[Radiance-general] Sky and Solar efficacy

[John Mardaljevic]

Kouichi (?),

> When I use Brightdata with measured data , I use this equation;
> 
> ( sky and solar )radiance=  luminance/179 
> 
> solar luminance is calculated from  measured direct normal illuminance.
> sky luminance is measured data.
> 
> Is this correct methods?
> I think efficacy is not always 179.
> But!!it seems to be unified into 179 in the RADIANCE system.

The 179 lm/W value is Radiance's own 'internal' luminous efficacy.
It should not be confused with actual daylighting efficacies
which cover the range 50 to 150 lm/W (these are extremes, typical
values are around 100 to 120 lm/W).

> And ,one more question.
> When   accurate illuminance value in the room is required, should  I take 
> both ground plane and ground dome into consideration? Or,ground plane 
> around the model only?

A ground plane will usually be needed for high accuracy work.  This is
becuase a (correctly specified) ground glow cannot account for the shading
of the ground plane by the building (see Fig 6.8 in Rendering with Radiance).
No harm in having a ground glow as well as a ground plane.

Both of your questions are addressed (with examples given) in the
newly reprinted Rendering with Radiance book.  See earlier threads
for availability.

-John

-----------------------------------------------
Dr. John Mardaljevic                     
Senior Research Fellow
Institute of Energy and Sustainable Development
De Montfort University
The Gateway
Leicester
LE1 9BH, UK
+44 (0) 116 257 7972   
+44 (0) 116 257 7981 (fax)

jm at dmu.ac.uk   
http://www.iesd.dmu.ac.uk/~jm