When Revit 2019 came out, it included an enhancement that corrected the Coefficient of Utilization Calculation for Absolute Photometry. However, there are still some things to be aware of, which I will cover using Revit 2024 in today's post.
While the previous issues with absolute photometry (i.e. LED light sources) has been addressed, there are still some calculation mysteries to be aware of. Let's explore this...
Setup
Four square spaces (25’ x 25’) with 8’ ceilings. Identical apart from the luminaire type (tip: click to enlarge images below).
- “Absolute” space has 9 instances of 2x2 troffers. Uses the published photometry of Cooper’s “Encounter” product.
- “Relative” space uses the same fixtures, but with the “lamp lumens” in the IES file changed from -1 to 2909 (to match the calculated luminaire lumens).
- “Relative-Bogus Lamp Lumens” space uses the same fixtures but with the lamp lumens in the IES file changed to 6,000 (arbitrary value, roughly 2x actual luminaire lumens).
- “Mixed” One instance of the Absolute fixture, and one instance of the Relative fixture. Used to evaluate the Revit rendering. More on this one later in the post.
Coefficient of Utilization
Appears to be (roughly) correct - see the schedule below.
- In theory, the absolute and relative fixtures should have identical CU values. But in reality, they vary a little bit in Revit. Relative CU = 0.862197 vs. Absolute CU = 0.863384. Close enough to be a rounding error.
- In releases prior to Revit 2019, the Absolute CU would have been a large negative number.
Remaining Calculation Mysteries
Here are a few things to keep in mind.
- Calculation of the “Average Estimated Illumination” space parameter depends on what the “Initial Intensity” parameter is set to on the Lighting Fixture family. This is nonsensical for families with an IES file specified. All information necessary for the Zonal Cavity method is contained in the IES file. Requiring the user to manually specify an additional parameter is unnecessary, and will lead to confusion/ mistakes.
- It is ambiguous whether the user is meant to specify luminaire lumens or lamp lumens for the Revit “Initial Intensity” parameter. I’ve never found any documentation that says which one to choose, and the results depend heavily on making the correct choice.
- Revit reports the average illuminance for the “Relative-Bogus Lamp Lumens case” as roughly 50% of what it estimates for the other 2 cases. This is illogical because the only difference between the “Relative” and “Relative-Bogus” cases is that the IES file for “Relative-Bogus” specifies twice as many lamp lumens. So, if there is going to be a difference, you would expect that case to be higher, not lower. All 3 cases should give the same average illuminance.
Renderings
- The fixture on the left uses Relative Photometry. The one on the right uses Absolute Photometry. They are otherwise identical, the same distances from the wall, etc. But the Revit rendering engine shows the brightness to be different.
ElumTools
By way of contrast, here is what the graphic results look like in ElumTools, Lighting Analysts add-in for Revit.
Zonal Cavity Method
It is worth pointing out that Revit uses the Zonal Cavity Method to calculate the average illuminance.- This method is really only accurate for rectangular-shaped rooms.
- A radiosity calculation, using ElumTools or similar, is required for accurate results.
Conclusion
It is great that Revit 2019 (and later) has corrected the problem related to absolute photometry, seeing as the use of LEDs has become so prominent in our industry. Of course, there are a few challenges to be aware of, which I am sure will be dealt with in due time!For BIM Chapters updates, follow @DanStine_MN on Twitter or connect on LinkedIn
Lighting design professionals: check out my Revit & ElumTools training https://bit.ly/3NJjhCV
