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Jonathan O'Neill

Is there a general photometrics calculator?

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Hi, I'm trying to work out a compared database of photometrics for bebee lights, wendy, maxi-brutes, 18k's etc etc.

Is there a generic photometrics calculator / app / website, where you can key in the company supplied photometrics data of each light, then chose to see the reading at your desired distance?

Thanks a lot

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Autodesk makes a photometrics application that does just that, but it's meant for architects/engineers to design the lighting in the planning phase of building construction. Actually, I think there's several applications including Autolux and Revit and I'm not sure which one would be best in terms of cost/ease of use/customization for film lighting. 

My wife is a lighting engineer and does exactly these kinds of calculations, but for construction. Let me ask her if it's hackable and I'll get back to you.

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Not a standalone library, but the Illumination Engineering Society does have a photometric .IES file extension named for them. I've used a free program called IESViewer for these files. .LDT are also used, but I haven't played around with those at all. ARRI provides both file types for their current fixtures, and I believe ETC does as well. Not the prettiest UI, but it works.

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Posted (edited)

I just remembered, I also made this inverse-square law calculator: https://jsfiddle.net/polymetric/pent71uh/

Here's how you use it:

Grab an incident reading of a light at a specific distance and enter in the incident reading and distance in whatever unit you want (footcandles or lux, feet or meters), then you have two options:

1. Calculate the intensity from a particular distance

2. Calculate the distance you need for a particular intensity

It's an extremely simply calculator. It doesn't account for the effective difference in distance that you get from fresnels or other fixtures with lenses in them, but then again, I don't think Arri's calculator does either, or maybe it's just not very much. I tested this by putting values from Arri's calculator into mine and then solving for the same distance, and they gave me very similar results. I'll test it on actual lights when I get a chance. Also, it should pretty much work perfectly on open faces and obviously point sources like bare bulbs.

I probably could have done this in a spreadsheet. Actually I definitely could have and should have, but I wanted to mess around with JavaScript, so there you go.

Edited by Brian E. Rutan

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I find a good way of guessing lux or fc based on a distance is using the root of two scale, which are basically the lens f stop numbers (1, 1.4, 2, 2.8, 4, 5.6 etcetera). You lose half your intensity (or a stop) for each of these numbers, whether its feet or meters.

So if you know you have 3000 lux at around 2.8m you will have 1500 lux at 4m and 187 lux at 11m. Same goes for feet and fc, so footcandles would be halved between say 11 and 16 feet or quartered between 5.6 and 11 feet

Around 250 lux or 25fc will give you a T2.8 @800 iso which can help you do a rough estimates on what size lights you need for a given space, if you only have basic data for a light.




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