[Radiance-general] R: Radiance-general Digest, Vol 84, Issue 6

[caria.roberto at tiscali.it]

Ciao Thomas,
thank you for your reply.

>So you really want to know internal illuminance values?for a given
>scene for the whole year? (I hope you know?you can get the external
>illuminance from the epw file.)

R: I know I can get the external illuminace from epw file but no all 
locations have these data values (if you open ITA_Cagliari Elmas you 
can see it is that)

I think if I consider the Illuminance values taken from epw file I 
can use gensky and insert them as -B and -R parameters (dividing them 
to 179, before) and add +s when I have both values and -c when I have 
only the diffuse Illuminance. (Can you confirm it?)

   

>My summary of your problem:

>Your idea is to use Daysim (or Radiance) with a climat data file to
>calculate the internal illuminance values and plug these into E+ or
>whatever. Your first step is to calculate external values in an 
empty
>scene to "validate" your sky model. Doing this in Daysim and 
Radiance
>(with gendaylit) you got two sets of results with significant
>differences.

I did it as you write.

>I hope you did have a -I option in there some where. You also only
need to increase your "-ab" to 1 to get the "diffuse" part.

I used -I option. 

>Now the interesting bit:
>
>If you look at your results or the global horizontal values you 
will
>see that the Daysim values are roughly 25%-30% higher than those
>calculated in Radiance. However, they are consistently higher 
(which
>is good). I would expect that the difference is a result of the
>approach the two applications have:

Why is good to get higher values? 


>1) Daysim calculates a sky component and just maps the sky onto 
this
>distribution to get a particular result.
>
>2) Radiance used the sky directly in it's raytracing calculation.
>
>Both sets of results are bound to be different from the actually
>observed illuminance on the ground. You can extract the illuminance
>values from the EPW file and compare them to the calculated 
results.
>If you find that one of the sets is reasonably accurate go with 
that.

I did it and there is a small difference between epw and daysim data 
illuminance (about 2-3%), so I suppose I can use illuminance values 
provided by Daysim.   

So, using the same input (Irradiance values comes from epw file) the 
sky created by Daysim works better than a sky generated by gendaylit? 
Iin which conditions can I use gendaylit?

So, if i want to show particular illuminance condition in radiance (i.
e. 12:00 on 21 March) by mean a falsecolor image with illuminace 
values, is better to use gensky with +s or -c 
depending on if there is direct and diffuse illuminance or only 
diffuse one, using the illuminance data values from daysim or epw (if I 
have it)?

If my weather file has not illuminance data values, I must use Daysim 
to generate the illuminace external values and use them in Radiance to 
make analysis (by means gensky); I can`t use gendaylit (starting from 
Irradiance value provided by the weather file) because it provides 
illuminance values are not correct compared to measured ones.

Can you confirm it?

>You can also use the measured values to 'calibrate' your 
calculations
>and scale the internal results according to a reference calculation
>outside. If your reference point value is different from the EPW
>record you increase or decrease the calculated illuminance to
>compensate for the overall difference.

Ok, I can do it if I have measured data. 


Sorry for many questions and thanks a lot in advance.

Ciao

Roberto




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End of Radiance-general Digest, Vol 84, Issue 6
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