Lux, lumens, foot candles...most accurate way to determine a light's brightness???

N

Nicholas Natteau

Veteran
For an LED flood light, I'm pondering whether to go with a Litepanels 1x1 flood or an Ikan IFD 1024.

But I need to know what method to use to measure a light's brightness. What's the preferred measuring standard in the film industry when comparing brightness among lights? Is it Lux? Lumens? Or foot candles?

Are they all equally valid? Or is one of them more accurate than the others?

Thanks very much in advance.
 
Footcandles in (North?) America, Lux elsewhere. I think it's something like 9.3 lux = 1 footcandle. They're both the same type of measurement, just different units. Lumens isn't a very useful measurement for us since it says nothing about the distribution of the light, whereas footcandles and lux are almost always accompanied by a beam diameter at a given distance.
 
Nicholas Natteau said:
For an LED flood light, I'm pondering whether to go with a Litepanels 1x1 flood or an Ikan IFD 1024.

But I need to know what method to use to measure a light's brightness. What's the preferred measuring standard in the film industry when comparing brightness among lights? Is it Lux? Lumens? Or foot candles?

Are they all equally valid? Or is one of them more accurate than the others?

Thanks very much in advance.
Click to expand...

The 'correct' units for most photo work is footcandles(fc) or lux(lx). Lumens is the 'total output' of the lamp, and as such much calculation is required to find out how many photons are falling on a surface, which in turn is reflected, which in turn yields the image on the sensor...

The pro light manufacturers usually will give 'photometric' specs for their lights, in terms of fc or lx, and at various distances from the light.

However, a light meter is strongly recommended to measure the light falling on a set/scene/subject, which will be much more accurate than some theoretical calculations.

I have used reflective meters for years, mostly spot meters, but for 'film things' I found that having a incident meter beneficial, which reads out footcandles or lux directly.

For example the 'Rule' of f/2.8 @ 1/48 @ 100 ISO @ 100 footcandles, is much more easily checked for other f-stops/ISO/speed values, if one has a meter reading off in fc/lx

Most ordinary 'store bought' bulbs are listed in lumens, and so one would have to perform the following caclulations...

Assume the bulb is spec'd at 1600 lumens (usual for 100W house light bulb...).

Recall from geometry that a sphere has a surface area of 4 * pi * r^2 (everyone has that at their finger tips... right...).

Now assume the 'bulb' is illuminating in an approximate 'sphere' pattern (not quite... but sort of close)... so that means there are 1600 lumens passing through the surface of the entire sphere around the bulb.

At 1 foot radius, the resulting area of the surface is 4*3.14 (approx...) or about 12 sq feet. that gives

1600 / 12 ft^2 = 130 lumens / ft^2, and recall... everyone does... right... that 1 footcandle == 1 lumen / ft^2... so one has 130 footcandles at one foot from a 100 W light bulb.

Using the 1/r^2 rule, then at 2 feet one has 130/4 = 30 fc, and at 4 feet one has 130/16 or about 8 fc @ 4 feet from the source.

All of the above does have use of 'rounding', and 'simplification' of the setup, but there's the idea.

If the light source is not a 'sphere', then the calculation requires one to calculate a 'cone' and the area of the surface of the base of the cone, which of course everyone remembers form highschool geometry... right...

All in all... it's just simpler to take out a meter and read off the actual value (relative to the accuracy of the meter...), than all that theoretical calculations.

The incident meter I bought was the Sekonic L-308DC and from a quick search seems to be going for $271 US at B&H.
 
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