Candlepower vs Lumens -- Mag Recharge

Ok, I now have a 40,000 candlepower bulb in my Maglite Rechargeable.

Which brings up the question ... how many lumens am I now emitting ... According to what I found on THELEDLIGHT:

Quote------

Now, lets convert the lumens, a metric unit of light measurement, to candlepower.

We understand a candle radiates light equally in all directions, its output, in this consideration is not focused by any mechanical means (lenses or reflectors). Pretend for a moment that a transparent sphere one meter in radius surrounds your candle. We know that there are 12.57 square meters of surface area in such a sphere. Remember your Solid Geometry classes?

That one candle (1 Candlepower/Candela) is illuminating equally the entire surface of that sphere. The amount of light energy then reflected from that surface is defined thusly:

The amount of energy emanating from one square meter of surface is one lumen. And if we decrease the size of the sphere to one foot radius, we increase the reflected energy 12.57 times of that which fell on the square meter area.

LUX is an abbreviation for Lumens per square meter. Foot-candles equal the amount of Lumens per square feet of area.

So, that one candlepower equivalent equals 12.57 lumens.

-----Unquote theledlight

Do I understand that to calculate the lumens of the 40,000 candlepower bulb, I multiple 40,000 times 12.57 = 502,000 lumens of light?

Lumens and candlepower cannot be compared. There seems to be confusion over terminology: Candela, candlepower, lux, lumen, Spherical Candlepower, etc etc.

--dan

Dan is right.

It is better to ask how a flashlight compares to others. This gives you a better idea of beams and outputs then numbers and ratings.

One of the great things about CPF is that there will be some member who have the flashlights you want to compare!

Respectfully,

Al

<CP -vs- Lumens> They can't be compared.

It's like saying I have the highest x.xx GHZ Pentium 4 and you have the fastest Mac. Which computer is really faster? There is no way to use the speeds of these computers to tell which one will run a specific program faster. They must be compared side by side, using the same method for both. The same is true for lights, side by side comparison is the best way to compare them.

But also note that the eyes can deceive - side by size is fine for subjective "which is brighter" type tests, and for the most part relative brightness is good enough - but if you're really pedantic you'll want some sorta lightmeter to gauge the outputs.

Originally posted by Onyx:
But also note that the eyes can deceive - side by size is fine for subjective "which is brighter" type tests, and for the most part relative brightness is good enough - but if you're really pedantic you'll want some sorta lightmeter to gauge the outputs.

Click to expand...

<font size="2" face="Verdana, Arial">I don't know that I agree with this. Your eyes are what should be judging the lights because thats how flashlights are used. If you're not capable of seeing a difference between the two beams, buy the cheaper one...

Same applies when buying speakers. If your ears can't tell the difference between the $40 ones and the $400 ones, then the choice is obvious regardless of how well the $400 one "metered."

Mark

When you have a light source that emits light in all directions, i.e., equally in a perfect sphere, you can simply multiply candela by 4 pi (12.57) to get lumens. When the light source doesn't radiate evenly in all directions, you have to perform the actual integration of amount of light (candela) per unit of illuminated area. Or, amount of light projected over a specified beam angle. You can approximate this using the formula:
lumens = candela * 0.00024 * (beam angle)^2 with "beam angle" in degrees. You should memorize this formula because it's the best way to compare LED specs to incandescent bulbs. (LED output is typically spec'd with beam angle and candela or millicandela (mcd). Incandescent bulbs are usually rated in lumens, as they are assumed to radiate spherically.)

I've never found a reasonable way to relate flashlight "candlepower" ratings to lumens, because they never specify what beam angle corresponds to the quoted candlepower value. If you knew this angle, then it would be entirely reasonable to compare different flashlights to each other as well as to other light sources.