Friday, April 20, 2018

Lighting in Revit 2019 + Lightfair 2018!

Next month I have the privilege of speaking at Lightfair International 2018 in Chicago. #LFI2018
Lightfair is "The world's largest annual architectural and commercial lighting trade show and conference."
More on my specific session in a later post!

Speaking of lighting, Revit 2019 just came out and includes an enhancement related to lighting. Specifically, they corrected the Coefficient of Utilization Calculation for Absolute Photometry.

At a high level, it does seem that they have fixed the major problems with absolute photometry. But there are still some significant items to be aware of. Read on to learn more...


4 square spaces (25’ x25’) with 8’ ceilings.  Identical apart from the luminaire type.
  • 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 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 previous Revit releases, 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 silly 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 they only difference between the “Relative” and “Relative-Bogus” case 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.


Now lets compare Relative and Absolute lighting fixtures side by side in a Revit-generated rendering.

  • The fixture on the left uses Relative Photometry.  The one on the right uses Absolute Photometry.  They are otherwise identical, same distances from the wall, etc.  But the Revit rendering engine shows the brightness to be different.


By way of contract, here is what the graphic results look like in Lighting Analysts add-in for Revit - ElumTools.

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.
  • Check out this link for an overview of this calculation method: click here.


It is great that Revit 2019 has corrected the problem related to absolute photometry, seeing as the use of LEDs id becoming so prominent in our industry. This enhancement will be very helpful to lighting designers using this design platform. Of course, there are a few challenges to be aware of, which I am sure will be dealt with in due time!

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