# actual radiance value

**URL:** <https://discourse.radiance-online.org/t/actual-radiance-value/3103>\
**Category:** radiance-general\
**Created:** [July 16, 2011, 4:16am UTC](https://discourse.radiance-online.org/t/actual-radiance-value/3103 "2011-07-16T04:16:17Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Hu\_Jia](https://discourse.radiance-online.org/letter_avatar_proxy/v4/letter/h/34f0e0/32.png) [@Hu\_Jia](https://discourse.radiance-online.org/u/Hu_Jia)\
**Post date:** [July 16, 2011, 4:16am UTC](https://discourse.radiance-online.org/t/actual-radiance-value/3103/1 "2011-07-16T04:16:17Z")

</div>

Hello experts:

Is the following understanding of "radiance" in the Radiance program  
correct?

If I input irradiance (W/m2) values in Gendaylit, the inputted irrdadiance  
values will be converted to zenith luminance (cd/m2) according to Perez's  
model (1990), then luminance will be converted to "radiance" (W/m2\*sr) value  
in the visible range by dividing 179 (I did not see "179" in the  
gendaylit.c).

So, the irradiance value calculated by the Radiance program should be  
different from irradiance value in weather file even though the actual  
irradiance values in the weather file are used as an input of  
gendaylit (assuming Perez's model is perfect). But the illuminance /  
luminance values calculated by Radiance should be accurate in theory (also  
assuming Perez's model is accurate).

PS, my school library does not have permission to access the full text of  
the Perez's paper (1993) referred to by Gendaylit program (url:  
[http://www.sciencedirect.com/science/article/pii/0038092X9390017I](http://www.sciencedirect.com/science/article/pii/0038092X9390017I)). If  
allowable, could someone send me this paper?

Thank you,

Jia

---

<div class="post-metadata">

**Author:** ![Hu\_Jia](https://discourse.radiance-online.org/letter_avatar_proxy/v4/letter/h/34f0e0/32.png) [@Hu\_Jia](https://discourse.radiance-online.org/u/Hu_Jia)\
**Post date:** [July 16, 2011, 4:22am UTC](https://discourse.radiance-online.org/t/actual-radiance-value/3103/2 "2011-07-16T04:22:55Z")

</div>

Sorry to interrupt you, I can access the paper now. Thanks.

> **···**
>
> On Sat, Jul 16, 2011 at 12:16 AM, Jia Hu \<hujia06@gmail.com\> wrote:
> 
> > Hello experts:
> > 
> > Is the following understanding of "radiance" in the Radiance program  
> > correct?
> > 
> > If I input irradiance (W/m2) values in Gendaylit, the inputted irrdadiance  
> > values will be converted to zenith luminance (cd/m2) according to Perez's  
> > model (1990), then luminance will be converted to "radiance" (W/m2\*sr) value  
> > in the visible range by dividing 179 (I did not see "179" in the  
> > gendaylit.c).
> > 
> > So, the irradiance value calculated by the Radiance program should be  
> > different from irradiance value in weather file even though the actual  
> > irradiance values in the weather file are used as an input of  
> > gendaylit (assuming Perez's model is perfect). But the illuminance /  
> > luminance values calculated by Radiance should be accurate in theory (also  
> > assuming Perez's model is accurate).
> > 
> > PS, my school library does not have permission to access the full text of  
> > the Perez's paper (1993) referred to by Gendaylit program (url:  
> > [http://www.sciencedirect.com/science/article/pii/0038092X9390017I\](http://www.sciencedirect.com/science/article/pii/0038092X9390017I%5C)). If  
> > allowable, could someone send me this paper?
> > 
> > Thank you,
> > 
> > Jia