Illuminance Calculation

Published : 1st August 2006

Hi all,

We all know the formula to calculate the illuminance of a ponctual source : lx=cd x dist x dist where lx is the illuminance in lux, cd is the luminance of the source and dist is the distance in meters.

But does someone know how to calculate the illuminance when using a diffusion? Gel, Frame or butterfly?

Thank you

Regards

Eric

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Eric André - Gaffer - Eu/Switzerland based
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Check on the www.lighttools.com site there is a program just for this sort of thing.

It is called egg calc and it is down loadable

www.lighttools.com/eggcalcweb/default.htm

Ed Barger
BARGER-BAGLITE
www.barger-baglite.com


I honestly don't know how you could begin to calculate it since there are so many factors. You'd need to account for the type of diffusion, thickness of the material, distance from the source, coverage on the material, loss of light reflected back by the diffusion, refraction by the material and Lord knows what other factors I'm forgetting.

I recall having my viewfinder adjusted by a camera tech once. He'd turn a screw a bit and then check it on a collimator, turn it again and check and so forth. After the fifth or six time asked if it would be easier to calculate the proper amount. He responded that he could probably sit down with a calculator and work out what it would take to throw a ball to someone across a park using various formulas to account for wind speed, gravity, acceleration and the curvature of the Earth, but in the end it was faster to toss the ball a few times until he got it right.

I don't know what application you have in mind but I would say that most industrial designers likely use models and practical experience to find this type of answer.

Mitch Gross
NYC DP


Salut Eric,

The source is either diffused or reflected. It does not really change in the formula.
Also surface is considered perfectly diffusing.
You need to calculate the reflection factor first.

Reflection factor = Reflected light / Incident light

Then

Luminance in candelas per square foot = (Reflection factor * Illuminance in foot candles)/ Pi

this also valid as

Luminance in foot lamberts = Reflection factor * Illuminance in foot candles

It's good to have old schoolbooks close at hand.
(Photographic materials and processes, Stroebel-Compton-Current-Zakia,
Focal Press 1986, p223-226) .

A+

Manu, Munich

Assistant Caméra - Camera Assistant - Kamera Assistent
BVK- European based
Munich +491608036889 - London +447910034443
cml-listmum


Eric

Wow! In all my years of lighting, I can honestly say I have never had to refer to those calculations and the word ponctual doesn't come up in my Webster's dictionary. But then again I never was much of a scientist or a mathematician! I'm sure someone here will have a great answer for you--but I was just wondering why and how you were applying this to set lighting?

Most lighting fixtures, bulbs, gel, and diffusions have manufacture luminosity / transmission specifications (photometrics) that I usually rely upon if I need to calculate what light to use where. But there's no replacement for good old "hands on" experience for me.

Looking forward to some enlightenment!

All the best!

Mike Ambrose
Gaffer
LA Based


"the word ponctual doesn't come up in my Webster's dictionary"

Hi Michael,

Sorry, my English! Ponctual was a Frenchisism, I mean a hard light source, we say punctual source for that in French.

"I was just wondering why and how you were applying this to set lighting? Most lighting fixtures, bulbs, gel, and diffusions have manufacture luminosity/transmission specifications (photometrics) that I usually rely upon if I need to calculate what light to use where. But there's no replacement for good old "hands on" experience for me."

For example, if you are going to use a half grid butterfly with a 6kw and would like to know by advance how much you get at x meters and so on, you need that kind of formulas, or to calculate the diminution of light of a flo out of the distances given in photometrics etc.

I am also basing my work on "hands on" experience since about 15 years but want to go further in theory to become more precise, also by curiosity.

Best regards

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Eric André - Gaffer - Eu/Switzerland based
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Eric,

It sounds as if you basically want to utilise photometrics to give you more accurate exposure calculations for various fixtures. Rather than spending a lot of time going thru multiple equations to give you an end result; you might like to try using something like a photometric calculator. Arri have one available on their website

http://www.arri.com/prod/lighting/index.htm

It will allow to pick a fixture, set the distance, flood/spot, then enter the usual frame rate/shutter/ASA & give you the exposure. There are other calculators available that will give a correction for gel or diffusion/grid etc. You can manually correct for the various gels/diffs as manufacturers like Lee give absorption details for their various products.

Unfortunately as most silks differ from manufacturer you tend to judge these from past experiences as you have already said. There is also the Maxim Fords cinematography program available as a download from the CML website, also useful is David Eubank pCine program for the palm, as this also has an exposure calculator.

For the last five years or so I’ve been using various versions of the above, & with a few quick measurements (Distance/Height of fixture), I have to say its a very comforting feeling arriving on set knowing that you’re never more than a few tenths of a stop from the exposure that you've plan to shoot at for that night exterior or rig that’s been pre-rigged on the stage.

Hope some of this is of some use.

Regards,

James Mc Guire
Gaffer
Dublin


I have read the responses to this question and I am sceptical of using illumination calculators when trying to predict light transmission through dense diffusion. If the diffusion is dense enough doesn't it at some point become the source? Therefore shouldn't measurements of intensity vary depending upon the placement of the diffusion relative to the subject?

If you place a dense piece of diffusion immediately in front of a punctual source (I like that term) and take a meter reading at a given distance, that reading will change as you more the diffusion closer to the meter.

I just did a crude experiment in my garage and got varying readings of just over one stop as I moved a silk from the source to my (fixed position) meter. What my intuition did not tell me was that the readings were brightest with the silk located at the source and immediately adjacent to the meter. The lowest levels were in the middle of the path. Anyone have a calculator for that?

Jim Iacona
DP
San Francisco
www.lightbreak.com


Jim,

I'm curious, were you using the ball diffuser on your incident meter or the disk? If using the ball, I can see why you would get an increase in readings as the silk began to fill in the sides of the diffuser on your meter. I'd be curious to see if the same would happen if you were taking spot readings off a grey card under the same conditions. I'll have to give it a try. I'm assuming you didn't change any properties of the light source during the test (spot / flood for example).

One of the most valuable lessons I ever received in lighting was a day spent with some friends trying to test 24 different lighting ratios on 6 different film stocks. We only had 3 cameras so we were replicating the setups over and over throughout a long but fun day. That was the day I learned you could get a friends hair to smoke with a spotted in 2K! :) Filmstox.com was originally intended to be a display of those tests but I lost heart when the ASC published their stock tests in American Cinematographer a few years back, their model was a lot more attractive then the three of us.

Tons of fun though.

........And Ed,

I'm a huge fan of your light. Great product.

David C. Smith
LA / OC D.P.
www.filmstox.com


*****I'm curious, were you using the ball diffuser on your incident meter or the disk? I'd be curious to see if the same would happen if you were taking spot readings off a grey card under the same conditions.*****

I used the ball diffuser. Good question about spot readings. At some point the inverse square law has to come into play - doesn't it?

*****Check on the www.lighttools.com site there is a program just for this sort of thing. It is called egg calc and it is down loadable*****

Thank you Ed. This seems to be the answer that we were seeking.

Best regards,

Jim Iacona
DP
San Francisco
www.lightbreak.com



 

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