Obtaining accurate illuminance values in iRay (RTX Accurate)

Hi, I’m an architectural lighting designer, and assessing workability of Omniverse Create as an accurate renderer for this use. My daily driver is Bloom Unit (iRay in SketchUp) which I still have access to…

In testing the iRay Illuminance canvas/lpe in create with a simple black test room and a single disc light with various IES photometric files for light shaping, I’m finding I have to set the intensity to arbitrary values which seem to have no rhyme or reason.

Some intensity values I’ve had to set to get correct illuminance values for various different luminaire IES files:

  • Intensity 7,277 (0.72X luminaire lumens, or 0.39X actual peak candelas, or 0.87X peak candela raw value in IES file)

  • Intensity 1,971 (3.3X luminaire lumens, or 0.05X actual peak candelas, or 0.05X peak candela raw value in IES file)

  • Intensity 4,619 (0.46X luminaire lumens, or 1.33X actual peak candelas, or 1.33X peak candela raw value in IES file)

  • Intensity 307.9 (0.42X/0.59X lamp/luminaire lumens, or 0.53X actual peak candelas, or 0.53X peak candela raw value in IES file).

All of the IES files from the examples above are well-formed files which give correct values in other rendering and calculation software. They are all (in this case) radially symmetrical with peak intensity occurring at nadir. Beam angles vary from super-narrow 5deg to about 120deg in the various examples.

The above empirically-derived intensity values are based on having light colour temperature turned off. Turning on colour temperature and setting at different values also yields slightly different values, e.g:

  • Colour temp off = 2,528 lux
  • At 6500K = 2,590 lux (+2%)
  • At 3000K = 2435 lux (-3%)

Light settings used to obtain these results:

  • Transform rotation z = 90 (so that the light points down)
  • Exposure 0.0
  • Scale 1.0 (alll axes)
  • Disc light radius 0.5 (5mm)
  • Normalize power = on (otherwise output values vary according to disc radius)
  • All multipliers set to 1.0
  • Cone angle left at default 180deg (setting to 360deg doesn’t change results)
  • All other shaping adjustments set to 0
  • Shaping normalisation = off.

Is there something buggy here, or is there a way to determine the correct intensity value to correspond with the IES file (and thus the real-world luminaire)? I would have expected the correct behaviour to be either:

  • Intensity value is the luminaire lumen output (ideally defaults to value calculated from the IES file on loading); or
  • Intensity value is multiplier of IES file values (default value 1.0);
  • (Non-preferred) actual peak luminous intensity value (candelas) irrespective of the angle where this peak occurs.

As an aside, “intensity” as a term is really problematic for lighting designers. Intensity should specifically mean luminous intensity (candelas) in a specific direction. Total output is “flux” or “output” (lumens).

Any advice would be much appreciated!

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