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Photometric Calculator


j.g.b.marion@...
 

Hi CML!

I’m looking for a photometric calculator similar to the 
Arri Photo Calc but wanted to enter my own light power or incident light values in lumens/lux as well as beam angle.

Can anyone recommend a brand agnostic calculator/phone app that will do the trick? Something also with exposure values like the Arri calc would be fantastic!

I’m hoping to be able to estimate the light power required for a given scene before setting up. My guesstimation tollerance is not yet quite down to 1x ballpark. 

I’ve put more info into a DP Review Forum Post if required.

Cheers
Bruno


Axel Gimenez
 

Hi Bruno,

That photo calc is just a basic calc - nothing special. As I assume you know, the light fall-off is calculated with the inverse square law. The Arri calc just uses that formula to calculate the photometric. However, this formula is flawed - overly simplistic - and will not give you actual results, only rough ballpark. The reason why is that the formula calculates light fall off for a point light. And in the real world that is theoretical. There is no such a point-light. Even a globe in a lamp is a small area light if you think about it. Thus the calculation falls apart.

Thus, to rough in calcs, just find an inverse square law formula on Google and do the math. But know that your results will vary wildly based on what light sources you're trying to calculate. The more area light your source is the worse the formula will work for you. 

Real calculations for area lights are very complicated, FYI and beyond most of us.

Hope that helps somewhat.

--
Axel Gimenez
Commercial Auteur
NYC
https://axelgimenez.com/


matt.choules@...
 

Hi Bruno,

I believe pCam (iOS app) allows for this functionality.
I tend to use the app as a Depth of Field Calculator and to show DPs / Directors comparative shot sizes for different formats but I'm told it has a lot of functionality for more than just us assistants.

Hope this helps,

Matt Choules (GBCT)
1st AC
London, UK.


denault.jim@...
 

Indeed. pCam has a photometric tool called "Scene Illumination." You need to know the peak beam intensity in candelas, which can usually be found in the manufacturers docs. This tool works for ellipsoidal, fresnel and open face sources, but once you add diffusion or use a diffuse source like Kino-Flos or Skypanels, the physics get a little more complicated. The best bet for those is the manufacturers data sheets. The numbers you get from pCam are kind of an idealized version. If you think the Source Four with five years of truck dirt on the lenses is going to perform the way the brand new one did for ETC's photometric tests, you will be disappointed.

If you are dealing with multiple sources, or want to be able to previsualize angles, one reasonably priced tool is LX Beams. http://lx.claudeheintzdesign.com/lxbeams.html. There is a discount for IATSE members.

Vectorworks is even more powerful. You can use the manufacturer's .ies file to create very accurate renderings of the beam shape and intensity of almost any light. It is expensive and has a bit of a long learning curve, but many lighting console programmers have the software and could probably help you out.

Jim Denault, ASC
DoP
Pasadena, CA


Ben Mcconnel
 

Hi Bruno,

(Caveat; My knowledge is very limited as I only take an interest in this area and community), that said, I came across this app in the AppStore that on the surface might be worth checking out for your needs.  


Be good to hear what you think?

Cheers
Ben

On 13 Feb 2020, 03:38 +0000, denault.jim@..., wrote:
Indeed. pCam has a photometric tool called "Scene Illumination." You need to know the peak beam intensity in candelas, which can usually be found in the manufacturers docs. This tool works for ellipsoidal, fresnel and open face sources, but once you add diffusion or use a diffuse source like Kino-Flos or Skypanels, the physics get a little more complicated. The best bet for those is the manufacturers data sheets. The numbers you get from pCam are kind of an idealized version. If you think the Source Four with five years of truck dirt on the lenses is going to perform the way the brand new one did for ETC's photometric tests, you will be disappointed.

If you are dealing with multiple sources, or want to be able to previsualize angles, one reasonably priced tool is LX Beams. http://lx.claudeheintzdesign.com/lxbeams.html. There is a discount for IATSE members.

Vectorworks is even more powerful. You can use the manufacturer's .ies file to create very accurate renderings of the beam shape and intensity of almost any light. It is expensive and has a bit of a long learning curve, but many lighting console programmers have the software and could probably help you out.

Jim Denault, ASC
DoP
Pasadena, CA