[Radiance-general] Approximating the gross daylight flux through venetian blinds.

[Greg Ward]

Hi George,

That's quite an e-mail.  At least you've done your research.  However, 
I don't think you'll get anyone to "bless" your calculation based on 
your current approach.  I won't even hazard a guess as to how close 
your results are to correct.  (I hesitate to make any claims about my 
own calculations for that matter.)

Given the amount of time you must have spent on this, even on composing 
this e-mail, don't you think you should spend a little time getting a 
newer version of Radiance onto your computer?  Just a thought.

Here are some others:

> - modelled a small Test Room A with the triple façade modelled
>   accurately in geometric detail, using Optics5 data for each sheet
>   of glass, and assumed reflectances and detailed geometry for the
>   venetian blinds.

Does Optics5 do ray-tracing to figure out the spatial distribution of 
light coming from scattered sources?  I had no idea it had such a 
capability!  I really should talk to my compatriots at LBNL more 
often...

>  - modelled a Test Room B of the same dimensions, but with a
>   pseudo-façade of simple plane elements with BRDFs,

Using BRTDfunc, or what?

>  - tweaked the BRDFs until the renderings compared well between
>   Rooms A and B under 3 outside environments, with external
>   obstructions, materials and ground planes,

This is the part that has me most worried.  Getting a reasonable match 
in a few different scenarios is no substitute for good modeling.

>  - applied the façade and BRDFs of Test Room B to my big models,
>   so I can crank out the illuminances and extract the daylight 
> statistics.

It should be possible to arrive at a reasonable BRTDfunc approximation 
to blinds based on a particular slat spacing, angle, and reflectance.  
It would take me a week or so to do it, though.  I wouldn't trust 
plasfunc or any of the other types, because they don't include a pure 
transmitted component, which you will certainly have with venetian 
blinds.  In short, I think for the time being at least, you should 
stick with a mkillum-driven calculation, or rely on the basic Radiance 
interreflection calculation with some large values for -ad and -as if 
all you want are a few data points per time step.

That's my two cents.  I hope the others have something more to say, 
because I know there are users out there with more experience with 
daylight simulations than I have....

-Greg