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What of this words define the intensity of the light of a bulb? Lumens or watts? I don't know if I am right. Lumens means intensity of the light and watts the consumption of the bulb

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Lumen is the unit of luminous flux and expresses the amount of visible light emitted by the source. Watts can be used to describe both the energy consumption or the radiant flux, be it visible or invisible.

It is possible to calculate between watts and lumens, in that case spectral sensitivity of the human eye and spectrum of the source have to be taken into account because some colours are more visible than others.

But no bulb has specified radiant flux, that figure always means power consumption.

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  • \$\begingroup\$ If I need to build a high brilliant bulb I need to make it with a high quantity of lumens? \$\endgroup\$ – John Alexander Betts Aug 27 '14 at 21:23
  • \$\begingroup\$ @JohnB Yes. And if you want to use it to cast light onto a surface, you need a lot of illuminance (luxes) which you can gain by using a reflector. \$\endgroup\$ – venny Aug 27 '14 at 21:29
  • \$\begingroup\$ I'm not clear what you mean by saying watts can describe the radiant flux. Do you mean when comparing similar types of strobes or light sources? Then watts would have a meaningful relationship, but you can't (realistically) use watts to compare bulbs of different efficiencies. Or am I missing something? \$\endgroup\$ – JYelton Aug 27 '14 at 21:45
  • \$\begingroup\$ @JYelton The radiant flux can be viewed also as the difference between input power and heat losses. This rating is used for example for infrared LEDs that obviously emit no visible light. Or for lasers, seriously powerful \$\mathrm{CO}_2\$ laser (which is also invisible) needs hundreds of electrical watts to produce 12 watts of radiant flux. Radiant flux of bulbs is unknown and in most cases irrelevant. \$\endgroup\$ – venny Aug 27 '14 at 22:05
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You are mostly correct. Watts is the power consumption (volts times amperes) of the bulb, which is a resistive load. Lumens is the total light output of the bulb.

There are several terms that deal with light (summaries are simplified, links are Wikipedia):

  • Luminance: A measure of the luminous intensity per unit area of light traveling in a particular direction.
  • Luminous Intensity: A measure of the power emitted by a light source in a particular direction.
  • Luminous Flux: A measure of the perceived power of light. (As opposed to radiant flux.) Luminous flux takes into consideration the sensitivity of the human eye.
  • Illuminance: Total luminous flux on a surface area.

And units associated with these terms:

  • Lumens: Unit of luminous flux. The amount of light that is radiated from a source.
  • Candela: Unit of luminous intensity. The intensity of a light source.
  • Lux: Unit of illuminance and luminous emittance. The illumination intensity of an area; or flux of photons passing through a given surface area (lumens per square meter).
  • Nits: Candela per square meter (cd/m2)

A bulb will consume a certain amount of power, measured in watts. Some of the power is converted to light (the objective), but a certain percentage will be lost as heat. Therefore, wattage is the total power consumption of the bulb, including losses as byproducts. A 15W LED bulb may exceed the luminous flux of a 60W incandescent bulb, because it more efficiently converts electrical power into light.

For more information about the complicated ways in which to measure and regard light, see also:

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Lumens are a difficult unit to work with if you are trying to count the number of photons in a light beam. This is because a Lumen is a measure of the human eye response to light. So "real" intensity (measured in Watts) is converted to lumens, following some chart of the average human eye. The maximum is somewhere in the green, all other wavelengths have fewer lumens per watt. On the other hand, Watts to photon flux is easy, (assuming you know the wavelength.)

Lumens are useful if you are laying out lighting for an office area, for instance.

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  • \$\begingroup\$ Starting an answer with "I like..." or "I hate..." doesn't seem like a good format. You're certainly welcome to elaborate in your answer about your opinions, but as it stands, I think I'd re-word this answer. \$\endgroup\$ – JYelton Aug 27 '14 at 22:15
  • \$\begingroup\$ @JYelton, OK less grumpy now. (As long as we are on the subject, why is the Lumen part of SI?) \$\endgroup\$ – George Herold Aug 27 '14 at 22:28
  • \$\begingroup\$ I feel your pain with regard to lighting units! \$\endgroup\$ – JYelton Aug 27 '14 at 23:23

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