Who is this for: Architects, Engineers, Lighting designers
Author: David Scheer - Product Owner, Architect - BPA, Autodesk, Inc.
It's been a while since our last update about the illuminance feature from Autodesk Rendering for Revit, Vasari and AutoCAD, but we've been working hard on updates and upgrades to make it more useful.
The latest upgrade is a provisional workaround for the automatic scale applied to illuminance renderings. The previous incarnation set the scale to range from the minimum illuminance in the scene to the 95th percentile highest illuminance in the scene with 10 proportional subdivisions. This works alright for lighting design, where illuminance levels are designed to range between 0 and ~1000 to 2000 lux, but it doesn't do so well in scenes with direct sunlight.
Remember that Illuminance renderings are not meant for solar load analysis using exterior renderings. Illuminance is only applicable for visual perception analysis, and is usually only used for interior scenes.
Humans perceive light intensity in a complex way. We can function and differentiate differences between very low levels of light (hundredths of lux), but we can still function even when light intensity reaches 10 million times (10,000,000x) that intensity as our eyes dilate and and shrink. Check out this article for more info.
Direct sunlight is 3 to 5 orders of magnitude (~103 to 105 or 1000x to 100,000x) more powerful than what humans need to function. Because of this, if the scale of the illuminance rendering spans the full range in a scene with direct sunlight, the levels we are interested in, between 0 and 2000 lux, all fall within one step of the color scale and are undifferentiated. There's no way to tell if we are meeting a 200 lux target or a 300 lux target.
So for now we've changed the scale to logarithmic so that lower range is more differentiated. In the example above, you can see that now it's possible to distinguish 7 color steps in the range between 0 and 2000 lux. A logarithmic scale also better represents how we perceive levels of brightness, as discussed in this article.
Tell us what is most useful for your work
We know the log scale is not an ideal solution to the problem. In the long run users need to set their own scale in order to do comparisons and to have flexibility for a variety of analyses. In the meantime, what scale would you find most useful? Full range? Logarithmic? What if it were locked at 0-2000 lux? Do you need to differentiate between intensities in the upper ranges? What fixed range do you use most of the time?