Lumens/Candlepower

Hi jinng, welcome aboard

Another UK member joins CPF !!

As to your question, please look through the Topic headings in "General Flashlight and Headlamp Discussion".

The subject arises quite frequently, and the answers are many.

Basically, a Candlepower measurement is done at a point in the beam, and represents a brightness at that particular part.

Lumen ratings are a measurement of the total, the whole amount of light produced.

Just as an example of the relationship between the two, without any particular accuracy in the numbers chosen -

Let's say a 1,000 Lumen beam could be rated at 100,000 CandlePower, if it is tightly focused.
But if you take that same lamp, so the same amount of light and change it to a wide focus, it would still be 1,000 Lu, but the CP would drop to a figure of perhaps 5,000.
Still the same amount of light, but not nearly the same measurement of light at just one chosen point.

At the other extreme, a laser will project a bright point of light for a good distance. Even an ordinary red laser has a very high CP rating, because it's a bright spot.
But it's Lu rating is very low - you don't have any real illumination, you couldn't light up a room to see very much with it.

If you are lucky enough to get a Green laser, you can project a point of light well over a mile. And if you shine it at the ceiling of a room, it will throw a small amount of light to see by. So here, you have a combination of a massive, enormous CP rating, and a low Lu rating.
I reckon the Green laser I borrowed from His Most Gracious Telephony has about 3+ Lu.

lightlover

Is there a way to convert one to the other, ie lumens to candlepower and vice versa.

Would make comparison of manufacturers' quoted output much easier.

Thank you so much for such an illuminating reply - I sure appreciate that.

something ridiculous ? is quite correct in the description of the difference between candle power and lumen. Think about it to get a gut feeling for what the math below will describe.

A very useful website is
Dictionary of Units of Measure, which describes all sorts of units of measure. The site has a very good description of the various lighting units. In a nutshell:

The CANDELA is the SI (metric system) unit of light _intensity_. It is defined as the intensity of a particular amount of photons of a particular color filling a particular solid angle of space. But the intensity is measured only at one single angle, not over this entire region. Essentially what the definition is saying is that you take a particular total amount of light, spread it over a defined area, and then measure the intensity at a single point. An analogy would be to pour a gallon of water over a square meter, measure the depth of the water, and use that measurement as your unit of water depth. The definition of intensity includes the sensitivity of the human eye; by definition lots of IR photons will not produce any candela (unless they ignite something else .

The LUMEN is the SI unit for flux (total amount) of light. A light source with a uniform intensity of 1 candela (meaning that the intensity is the same in all directions) causes a flux of 1 lumen per steradian.

A STERADIAN is a unit of solid angle, it is analogous to area, but for solid angular measure; if you imagine a cone, the steradian would be the unit that you use to measure how broad the cone is. A unit that is somewhat more comfortable is the 'square degree'; this is a very sharp pyramid with sides that have a 1 degree angle at the tip. 1 steradian is 41,253 square degrees

What does this all boil down to? Say I have a light which produces a beam that is 5 degrees across, and is rated at 100,000 candela, and the light is perfectly evenly distributed. A 5 degree cone is a solid angle of about 20 square degrees, or about 0.0005 steradian. Multiply the intensity in candela by the solid angle in steradians, and you get the total light flux, about 50 lumen. Note that I did a bunch of approximating here; a real lamp would not produce an even beam, and you would have to integrate across the beam rather than just multiplying, but this gives you an idea of the conversion.

-Jon

Why not standardize a two-fold light rating?

1) LUMENS
2) CANDLEPOWER at optimum ("brightest") point within an area of defined diameter at a defined distance from a (presumably white)
target.

IMO this would be a simple way to give a better over-all idea of a light's performance.

Best regards,
Brightnorm

Perhaps Forget CP

Lumen rating.
Central beam angle & Colour Temperature.
Total beam angle.


What else to I need to know?

When we've got a regulated constant output runtime, so there's no need for peak this, and maximum that...

Al

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by brightnorm:
[QB]Why not standardize a two-fold light rating?

1) LUMENS
2) CANDLEPOWER at optimum ("brightest") point within an area of defined diameter at a defined distance from a (presumably white)
target./QB]<HR></BLOCKQUOTE>

In other words nr 2) is pretty close to LUX

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by recercare:
In other words nr 2) is pretty close to LUX<HR></BLOCKQUOTE>

In re-reading this thread I realize that you're right.