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Light Output Calculations

Published : 13th November 2003


Bonjour

Has anyone ever, or know how to, calculate the light output of a Dino-Light, Mini brute or things of the sort?

A client is trying to calculate what light output he can expect from these large sources at a given distance using either flood, spot or intermediate bulbs.

Is there a way to calculate this scientifically? Knowing that a single 1000W Par64 bulb will give us "X" amount of lumens (depending on front lens) at "Y" meters, how would you calculate accurately the sum of a 12, 16 or 24 bulb fixture?

I have a feeling it must be a combination of "X" times "amount of bulbs" by "size of fixture Z" divided by "Y", with a couple of square roots and logs thrown in.

Any help would be appreciated.

Best Regards

Danys BRUYERE
Dir. Operations
Groupe TSF / Paris



>Has anyone ever, or know how to, calculate the light output of a Dino->Light, Mini brute or things of the sort?

Danys -

For Dino's I found photometric data at

http://www.ultralightmfgco.com/sales_light_detail_1.php?rec_num=17&page

(Merely did a Google search for Dino light and found that).

The company with that info is

http://www.ultralightmfgco.com - which I was also very surprised to see that they offered Wendy lights in the US (company is based in Las Vegas) - very nice information.

Hopefully that helps.

I have no affiliation whatsoever with Ultra Light or Google.

All the best,

Jay Holben
Director of Photography
Los Angeles, CA
www.imaging-the-future.com



Jay Holben said:

>The company with that info is http://www.ultralightmfgco.com - which I >was also very surprised to see that they offered Wendy lights in the US >(company is based in Las Vegas) - very nice information.

Well, I'm not into bashing in public but just be warned - I own a few Ultralight fixtures and well . . . they're not of the best manufacture.

I'm constantly having to fix and rebuild their lights. I hope the info Jay pointed out is valuable - but anyone lured by their prices ought to think twice.

Roderick Stevens
Az. D.P.
www.cinema-vista.com



Hello Anders :

You replied to Edwin Myers :

If you have a light that gives 10 footcandles and add another light that gives 10 fc’s, you will have 20 fc’s. Footcandles are a straightforward unit of measurement.

I disagree. Edwin is correct, basically, considering to the distance of the lights from the subject. The inverse square law dictates that the energy twice as far from the source is spread over four times the area, or one fourth the intensity. Footcandles are NOT cumulative per fixture.

The farther the instruments from the subject, the less cumulative they are.

Best Regards,

Nick Mueller
Director of Photography
Washington, D.C.



This is true, for point sources :

>energy twice as far from the source is spread over four times the area, or >one fourth the intensity.

This is not true :

>Footcandles are NOT cumulative per fixture. The farther the instruments >from the subject, the less cumulative they are.

All other things being kept the same, at a set distance between a fixture and the object being measured, doubling the number of PAR bulbs will double the light, whether you measure it in footcandles or convert it to stops. This has nothing to do with the inverse square law. The inverse square law will help describe what the fall-off will be as you vary the distance between the light(s) and the subject.

Lots of factors will affect what you will read in the field as you add PAR bulbs on a Dino light, however. Dino's do not point all their bulbs at the same point in space. Which way around you have the bulbs aligned will radically change your readings at some distances, as will how you align the vertical banks of bulbs.

For instance, if you are relatively close to the fixture, and if you are using very narrow bulbs, and if they are aligned with their filaments horizontal, you will never get the hotspots of a given bank to overlap, so depending which bulbs you turn on or off, the stop change will NOT follow the mathematical model.

On the other hand, if you are a couple of hundred feet away, and if you are reading and averaging over a small area (to smooth out the alignment issues a little bit) you will find that doubling the number of bulbs that is lit will roughly double the number of footcandles that you are getting at your end of the world, and that will roughly add one stop to your exposure.

Mark Weingartner
LA based



Hello Mark,

Your illustration with the Dino instrument is very good. The photometrics do change quite a bit depending on the globes installed. Concerning how inaccurate I am about footcandles not being cumulative and your statement that, “. . .you will find that doubling the number of bulbs that is lit will roughly double the number of footcandles.,” I guess it depends on how ‘rough’ a calculation your production values will accept.

The quickest, simplest evidence I can offer is to invite anyone to examine the photometric charts for the Molefay instruments, available on the Mole-Richardson website. There one will find that, depending on the globe used, the footcandle measurements, for example, of a 6-lamp unit do not double if one were to employ a 12-light unit. In fairness, some globes, at a given distance, double exactly. In other instances doubling the wattage fell 100 – 200 footcandles short.

In higher wattage instruments, the disparity can be greater. On an outside production, sometimes the differences can seem minimal. On a soundstage, however, they can be much more critical.

Best regards,

Nick Mueller
Director of Photography
Washington, D.C.



>I guess it depends on how 'rough' a calculation your production values >will accept.

I do a lot of visual effects. I live in the real world. I do a lot of calculations, some rough, some to many decimal places. Then I measure the light and adjust. I measure the light very accurately. When you are shooting miniatures that take a long time to build and a very short time to blow up, you measure very accurately.

I will say that if you have one Dino and you turn half the globes off, you will lose more than a stop, a stop, or less than a stop, depending on where your meter is and how the globes are aimed. If you have 6 Dino's clustered together and you turn half the globes off (in an even pattern, of course) you will darned near lose one stop.

The physics is very straightforward.

The application of it requires understanding :

1. Where your "real-life" situation differs from the theoretical situation
2. How to adjust your estimates based on those differences.

Or, when in doubt, order more stuff.

Mark Weingartner
LA based



>Your illustration with the Dino instrument is very good. The photometrics >do change quite a bit depending on the globes installed.

Hi guys,

My initial point was that footcandles are a straightforward unit of measurement. Double the footcandles and you will double the light. My example was oversimplified out of necessity, different fixtures at different distances will read differently in different places (Mark and Nick both illustrated this very well). This discrepancy is due to factors that have no relationship to the additive qualities of footcandle units, but rather physical and environmental factors that redistribute or disseminate those footcandles (like beam spread and overlap, etc.).

If you measure 50 fc, then add another 50 fc, you will indeed have 100 fc, which is twice the amount of light (one stop) that you had with 50 fc.

Marks practical summation is a good one :

The physics is very straightforward. The application of it requires understanding:

1/. Where your "real-life" situation differs from the theoretical situation
2/. How to adjust your estimates based on those differences.

Best Regards,

Anders 'one foot in the candle' Uhl
cinematographer
ICG, New York



Hello Anders and Mark,

Gentlemen. . .I believe we are in absolutely agreement.

Mark, I would love to see your staging on the miniatures. Sounds like my kind of toy store.

Anders, I never meant that if you have 10fc and add another 10fc, by whatever means available, that you don't achieve 20fc. You certainly do. I have enjoy the discussion. Thanks to you both.

Best regards,

Nick Mueller
Director of Photography
Washington, D.C.



John N. Mueller (Nick, please) wrote :

>Gentlemen, I believe we are in absolutely agreement.

Right! Wouldn't think such a cordial discourse could take place these days. Too bad we aren't heads of state. . .

>Anders, I never meant that if you have 10fc and add another 10fc, by >whatever means available, that you don't achieve 20fc.

I realize that, and so, was just trying to clarify. Thank you.

Best Regards,

Anders Uhl
Cinematographer
ICG, New York



1 + 1 = 2

Add two sources from the same position and you get twice the number of footcandles from that source. This part is simple math. Inverse square law applies to light fall off (or gain) with changing distance. That will help you predict how much change you can expect if you move closer or further.

One cautionary note :

Manufacturer photometrics are almost always exaggerated. Take their figures and reduce by 25-30%. Also remember that the light output is not even across the entire field.

The "beam angle" intensity marks the area which is at least 50% of the peak value. The "field angle" is the area which is at least 10% of the peak value. In other words, the light will feather out from the hottest spot. This means that when you calculate total aggregate light output, you have to consider these feathering zones as you build a coverage area of overlapping beams of light from individual lamps.

It is possible to accurately predict the amount of light you will get over an area, but only if you do a point-by-point calculation as you add more individual lamps.

Favourite Hint for Working with PARS :

When building up area coverage with multiple PAR 64 fixtures, leave a donut hole in the middle. The middle always builds up from the multiple overlapping sources and has a tendency to get hotter than otherwise expected.

I hope that helps.

Bruce Aleksander
Lighting Designer +
ABC/Disney
Houston



Bruce wrote :

>The "beam angle" intensity marks the area which is at least 50% of the >peak value.

Very important point, Bruce. Though certainly in the 'small print’.

Mole-Richardson does have that warning asterisked at the bottom of the page for the molefays. Good hint on the PAR 64 'donut.' How big of an area would one be lighting to consider taking advantage of the overlap build up?

Nick Mueller
Director of Photography
Washington, D.C.



Nick Mueller wrote :

>"Good hint on the PAR 64 'donut.' How big of an area would one be >lighting to consider taking advantage of the overlap build up?

The "donut" is the whole that you plan in when you are focusing in a set of PAR lights to allow for the tendency of the centre getting hotter than the edges. Concerning how big an area is involved to require the donut, I would think that anything involving more than 4 lamps. This experience is based on lighting telethons and large concert performances.

Here's an attempt at a diagram the technique with the numbers (1 - 6) representing the "focus points" on the stage for the lights (1a-6a & 1b-6b). Let's say the points are about 6' apart at the edges and about 12' apart in the
middle between 3 & 4.

----1----2------3----------4------5----6----

1a2a3a4a5a6a--------------------1b2b3b4b5b6b

If I had two "6 bars" of PARs that were split in the middle, I would focus them on each of the points across the stage (lamp 1 to focus point 1, lamp 2 to focus point 2, etc. from each side) with two lamps hitting each of the 6 focus points. The middle will become hotter than you might expect due to the beam edge overlap. The "donut" hole compensates for this tendency and helps maintain an even wash of light across the entire stage. As you build several rows of focus points for a deeper stage you get a more pronounced hole in the middle. That's the "Aleksander Donut Hole Focusing" approach to lighting large areas with multiple PARs.

Bruce Aleksander
Lighting Designer +
ABC/Disney
Houston, Texas