[Radiance-general] RE: gendaylit

Mon, 17 Jun 2002 09:33:26 +1000

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Hi Martin,<br>
<br>
You are right that the irradiance data fed to gensky should be treated
with caution.&nbsp; The reason for this, however, is Radiance's use of a
constant luminous efficacy of 179 lm/W.&nbsp; If you want accurate
interior daylight levels using gensky, you should use illuminance data if
you have it, rather than irradiance data.&nbsp; simply take the
illuminance data and divide it by 179 lm/W to get 'pseudo-irradiances' to
feed to gensky.&nbsp; it is highly likely that this will not equal
measured irradiance data as realistic sky luminous efficacies are between
90 &amp; 150 lm/W.&nbsp; if you have irradiance data only, that should be
first multiplied by a reasonable luminous efficacy (refer for example,
Lam, J.C. and Li, D.H.W. (1996), <i>Luminous Efficacy of Daylight under
Different Sky Conditions, </i>Energy Conservation and Management<i>,</i>
37 (12), 1703-1711) to get illuminance data.&nbsp; then follow the same
process to get your 'pseudo-irradiances' for input to gensky.<br>
<br>
Regards,<br>
Phil Greenup.<br>
<br>
<br>
<blockquote type=cite cite>The important difference between gendaylit and
gensky is the input of irradiance data. If those data given to gensky -B
-R are integrated over the full spectrum, not just the visible range,
interior daylight levels will double! Gensky wants W/m^2 in the visible
range only, whereas gendaylit uses the actual irradiance data from
weather stations, which are approx. twice as high. <br>
&nbsp;<br>
Martin</blockquote></html>