# lumens??

#### monsterdawg725

##### Newly Enlightened
sorry im a newbie flashaholic but what is a lumen?

#### clipse

##### Enlightened
Measurement of light output.

#### monsterdawg725

##### Newly Enlightened
ok i mean how many candlepower is one lumen equal to?

#### cratz2

##### Flashlight Enthusiast
Actually:

http://en.wikipedia.org/wiki/Lumens

Edit - and welcome to CPF!

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#### cratz2

##### Flashlight Enthusiast
Imagine about 1 Lumens would be the minimum you'd want to have to walk around a comploetely darkened room with night-adapted eyes. 10 Lumens would be more than enough to light up a reasonably-sized room. 60 Lumens would be enough for almost any outdoor task with, say 25 feet.

#### monsterdawg725

##### Newly Enlightened
thanks cratz2 thats what i was looking for i appreciate it

#### Ra

##### Flashlight Enthusiast
Hi Monster,

Welcome to these forums.. and hide your wallet !!

Let me enlighten this a bit for you:

Indeed lumens is a measurement of light-output, the quantity of light.
With CP, there are two types: MSCP (Mean Spherical CandlePower).. And BCP (Beam CandlePower)

MSCP is a measurement of the omnidirectional output of a bare bulb without reflector or lens. This mostly is recalculateble to lumens-output:

1 MSCP equals 12.57 lumens

Things change if you use a reflector: You are concentrating most of the lumens at a point at infinnity, so the output of the bulb-reflector combination no longer is omnidirectional.

Beam CandlePower is measured in the "hottest" part of the collimated beam: You measure the lux at a certain distance and recalulate to CP.

Beam-CP is not related to lumens-output: You can have a flood-light, producing alot of lumens, reading 1000 lux at 10 meter distance (100,000 cp)
You can also have an extreme narrow spot from a low lumens light that also reads 1000 lux at 10 meters. In these two cases there is a big difference in lumens output, but no difference in CP-output.

Regards,

Ra.

#### 2xTrinity

##### Flashlight Enthusiast
Another thing you may find confusing is when people speak of lumens per watt. Lumens are essentially the power of the radiated light in watts (coming out of the light, not electricity input) weighted by how sensitive the eye is at that particular color/wavelength according to the Luminosity Function.

The peak sensitivity is at 555nm, a yellowish green color, 1 watt at that wavelength is defined as 683 lumens. The further away you get from this (toward the red, or toward the blue) this number starts to decrease. Once you get all the way to IR or UV, the amount of lumens is zero no matter how many watts you are radiating -- this is why incandescent lamps are more efficient as heaters than as light sources, most of their output is IR.

This is useful to know, as the most efficient LED in terms of radiant efficiency (power in vs. power out) are blue and red, but they are not the most efficient ones in terms of lumens per watt, since the eye isn't as sensitive there. Blue-green (cyan), or red-orange LEDs, will generally appear brighter as they are closer to the sensitivity peak. The true green LEDs that are out now are actually very inefficient, so you'll get the most lumens per watt out of Cyan, or cool white (blue where some of the light is converted to other colors with a phosphor).

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#### monsterdawg725

##### Newly Enlightened
thanks RA, and 2xtrinity

#### chevrofreak

##### Flashlight Enthusiast
2xTrinity said:
Another thing you may find confusing is when people speak of lumens per watt. Lumens are essentially the power of the radiated light in watts (coming out of the light, not electricity input) weighted by how sensitive the eye is at that particular color/wavelength according to the Luminosity Function.

The peak sensitivity is at 555nm, a yellowish green color, 1 watt at that wavelength is defined as 683 lumens. The further away you get from this (toward the red, or toward the blue) this number starts to decrease. Once you get all the way to IR or UV, the amount of lumens is zero no matter how many watts you are radiating -- this is why incandescent lamps are more efficient as heaters than as light sources, most of their output is IR.

This is useful to know, as the most efficient LED in terms of radiant efficiency (power in vs. power out) are blue and red, but they are not the most efficient ones in terms of lumens per watt, since the eye isn't as sensitive there. Blue-green (cyan), or red-orange LEDs, will generally appear brighter as they are closer to the sensitivity peak. The true green LEDs that are out now are actually very inefficient, so you'll get the most lumens per watt out of Cyan, or cool white (blue where some of the light is converted to other colors with a phosphor).

I think your description is just going to confuse people. Here on CPF and in the rest of the lighting world lumens per watt is a measure of efficiency. The higher the lumens per watt the more efficient the light source is. The lumens are measured as the light coming out of the LED or bulb, and the watts are measured as the power being used by the LED or bulb.

If an LED being fed 2.5 watts is putting out 174 lumens, then it has an efficiency of 69.6 lumens per watt.

#### 2xTrinity

##### Flashlight Enthusiast
chevrofreak said:
I think your description is just going to confuse people. Here on CPF and in the rest of the lighting world lumens per watt is a measure of efficiency. The higher the lumens per watt the more efficient the light source is. The lumens are measured as the light coming out of the LED or bulb, and the watts are measured as the power being used by the LED or bulb.

If an LED being fed 2.5 watts is putting out 174 lumens, then it has an efficiency of 69.6 lumens per watt.
Wow, I was actually going to say that but forgot to add that in the first paragraph -- there are two:

lumens per watt of input (used to talk about flashlights, most commonly used around here)
lumens per watt out output (to talk about how sensitive the eye is to a wavelength)

#### DM51

##### Flashaholic
Welcome to CPF, monsterdawg (I trust from your handle you do have one - please let us see a photo of it.)

That is some good information above, but there are only two important things you will need to bear in mind about lumens if you are to survive here:
1. You can never have enough of them.
2. They can be very expensive.