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
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.
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:
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.