[Radiance-general] Sky and Solar efficacy

[Jan Wienold]

Hi all,

in my opinion it is a question of the used spectrum. RADIANCE itself 
calculates completely within the visible spectrum. So the internal luminous
 efficacy of 179 lm/W is from visible to visible spectrum.

The commonly used  luminous efficacy is from the full spectrum (as e.g. 
measured by a pyranometer) to the visible. This efficacy strongly depend 
on the actual sky state (Blue sky about 90 lm/W, overcast sky about 130 
lm/W).
If you use "gendaylit", the perez-sky luminous efficacy model is used to 
"transfer" full-spectrum data into the visible range and gives good 
results for the most sky situations.

Jan



John Mardaljevic wrote:

>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
>
>
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>
>  
>

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  | \            Jan Wienold
                 Fraunhofer Institute for Solar Energy Systems
                 SOLAR BUILDING DESIGN GROUP - DAYLIGHTING
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