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Doyle LaCrua

Light Meter Reading Preferences and Whys

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Greetings all,

 

I'm excited to be a part of a community so generous with it's knowledge.

 

I'm really trying to wrap my brain around the Sekonic L-758cine light meter and it's advance capabilities. This might well be a very tremendous question...

 

Footcandles, Lux, Foot Lambert, and cd/m². I'm hoping to understand in what situations it'd be best to use these light measuring features. Are these just a preference? Where and why would you use them?

 

Thank you sincerely for your time.

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Hi Doyle,

 

Footcandles and lux are two different units of the same measure : luminance, which is the light level (incident readings). In the US you will use footcandles, and in any case, it's very easy to do the conversion as 1 fc approximately equals to 10 lux.

It's useful to think in terms of footcandles instead of t-stops. If you say you need x footcandles for your key light, a gaffer knows how much fc a light outputs at a given distance, and it's easier for everyone. It takes the camera (ISO, lens, etc...) out of the equation.

 

Foot lambert and cd/m2 are two different units of reflected light (spotmeter reading). It is a measure of the light that is emitted for a given area, like computer screens (or movie screeens) for example.

 

 

G.C.

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Thanks for the reply.

 

Interesting. So it seems footcandles/lux are used when you want to express a "fixed" unit of incident light measurement, but regardless of any other factors.

 

The other two guys, foot lamperts & cd/m², also measure with a "fixed" unit of light but they measure reflectived light (or light being emitted by a particular object). And since we measure with "m²" that also is typically used outside the U.S.

 

I guess they are just an extension to the idea of incident and reflective light measurement.

 

Please let me know if I'm wrong! Other than that, splendid! Thanks for the reply.

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So it seems footcandles/lux are used when you want to express a "fixed" unit of incident light measurement, but regardless of any other factors.

The other two guys, foot lamperts & cd/m², also measure with a "fixed" unit of light but they measure reflectived light (or light being emitted by a particular object). And since we measure with "m²" that also is typically used outside the U.S.

I guess they are just an extension to the idea of incident and reflective light measurement.

 

That's correct :-)

 

G.

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Foot-lamberts are used for measuring the brightness of a projected picture. It's measured without film in the gate at the centre of the screen. Too bright is as bad as too dark; it causes visible flicker. You sometimes notice it on a blank screen with Super-8 at 18fps.

On my wall I have a copy of a letter from Stanley Kubrick to projectionists asking them, among other things, to ensure that 'Barry Lyndon' is screened at between 15 and 18 fL. He cared about stuff like that.

Edited by Mark Dunn
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Actually, footcandles and lux are not units of Luminance, but rather units of Illuminance.

 

There are four main measurements of photometry:

-Lumens: Total Power of Light (weighed to human eye response) that a light gives off. [lumen]

-Candelas: Total Intensity (Luminous Flux) given off PER solid angle. [cd or lumens/steradian]

-Illuminance: Total Power Incident on the surface of a square meter. [lumens/m2 or LUX]

-Luminance: Total Intensity per Solid Angle from a light with a given size. [cd/m2]

 

Footcandles are the US (read non-metric) version of Illuminance, measured in lumens/ft2.

Footlamberts are the US version of Luminance, IF the luminance is measured from a Lambertian (perfectly diffusing) Surface. This does not readily apply to screens with gain other than 1.0. Often cosine falloff adjustments are needed for the proper interpretation of the levels read.

 

Incident Light Meters give you an exposure for a surface that would be a given percentage of the light incident. Often this percentage is erroneously called 18%, but to find the actual percentage you plug in this equation for the meter's calibration constants ( [K*PI/C] ). Sekonic gives you readings for 15.7% on the incident meter, for instance. What this translates into on your camera is dependent on the ISO speed and manufacturer's deviations, of course.

 

Footcandles, Lux, Foot Lambert, and cd/m² are great for the reasons listed above and serve to give you "absolute values" of the scene, etc. EV, Aperture Priority, Shutter Priority and camera-centric values give you "relative values" to a given meter and camera calibration.

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Luminance vs illuminance : Joshua's right, I misspelled it.

 

So, to sum up :

 

- LUX, Footcandle, Lumen/ft2, lumens/m2 = they are all units of Illuminance which are measured by incident metering (we hardly ever use the other units such as lumens/ft2, nor the... meter candle !!)

 

- Incident light meters will give you a reading for something between 12% and 18% gray, but only if you use them in a "camera-centric" mode (to display an f-stop). If you take an incident reading in a footcandle or lux mode, you'll get a raw measurement of the illuminance, in one of the previously mentioned units. Still, in order to get a correct reading you need to pay attention to the placement of the sphere (usually turned towards the camera's lens), and whether you want to use the dome or the retracted (flat) mode (learning how to use a lightmeter is off-topic ;-)

 

- Foot-lamberts, cd/m2 are units of Luminance, measured by reflected metering, and these units do apply to measure screen brightnesss. A movie screen's pure white should be 16 foot-lamberts bright. (btw Joshua, what do you mean by "a screen gain greater than 1.0 ?")

 

- The lambertian surface is a theoric perfect diffuse surface that has no specular highlights. Movie screens are designed to be as "lambertian" as possible so as not to reflect the projector's specular highlight.

 

- Cosine adjustments are only needed when the surface is viewed (or lit!) from an angle. It's useful for flat surfaces in general:

 

300px-Lambert_Cosine_Law_2.svg.png

In practice, since what you are measuring with your spotmeter is the actual surface, a spotmeter is already measuring the result of the cosine law. That is, as long as you measure from (roughly) the same angle as the camera (which is a good habit to take anyway).

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That's a great description of the application of the cosine law for use with a light meter!

The reason I mentioned the screen gain is because things can get just a little more complicated when using silver screens, which aren't used very much except for circular/linear polarized light in 3D films anyway.

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Thanks Joshua, and thank you for your contribution too !

Aw, silver screens.... The problem with these screens is if you're not in the middle of the theatre, the image is much darker! In other words: they're anything but "Lambertian" ! :D

Edited by Guillaume Cottin

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