[Radiance-general] Gendaylit Visible radiation

[Lars O. Grobe]

Hi Julien,

as Jan explained before, the inputs of gendaylit can be "visual irradiance" (which is probably achieved by dividing illuminance by Radiance internal luminous efficacy 179), irradiance (which is W/m2 as measured by a pyranometer without any weighing), or illuminance. You select according to your data using the -O option. If you use -O 0, gendaylit assumes that you have a typical data set as recorded by a pyranometer, which includes e.g. infrared. It will assume a luminous efficacy for sun and sky, and reduce the irradiance accordingly.

If you want to verify your method first, start simple. E.g. go with a simple model, where you assume that photo metric measurements were available. Create a basic sky with 1000lx beam (normal) direct illuminance and 1000lx diffuse horizontal. Use this with rtrace -I+ -ab 0, and you should get 1000/179 * cos(pi-sun altitude) W/m2. Render with -ab 1, and you should get this direct horizontal irradiance + 1000/179 W/m2.

The next step is to look into whether you have measured photo metric readings, or whether you need to use radio metric readings. In this case, you can let gendaylit reduce these using its luminous efficacy models.

Cheers, Lars.

Dipl.-Ing. Lars O. Grobe

Am 05.02.2013 um 10:13 schrieb Julien Boutillier <boutillier at estia.ch>:

> Hi Lars, 
> 
> Thanks for this explanation.
> 
> I tried again to put -ab 0 and effectively I only have sun contribution. I don't know what I have done last time (I thought I had tried this case already).
> 
> With 500 W/m2 for direct irradiance and 200 W/m2 for diffus irradiance I obtain 280 W/m2 with -ab 0 and 425 W/m2 with -ab 1 or higher.
> 
> With 0 W/m2 for direct irradiance and 200 W/m2 for diffus irradiance I obtain 0 W/m2 with -ab 0 and 136 W/m2 with -ab 1 or higher.
> 
> I am not sure to understand why the diffus contribution is not the same (425-280 = 145 W/m2 not equal to 136 W/m2). I  saw Perez parameters are not the same, so I think the created sky is a bit different, and this is why…?
> 
> Cheers, 
> 
> Julien
> 
> Le 4 févr. 2013 à 21:49, Lars O. Grobe a écrit :
> 
>> Hi Julien,
>> 
>> whatever result you get there, something is wrong with the scene which is not about the Perez sky model or its parameters.
>> 
>> If you do an irradiance calculation using rtrace -I+ -ab 0, glow sources do not contribute at all to the result. That means that whatever sky distribution you generated, it will not be considered in your calculation. If you really got the same results for your sky with -ab 0 and -ab 1, that means that you did not have any diffuse sky component in your scene. Maybe the input was out of the valid range of the Perez sky, and only the sun (which is modeled as light, not glow), was written to the sky description.
>> 
>> Try it out yourself - create a scene with a 180 degrees source, modified by a glow definition of RGB 1, 1, 1. This should lead to an irradiance of RGB pi. With -ab 0, you will get zero irradiance, with -ab 1 and anything higher you get the expected 3.1416.... W/m2.
>> 
>>> I have try with -ab 1 and the result is the same.
>> 
>> Cheers, Lars.
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