# Radiation analysis set solar heat gain of the glass

Hi everyone,

I am trying to run a radiance study where I measure the cumulative solar radiation on a grid over the year - (grid based simulation type 1)

However I am confused as to how to set the solar gain properties of the glass which is in front of the grid. How do I do that? As far as I can see only the visible light transmittance properties of the glass can be set.

If you use a climate-based sky and run the analysis for radiation, transmittance value will be used as the SHGC.

Mostapha do you mean the genCumSky component? I am doing a radiation study.

No. It has nothing to do with genCumSky. Let me try again > Transmittance values will be used as SHGC in radiation studies. Just set the values to SHGC and run the radiation analysis.

Hi Mostapha, hi Anton,

I tought in solar radiation calculations via Radiance you´re using the Tsol (direct value of SGHC) of the glazing (as transmittance value) and not the whole SHGC , SHGC=Tsol+SHGCindirect, I was not able to find it now, but I saw it in the past, can you clarify this ?

Best

Peter

Hah, I did not know that

It makes things so much clearer now.

Hi all,

I’ve been delving deeply into the code and I am posting my thoughts here in the hope of making things clearer.

What should be entered into the 3 inputs is the transmittance of the glass, however radiance actually needs the transmisivity in order to account for the internal reflections of the glass. There is a formula for this shown on page 7 of the radiance primer.

Mostapha has embedded this formula into the code so that for the radGlassMaterial component transmisivity is automatically written to radiance/daysim.

Furthermore transmisivity works for both daylighting and solar radiation studies where the input transmittance is visible transmission for daylighting and SHGC for the latter.

However what I am not entirely clear about is what is the purpose of theavrgTrans output where theavrgTrans = 0.265 * RedTransmittance + 0.670 * GreenTransmittance + 0.065 * BlueTransmittance. Mostapha I understand that this the photometric “visible transmittance” so why is it not used for daylighting simulations?

Let me know if I’ve got anything incorrect here,

Anton

Anton,

If you are asking about the multipliers for theavrgTrans, those are meant to relate radiative energy to the human visual system. Lighting calculations, as performed by Radiance/ AGI32/Dialux are basically photopic calculations that intend to describe the brightness of light sources. Although this wikipedia page doesn’t do justice to this topic, it is a good starting point to read more on subject: https://en.wikipedia.org/wiki/Luminosity_function

In very simplistic terms, the values of 0.265, 0.67 and 0.065 “weight” the radiation calculated by Radiance and conform them to our visual system. They are used almost everywhere in daylighting simulations inside Radiance when the results are being finalized from RGB triplets to a single Illuminance/Luminance value.

Sarith

Hi Sarith

Thanks for your reply, I do understand that theavrgTrans relates radiative energy to visible light (by humans) what I am trying to understand is how this is calculated within radiance.

I am guessing that within the daysim code for daylighting simulations the same formula (0.265, 0.67 and 0.065) is used to convert the radiative energy transmitted through the glass into visible light.

Is this correct?

Thanks

Hi Anton,

Yes.

It is that formula or a variation of that formula depending on context. For example in DaysimPS, the final step in calculating illuminance is something like: /C c:\daysim\bin\rcalc -e “\$1=\$1;\$2=\$2;\$3=\$3;\$4=\$447.4482686+\$5119.950519+\$6*11.6012125”

where the values of (47.44,119.95,11.60) are in the same ratio as (0.265,0.67 ,0.065).

CIE (or IES) calculations for lighting are focused on brightness based calculations and tend to disregard spectral information (which in the case of Radiance is just RGB) in favor of a single number. The calculations that are done in Radiance on Daysim in this regard are in line with the Trichomatic Color Theory and the concept or Long, Medium and Short range cone cells in the eye.

There is some discussion on this in the Radiance forum: http://radiance-online.org/pipermail/radiance-general/2011-January/…. (V-lambda or Y luminance curve or Photopic efficiency function are one and the same).

Sarith

Hi Sarith,

Again many thanks for your helpful reply, this does make sense therefore as Mostapha has said when doing a radiation analysis the R,G and B transmittance values of the glass should be the SHGC, while for a daylighting analysis they should be the visible transmission.

Anton