Lumens,Lux, and Candlepower

[HighLight]

I'm sure this is a basic question but is there an easy way to briefly explain the difference between Lumens, Lux and Candlepower and how they compare to each other? :thinking:

 

[eebowler]

http://flashlightreviews.com/features/lux.htm

 

[Marduke]

Also here:
http://www.candlepowerforums.com/vb/showthread.php?t=172991

 

[SilverFox]

Hello HighLight,

The Welcome Mat discussion covers Lux and Lumens, but that still leaves Candlepower.

Candllepower is often referred to as Foot Candles. It measures the same thing as Lux, but in feet. Lux is standardized to meters.

So, basically you have lumens as the total amount of light emitted from a flashlight, and lux or candlepower measuring the amount of light falling on an object that you are trying to light up.

The MagLite example in the Welcome Mat post is the best way to understand this.

Tom

 

[2xTrinity]

Hello HighLight,

The Welcome Mat discussion covers Lux and Lumens, but that still leaves Candlepower.

Candllepower is often referred to as Foot Candles. It measures the same thing as Lux, but in feet. Lux is standardized to meters.

Not exactly. Candlepower is an obsolete unit that refers to intensity in a particular angle. It has been replaced with the SI unit "candela".

The Candela is actually the base unit for photometry -- it is based on the apparent brightness of a sample of platinum precisely at its melting point. One lumen is defined as a beam with intensity of 1 candela, emitted into a solid angle of 1 steradian. A light source source emitting 1 candela isotroptically (uniformly in all directions) would be emitting 4pi lumens.

In general, if you were to take two sources producing the same amount of flux (Lumens), one projecting into a narrow angle, and another into a wide angle, the narrow angle light will have a higher intensity, or be producing more Candelas. Things like small accent spotlights, or 5mm LEDs usually specify brightness in Candelas -- as the relevant figure is how intense or bright the beam will be, rather than total output. Flood lights on the other hands usually advertize their output in lumens, or total flux, which is more relevant for general lighting.


Another photometry unit you may have heard of is called "Nits", commonly used to determine how "bright" computer monitors are. Nits are candela/square-meter. The reason this is used insetad of lux (lumens/square-meter) is because what you are generally interested is how bright the monitor will appear when viewed head-on, not how bright it is when looked at from off-axis.

 

[HighLight]

Not exactly. Candlepower is an obsolete unit that refers to intensity in a particular angle. It has been replaced with the SI unit "candela".

The Candela is actually the base unit for photometry -- it is based on the apparent brightness of a sample of platinum precisely at its melting point. One lumen is defined as a beam with intensity of 1 candela, emitted into a solid angle of 1 steradian. A light source source emitting 1 candela isotroptically (uniformly in all directions) would be emitting 4pi lumens.

In general, if you were to take two sources producing the same amount of flux (Lumens), one projecting into a narrow angle, and another into a wide angle, the narrow angle light will have a higher intensity, or be producing more Candelas. Things like small accent spotlights, or 5mm LEDs usually specify brightness in Candelas -- as the relevant figure is how intense or bright the beam will be, rather than total output. Flood lights on the other hands usually advertize their output in lumens, or total flux, which is more relevant for general lighting.


Another photometry unit you may have heard of is called "Nits", commonly used to determine how "bright" computer monitors are. Nits are candela/square-meter. The reason this is used insetad of lux (lumens/square-meter) is because what you are generally interested is how bright the monitor will appear when viewed head-on, not how bright it is when looked at from off-axis.

I was asking if there was an Easy way to explain the difference
Seriously though I think I got it..thanks!

 

[Size15's]

Not to make light of this conversation but it does begin to show that to see things clearly one needs to find the person bright enough to shed light on, erm, light.

Before anybody gets incandescent I'd like to point out that I consider CPF to have a diverse spectrum of members from whom the questioner can select the explanation(s) that help them. When it comes to light we don't tend to leave people in the dark.

Al

 

[HighLight]

Not to make light of this conversation but it does begin to show that to see things clearly one needs to find the person bright enough to shed light on, erm, light.

Before anybody gets incandescent I'd like to point out that I consider CPF to have a diverse spectrum of members from whom the questioner can select the explanation(s) that help them. When it comes to light we don't tend to leave people in the dark.

Al

what he said..

 

[SilverFox]

Hello 2XTrinity,

So, let me make sure I have this down correct...

If I measure a light at 1 foot and it registers as 1 foot candle on my light meter, that means that I have 1 candela?

Tom

 

[2xTrinity]

Hello 2XTrinity,

So, let me make sure I have this down correct...

If I measure a light at 1 foot and it registers as 1 foot candle on my light meter, that means that I have 1 candela?

Tom

From Wikipedia:

The unit is defined as the amount of illumination the inside surface an imaginary 1-foot radius sphere would be receiving if there were a uniform point source of one candela in the exact center of the sphere. Alternatively, it can be defined as the illuminance on a 1-square foot surface of which there is a uniformly distributed flux of one lumen.

To answer your question, yes. That relationship will only be true at the 1-foot distance. Also measuring lux at 1-meter is also synonymous with the candela intensity of a beam, assuming the light source is approximately a point-source at that distance.

Measuring footcandles a foot away from a fluorescent tube, or even many large flashlights would not give accurate readings at all. This is why with flashlights, a common way to calculate candela is to measure intesnity from much further away, then use inverse square law to calculate what it would be at 1m if the light were actually a point.

 

[SilverFox]

Hello 2xTrinity,

So, when I buy a light that is advertised as being able to produce 1500000 candlepower, if the claim is accurate, I can expect to read 1500000 foot candles at 1 foot, or about 139350 lux at 1 meter on my light meter.

Correct?

Tom