Diagram: Useful Daylight Illuminance

Useful Daylight Illuminance

How useful is the light coming into the space?

The following was posted on Max Pierson's Website.

Useful Daylight Illuminance (UDI): Another variation on Daylight Autonomy, UDI attempts to penalize for direct sunlight that falls into the space, on the theory that people will find this glary and distracting.  UDI is a set of three numbers for every point in the space, the percentage of time that a point was below a minimum threshold, between a useful minimum and maximum value, and above a maximum value, which could result in glare or thermal discomfort.  Often 10fc is used as the lower bound of useful illuminance, and 250fc on the upper bound.


  • Most of the advantages of DA, but an additional dimension for glare and thermal discomfort.
  • Good for comparing the performance of two design variations.


Since you’re generating three data points for every spatial point, it’s difficult to assess performance at a glance or communicate how well the space performs to someone who isn’t a specialist.



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