Lumens vs candlepower

[Eric Sedletzky]

Back in the Mid 80's I had a rechargable cop light that was rated at 30,000 candlepower. I remember it was pretty damn bright and would light up an alley like a car headlight. Back then I never heard of lumens as a rating for flashlight brightness.
Now everything is rated in Lumens and I never hear about candlepower anymore. So just out of curiosity I'm wondering if any body knows what the equivalent crossover would be (1 lumen = x candlepower)

Thanks

 

[lucca brassi]

Lumens to candela (cd) conversion calculator or LedRise - Candela / Lumen Calculator

depends on angle , distance are same , and calculation refers to even illumination

 

[tursiops]

Lumens tell you how bright the light is, in terms of the full amount of power being emitted in all directions. So a 1000 lumen video (flood) light will not appear to be as bright as a 1000 lumen narrow-beam light. In the latter, the same pwoer of the former is being directed all in pretty much one direction...so any surface it shines on will be more highly illuminated. The candlepower (now candela) is the power shining in one steradian (there are 4xPi = 12.57 steradians in a full sphere), or lumens per steradian. Basically the same thing, but referenced to a full sphere or to 1/12.57 of a sphere. Actually, you probably really care about how brightly a surface is illuminated, which is lux, or lumens per square meter.Tthat is why a spot-beam light of equal lumens to a video light appears to be brighter...those same lumens are focussed into a smaller beam, thus more lux on the surface you are looking at.

Your 30,000 candlepower cop light would be roughtly equivalent to a 30000/12.57 = 2400 lumens light today, if they spread their light over the same beam angle.....it would give the same lux on the surface you are looking at.

It is all about power, and beam angle.

The problem is, many of the lights today are not rated by the power emitted, but by the power going into the LED, so if the LED/system is (say) 80% efficient, then the light sold as 1000 lumens is really only producing 800 lumens of light. SOLA is honest, Light Monkey is honest, maybe some others. See whether the spec says "lumens output" or just "lumens."

Then there are lights rated in watts. This is most commonly a measure of volts x amps......so is about the power going into the light source. Light Monkey rates their 21W HID as 1500 lumens output.

 

[tbone1004]

and wattage is a terrible unit of measurement with LED's because it only works for the same emitters.

I.e. Light Monkey has a 32W LED that produces 2500 lumen. It's a 10 year old emitter and is horribly inefficient. UWLD only needs 20 watts to get the same output. It works when comparing most of the backup lights because they are basically using the same Cree emitter, but on primaries it's a lot different. It was more useful when everyone was using the same HID or Halogen bulbs, but with LED's there is a lot more going on so using power consumption is pretty useless.

The emitter output is more commonly what is coming out of the emitter, but not what is coming out of the glass, that is where the reduction usually is. The emitter may very well be putting out 1000lm, but because of efficiency losses in the lens, you're only getting 600-800, which is about all you can expect from a single emitter without a high AH pack behind it.

Lux is the number you want for spots, lumen is what you want for total light output. Problem with both, but especially lux is that it is damn near impossible to accurately measure, and the equipment needed to measure them is insanely expensive. Lux is tough because you can have a however many thousand lux light with a 6* beam, and you can have the same lux output with an 8* beam, and the 8* is putting out a LOT more light despite having the same lux number which makes it pretty useless since there are a lot of different beam angles on the market. It just measures how "hot" the hotspot is. Total lumen output and the beam angle is enough, better yet, have a picture of the beam patterns for comparison.

Compare | Underwater Light Dude
You can see some pictures there. Unfortunately they only had access to the Hollis 25w LED which is similar to the LM but not the same. You can see the the LM 12 vs the LD15 though for good comparison. The LM12 throws about 750 lumen which is comparable to most of the backup lights on the market right now and draws 12w. The LD15 is drawing 30% more power at 15W but throwing twice the light and it shows in the pictures.