[Radiance-general] Questions about spotlight and specularity

[Greg Ward]

Hi Joe,

Regarding reflectivity, I suppose you could say that for any opaque material, reflectivity==reflectance.  Since Radiance materials (with the possible exceptions of "dielectric," "interface" and "mist") don't really consider volumes, this equality generally applies.

I don't really know anything about SolTrace, but from your brief description, it sounds like it might be the right tool for your purpose.

The basic problem with Radiance is that as a light-backwards ray-tracer, it needs to know where to look for light sources, particularly tiny ones like the sun.  There is a trick you can use, which is the "mirror" material light, which enables a virtual light source search.  That brings other issue, especially once you go to a full array.  By the way, it employs the "spotlight" mechanism internally to avoid unnecessary source tests, so you had a good idea with that.

I remember clearly that someone covered this in a Radiance workshop presentation.  Can someone post the link for me?  I haven't managed to locate it with any of my searches.

Cheers,
-Greg

> From: Joe <solarjoe at posteo.org>
> Date: July 28, 2017 2:18:13 AM PDT
> 
> Hello Greg,
> 
> thank you very much for your quick response and the good answers.
> Abbout reflectivity / reflectance, I just found this
> https://en.wikipedia.org/wiki/Reflectance#Reflectivity
> 
> I am also using SolTrace, also from NREL, maybe you know it. It's a
> more technical raytracer (path tracing, starting at the light source),
> not considering colors, only energy.
> 
> Now I would like to make a rendering of a concentrating solar tower
> plant. But since I have only very little experience with rendering so
> far I can't get the materials right, or I am missing something else.
> 
> A solar tower plant consists of thousands of tracking mirrors reflection solar radiation
> onto a receiver at the top of a tower.
> 
> Tower and receiver:
> http://cspworld.org/sites/default/files/map/images/eSolarBabcockWilkoxReceiver.jpg
> 
> Heliostats scattered around the tower
> http://www.solartowersystems.com/mediapool/99/994662/resources/big_21459290_0_350-279.jpg
> 
> This is what I have so far
> 
> http://imgur.com/Vgf6Zuz
> 
> But I was not able to see a light spot of the heliostat on the receiver (large black cylinder with lid).
> I would have expected it at the spot marked in blue.
> 
> The scene is illuminated by a sun disk in zenith (0, 0, 1, from gendaylit) and a downward
> pointing spot (red) directly above the heliostat (green). Heliostat diameter is 10 m.
> 
> This is the sun I am using at the moment
> 
> void light sun
> 0
> 0
> 3 7.131e+004, 7.131e+004, 7.131e+004
> 
> gendaylit originally returned
> 
> void light sun
> 0
> 0
> 3 7.131e+006, 7.131e+006, 7.131e+006
> 
> but this oversaturates my scene completely and it looks like a white-out.
> 
> I tried several materials for the heliostat (glossy colors, glass with no transmittance, mirror).
> 
> So far I tried a red plastic material for the receiver and played a bit with the specularity
> and roughness but could not get it to reflect anything towards the camera.
> 
> Do you have some ideas how I could get this to work?
> 
> Kind regards,
> Joe