What is the watt equiv. for 1080 lux?

Question

lux is the "watts" of LED's... and if you know, is that alot of lux for LED's or relatively small, but the main question is the watt equiv.

Answer 1

You're basic premise is incorrect.

First, there are two types of units for measuring light -- commonly called photometry units, and radiometry units.  Radiometry measures light at all wavelengths, or the band or weighting is specified.  Radiometry units are generally in SI -- for instance, Watts, Watts/m^2, Watts/sr, Watts/m^2/sr.

Photometry units measure a spectrally weighted power -- weighted by the eye's normalized sensitivity to light.  This makes it more useful when you care about how bright things will appear to a person.  Units of photometry are lumen (candela*steradian), lux, lm/m^2, candela, candela/m^2 (same order as above).

So.. the photometric equivalent of Watts is actually lumen, not lux.  For a given watts (that is, radiant flux), the corresponding lumens is always lower, because the weighting curve has a maximum of 1.  For a light bulb, most of the watts fall outside of the eye's sensitivity -- so it is possible to have a LED that draws low power, but has higher lumens than a light bulb, because most of the watts produced are of a useful wavelength.

Now, as to your specific question about lux.

Lux is lumen/m^2 -- or power per area.  So to be meaningful, you'd have to know the conditions under which it was measured -- what distance from the source.  If you move the source closer to the detector, you will measure a higher lux.  Also, a more focused source produces a higher lux than a wide beam.

Answer 2

You cannot one does not convert to the other.

Lux is Illumination like:

Meter-Candle, Foot-Candle, Flame, Phot, Nox, Lux, Lumen/square Meter, Lumen/square Centimeter, Lumen/square foot.

Watt is power like:

Convert watt, kilowatt, megawatt, gigawatt, horsepower, pferdestärke, BTU/second, ton, calorie/second, calorie/hour, erg/second, pound-foot/second, joule/second.

Like all photometric units, the lux has a corresponding "radiometric" unit. The difference between any photometric unit and its corresponding radiometric unit is that radiometric units are based on physical power, with all wavelengths being weighted equally, while photometric units take into account the fact that the eye is more sensitive to some wavelengths than others, and accordingly every wavelength is given a different weight. The weighting factor is known as the luminosity function.

The lux is one lumen/meter^2, and the corresponding radiometric unit, which measures irradiance, is the watt/meter^2. There is no single conversion factor between lux and watt/meter^2; there is a different conversion factor for every wavelength, and it is not possible to make a conversion unless one knows the spectral composition of the light.

The peak of the luminosity function is at 555 nm (green); the eye is more sensitive to light of this wavelength than any other. For monochromatic light of this wavelength, the irradiance needed to make one lux is minimum, at 1.464 mW/m2. That is, one obtains 683.002 lux per W/m2 (or lumens per watt) at this wavelength. Other wavelengths of visible light produce fewer lumens per watt. The luminosity function falls to zero for wavelengths outside the visible spectrum.

For a light source with mixed wavelengths, the number of lumens per watt can be calculated by means of the luminosity function. In order to appear reasonably "white," a light source cannot consist solely of the green light to which the eye is most sensitive, but must include a generous mixture of red and blue wavelengths to which it is much less sensitive.

This means that white (or whitish) light sources produce far fewer lumens per watt than the theoretical maximum of 683 lumens per watt. The ratio between the actual number of lumens per watt and the theoretical maximum is expressed as a percentage known as the luminous efficiency. For example, a typical incandescent light bulb has a luminous efficiency of only about 2%.

In reality, individual eyes vary slightly in their luminosity functions. However, photometric units are precisely defined and precisely measurable. They are based on an agreed-upon standard luminosity function which is based on the measurement of many individual eyes.

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Those two bulbs are probably in around the same ballpark.

42 ^_^