KevinWuFaji
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What's the difference between the unit "lm" & "mcd"?
and for instance, how much of "lm" is the same as 20000mcd?
and for instance, how much of "lm" is the same as 20000mcd?
What's the difference between the unit "lm" & "mcd"?
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Cypher_Aod
Enlightened
Lumen is the measure of total light output in all directions from the emitter/torch.
Mcd means "MilliCandela", or thousandths of a candlepower, and this is the measure of point light intensity (just like Lux), which is the measure of point brightness, which is dependant on how focused the beam from the light is.
Mcd means "MilliCandela", or thousandths of a candlepower, and this is the measure of point light intensity (just like Lux), which is the measure of point brightness, which is dependant on how focused the beam from the light is.
That's not entirely correct.
mcd means indeed millicandela, Mcd Megacandela (vastly different range).
candela is a base unit in the SI system. It measures how much light a light source puts out in one direction at the specific wavelength of 555 nm. You can use it to compare different light sources, if you define the direction (it's not in the candela definition). Even so, this is only practical for similar light sources. It works quite well for comparing incandescents, but cannot easily be used to compare incans to CFLs or LEDs, since the light output at just one wavelength does not compare to the total light output in a similar way for different technologies. The candela definition specifies how to construct a specific light source, which can then be used to calibrate other light sources or detectors, but it is not very useful for comparing light sources that are not constructed to this specification.
Lumens are different. As stated, it measures the light output of the light source in ALL directions. It also measures ALL wavelengths (in the visible range of 400 .. 700 nm). Also computed in is the sensitivity of the human eye. If you want to compare different light sources, and how well they will illuminate your surroundings, you have to go with lumens.
You cannot calculate lumens from candela for different light technologies, as the spectra of the light sources are vastly different.
Both units have little to do with lux (which measures how much light ARRIVES at a certain spot and distance from the light source). To calculate that, you have to specify the direction from the light source (which is not specified in the candela definition), and the lumens number of the light source does not tell how the light is distributed around the light source (if the light source throws all light in one direction, you get a high lux number in THAT direction, but if another light source with the same lumens throws light in all direction, that same spot will have a low lux number).
mcd means indeed millicandela, Mcd Megacandela (vastly different range).
candela is a base unit in the SI system. It measures how much light a light source puts out in one direction at the specific wavelength of 555 nm. You can use it to compare different light sources, if you define the direction (it's not in the candela definition). Even so, this is only practical for similar light sources. It works quite well for comparing incandescents, but cannot easily be used to compare incans to CFLs or LEDs, since the light output at just one wavelength does not compare to the total light output in a similar way for different technologies. The candela definition specifies how to construct a specific light source, which can then be used to calibrate other light sources or detectors, but it is not very useful for comparing light sources that are not constructed to this specification.
Lumens are different. As stated, it measures the light output of the light source in ALL directions. It also measures ALL wavelengths (in the visible range of 400 .. 700 nm). Also computed in is the sensitivity of the human eye. If you want to compare different light sources, and how well they will illuminate your surroundings, you have to go with lumens.
You cannot calculate lumens from candela for different light technologies, as the spectra of the light sources are vastly different.
Both units have little to do with lux (which measures how much light ARRIVES at a certain spot and distance from the light source). To calculate that, you have to specify the direction from the light source (which is not specified in the candela definition), and the lumens number of the light source does not tell how the light is distributed around the light source (if the light source throws all light in one direction, you get a high lux number in THAT direction, but if another light source with the same lumens throws light in all direction, that same spot will have a low lux number).
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Cypher_Aod
Enlightened
2xTrinity
Flashlight Enthusiast
Elgarak, I will further clarify a couple points:
I think you may be getting candelas mixed up with optical power, in watts. Both candelas and lumens take into account the eye sensitivity. The only diference is candelas measure how focused light is, in terms of angle.
You actually can calculate lumens from candela if you know the beam profile, specifically the beam solid angle:
Candela = Lumen/Steradian
where steradian is a unit of solid angle. For reference, light going in all directions (ie, in a sphere) is spread out over 4*pi steradian.Light emitted equally into a hemisphere is emitted into 2*pi steradian. A light emitting 4pi lumens in all directions has an intensity of 1 candela.
In genearl, light emitted into a cone (typical of say a 5mm LED) of angle theta, will have a solid angle given by this formula:
So a narrower beam will have a smaller solid angle Omega. If the same number of lumens is focused into a narrower beam, the total number of candelas will be greater.
When comparing 5mm LEDs (where candelas are usually specified), the value refers to how intense the beam will look viewed head-on, and will be much higher for narrower angle LEDs, all else equal.
The problem with this is that almost no flashlight actually behaves like a point source only 1 meter away (typical reflectors are several cm across). If you wanted to actually measure the beam intensity with a light meter, the way to do it would be to go far from the light, to say 30 meters, then measure the intensity in lux out there. Then multiply that lux number by 900 (30 squared). That should be the beam intensity in candelas. This is OK because at 30m, the light will look like more than a point-source, so the inverse-square law will apply.
This calculated number is usually smaller than the lux value actually measured at 1m.
I think you may be getting candelas mixed up with optical power, in watts. Both candelas and lumens take into account the eye sensitivity. The only diference is candelas measure how focused light is, in terms of angle.
You actually can calculate lumens from candela if you know the beam profile, specifically the beam solid angle:
Candela = Lumen/Steradian
where steradian is a unit of solid angle. For reference, light going in all directions (ie, in a sphere) is spread out over 4*pi steradian.Light emitted equally into a hemisphere is emitted into 2*pi steradian. A light emitting 4pi lumens in all directions has an intensity of 1 candela.
In genearl, light emitted into a cone (typical of say a 5mm LED) of angle theta, will have a solid angle given by this formula:
So a narrower beam will have a smaller solid angle Omega. If the same number of lumens is focused into a narrower beam, the total number of candelas will be greater.
When comparing 5mm LEDs (where candelas are usually specified), the value refers to how intense the beam will look viewed head-on, and will be much higher for narrower angle LEDs, all else equal.
this is true. On the forums the term "lux" is often used interchangeably with candela. This is because in the special case where your surface is exactly 1m from a 1-candela point-source, the surface will be lit to an illuminance of 1 lux (or 1 lumen/square-meter).
The problem with this is that almost no flashlight actually behaves like a point source only 1 meter away (typical reflectors are several cm across). If you wanted to actually measure the beam intensity with a light meter, the way to do it would be to go far from the light, to say 30 meters, then measure the intensity in lux out there. Then multiply that lux number by 900 (30 squared). That should be the beam intensity in candelas. This is OK because at 30m, the light will look like more than a point-source, so the inverse-square law will apply.
This calculated number is usually smaller than the lux value actually measured at 1m.
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