Don't even think about trying to compare C.P. with Lumens unless you want to compare different flashlights from the same manufacturer and that manufactuer states both C.P. and Lumens (ie all the measurements have a good chance of being carried out using the same standard methods)
As far as I'm aware, no manufacturer does this across their whole range.
I'm gonna check out to see if there is light measuring equipment at my University...Then I'll do some proper tests on all the flashlights I can get my hands on.
It is simple to compare units of illuminance and irradiance. Assume a light is producing one candela (the candela is the base unit in light measurement, and is defined as follows: a 1 candela light source emits 1 lumen per steradian in all directions isotropically. A steradian is defined as the solid angle which, having its vertex at the center of the sphere, cuts off an area equal to the square of its radius.
This means that the one candela light produces one lumen/square foot at a distance of one foot. This is the same as a foot candle. The same light produces one lumen per square meter at a distance of one meter (as the light spreads out as it goes farther from the source). Go here for a full review: http://www.intl-light.com/handbook/ch07.html
However, these data do not contradict the above posts warning you of the pitfalls comparing levels of irradiance or illuminance. The problem is that the above data, as stated, depend upon ISOTROPIC radiation; that is, the radiation is equal in all directions. Obviously, this is not the case in a lamp with a reflector which produces a focused beam, or an LED which radiates in a given direction. Thus, comparison between different units, as measured by different methodology is difficult if not impossible.
There is no conversion or direct comparison between Lumens and candlepower. Candlepower is the measure of the brightest spot in the light beam. However this can vary depending on the focus of the beam, and the distance it is measured. No manufacturer lists the distance at which they measure the cp rating. Also they don't state the size of the hot spot they are measuring. It could only be a 1 inch bright spot in the edge of the beam. Lumens is the measurement of the total light output of a bulb, but not it's brightness. Most 60 watt incandescent light bulbs are rated at 800 lumens. This is more than even the brightest SureFire light, but compare the brightness of even the dimmest SureFire light to a desk lamp and the SureFire will be much brighter. Candlepower also changes by the focus of the beam, and the size of the reflector.
I believe a direct comparision is not possible because of the flashlight's reflector focusing the light. Using math, it maybe possible to convert Lumens to Beam Candela (Beam cd) [aka C.P.], but it's far harder, if not impossible to convert from C.P. to Lumens.
The reason that it may not be possible to convert C.P. to Lumens is becuase the C.P. is the output 'intensity' measured at a particular beam angle within the beam and at a distance from the lens. Both the angle and the distance are hardly (if ever) given.
The type and size of reflector and lens is also very important in determining if the beam is of uniform intensity. C.P. allows for selective non-representative very high values to be quoted. It's used as a hype tool I've found.
So the intensity of the light at a beam angle of 0.5 degrees is usually more than if measured at 3 degrees as most other flashlight brands have uneven beams with dark bits and rings. So CP ratings are to be taken with a large bucket of salt unless they given with more details such as beam angle etc.
SureFire gives ratings in Lumens because it gives the total light output including the surround 'flood' beam, not just the intense spot. I think SureFire beams usually have about 75% of the light focused into the beam.
I would have thought that because SureFire beams are very uniform (it'd have to be assumed that the beam was of uniform intensity at 1m), the area, and beam angle at 1m would be quite easy to measure. You then need to do some complex math!
Here is a more specific problem with distance and candlepower ratings. If I were to say measure my Streamlight Stinger for both lumens and cp at 1 inch and then at 1 foot, and then again at 10 yards the candlepower ratings would differ, but the lumen output would not.
Focus has the same problem. Again using a Stinger. Measure the candlepower and lumens with the beam at it's tightest, then as you widen the beam. Again the lumens will be the same, but the candlepower will differ each time.
With that Stinger, somewhere it sill measure 15,000 candlepower, but where? Is it with the tightest beam at only 1 inch away?