How to define reflectance of near infrared of solar radiation

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:

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:

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



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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