[Radiance-general] How to define reflectance of near infrared of solar radiation

[Lars O. Grobe]

Hi Natchan,

reflectance from glass is a function of at least wavelength and 
direction. So short answer is no, most surfaces will show an entirely 
different reflection in NIR than they do in the visible range. Most 
glazing manufacturers work on exactly this target - low NIR 
transmission, high visible light transmission. And while reflectance for 
visible light may be dominated by Fresnel laws, NIR reflection will 
probably be caused by different mechanisms due to the coating applied to 
the glass.

Please check some reflectance spectra to get an understanding of what 
that means. And look into measurement standards:

http://windowoptics.lbl.gov/data/standards/solar

Radiance does not really care much about wavelength. As long as it can 
be considered to behave as light, you can feed any kind of information 
on radiation into the three channels that are evaluated in parallel 
within Radiance. Most people use the three channels for color 
information, red / green / blue. Often, when only photometric results in 
terms of luminance or illuminance are required, you would set all three 
channels to the same value.

Radiance has several material types for glass, but most of them assume 
an uncoated glass (dielectric, glass) which may not be what you are 
looking for. In your case, I would recommend to first define the 
wavelength range you want to considere (what is NIR for you?). Then you 
need to find the reflectance for your glazing for that wavelength range, 
and for several incident directions. You may try to use Optics/Window, 
both provided by LBL, to try this out. And check Jack de Valpine's nice 
presentation on a very related topic, where he investigated reflection 
from (coated) glazings:

http://www.radiance-online.org/community/workshops/2009-boston-ma/Presentations/devalpine_visarc_glazing.pdf

Please note that you will also have to model the emission from sources 
in the _same_ NIR range as determined above.

Cheers,
Lars.

> Hi.I'm natchan.
>
>
> Im investigating reflected light of sun from glazing of buildings with 
> radiance.
>
> Radiance materials can be set reflectance of each RGB.
>
> Then, can I define reflectance of near infrared(NIR, unvisible light) 
> of solar radiation?
>
> Is the NIR reflectance equal to RGB reflectance?
>
> I apologize for my poor English.
>
> Thank you.
>
>
>
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