If you are doing lighting analysis with 3ds Max Design using photometric files there are a few things that can affect the results of your simulations to keep in mind. Its not because someone gave you a photometric file (typically an *.ies file) that this is an ”accurate” one.
Always verify your data otherwise your project can go wrong – here are a couple advices on the topic:
IES file can contain wrong information: double check them!
I have once seen a manufacturer using the wrong values in their IES files. I had to contact them and demonstrate that their data was wrong. It turned out that one of their engineer made a mistake with their database system where North American IES files where published using European measurements. That basically means that errors can come from anywhere in the process. Double check your work!
In the ideal scenario, you need to get a physical sample of the luminaire in your office, measure intensities with an illuminance meter and compare with the IES data provided to you in a simple test scene.
Do the numbers match up? You are good to go. They don’t match up? Keep reading..
Check the 3ds Max scene units:
This is a typical trap: wrong units, wrong light intensities reported on light meters!
3ds Max and Revit has bugs too!
There is a issue with the IES file reader that can affect you in rare occasions. The bug has to do with a certain “feature” of the IES file format that some manufacturers rely on. Let me explain:
The IES file format is divided in two main sections: a header section and a data section:
In the header section, there is a special ”bit” that instruct the software to either use the “raw” data or apply a multiplier to it. For example, a manufacturer might have measured a luminaire only once with a 100W lamp in it. Knowing that the same luminaire with a 200W lamp** would emit twice as much light, the manufacturer could publish a different IES file based on the same measurements and simply state that the “raw” data should be multiplied by a factor of 2.0 by changing this special “bit” in the file as illustrated:
For some reason, 3ds Max and Revit are ignoring this multiplier and always read the “raw” data from the IES files, resulting in incorrect simulations. The work around is to use the built-in multiplier (dimmer) feature of the photometric lights:
Note about linear and area lights:
In many occurences, I am asked to perform lighting analysis reports in spaces designed with cove lights. Unfortunately, this is still not possible to do precisely - and this problem has not been solved by the industry yet!
There are currently two major problems with linear and area lights in lighting simulation softwares:
- First, the IESNA standards for photometric measurements assumes that luminaires are measured from a certain distance, making any calculated points inside this distance mathematically inaccurate. If you search for “Near Field Photometry” or “Far Field Photometry”, you will find plently of information on the subject.
- Second, 3ds max (and Revit btw, as both share the same code for rendering) still has issues to deal with linear and area lights calculation making them unreliable photometric-wise.
As I write these lines, cove lighting situations cannot be mathematically correct with photometric files in 3ds Max: you will have to do some guess work when working with theses scenarios.
**I know, a 200W lamp does not emit twice as much light than a 100W lamp that was only for keeping the example simple.