikipedia explanation better:

"[/SIZE][/FONT]The candela is the luminous intensity, in the perpendicular direction, of a surface of 1/600 000 square metre of a black body at the temperature of freezing platinum under a pressure of 101 325 newtons per square metre."

Click to expand...

Even that appears to be out of date by almost 40 years.
"The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian." http://physics.nist.gov/cuu/Units/current.html

This page (http://www.sizes.com/units/index.htm) is also quite handy.

Dodge said:

According to the annoying article:

"Candlepower, or CANDELA is a measure of how much light the bulb produces, measured at the bulb, rather than how much falls upon the thing you want to light up."

Are they just completely wrong here? I guess they are. I like the Wikipedia explanation better:

"The candela is the luminous intensity, in the perpendicular direction, of a surface of 1/600 000 square metre of a black body at the temperature of freezing platinum under a pressure of 101 325 newtons per square metre."

Click to expand...

Wikipedia hit the nail perfectly. The key issue here is "in the perpendicular direction".
If I where president of the universe, the authors of the first site you mentioned should be put to work in a salt mine for a couple of weeks so they get some time to think about what they have done and what consequences misleading people might have.

winny said:

Wikipedia hit the nail perfectly. The key issue here is "in the perpendicular direction".
If I where president of the universe, the authors of the first site you mentioned should be put to work in a salt mine for a couple of weeks so they get some time to think about what they have done and what consequences misleading people might have.

Click to expand...

Very true, and much of the misleading persists to this day.

holy resurrection.

Million candlepower spotlights tend to be between 400 and 600 lumens from what I have seen. Lumens is a unit of the the total amount of visible light on a surface a distance from the light source(infrared and ultraviolet light have 0 lumens no matter what). One foot away, a 1 foot by 1 foot area lit up as bright as a candle one foot away (one foot candle) is equal to 1 lumen. Candlepower simply tells the brightness at the source compared to the brightness of a candle. It doesn't take into account losses in efficiency from the reflector, lens, or optics. Lumens does account for these losses. A million candlepower spotlight doesn't put out a million candlepower anyway. It is just marketing hype to get you to buy their bright spotlight.