# Modeling Photochromic glass using Radiance

Hello everyone,

This question isn’t really related to Ladybug/Honeybee, it is a Radiance question. I know there are a lot of daylighting experts here that can to help me out.

We want to simulate a red photochromic glass using Radiance. The UV-VIS spectroscopy analysis was conducted and optical characteristics of the red glass including transmittance and absorbance were recognized. Then, the transmissivity of the glass sample was calculated based on the following formula.

Transmissivity = (sqrt (0.8402528435 + 0.0072522239 * Transmittance ^ 2) - 0.9166530661) / 0.0036261119 / Transmittance)

For example, the Transmittance of red glass is 48 % and the transmissivity is 0.5232.

We can model the red photochromic glass in three different ways in Radiance. I appreciate if you clarify which one is correct.

1-First method

Void glass Red_Transmittance_48

0

0

3 0.5232 0.5232 0.5232

2-Second method

Void glass Red_Transmittance_48

0

0

3 0.5232 0 0

Note: In the second way, we remove the parameters of gtn and btn because of red color of the sample.

3- Third method

Void glass Red_Transmittance_48
0
0
3 0.211 0.086 0.196

Note: Some scholars suggested to add the wavelengths of lights (red, green, blue) in the definition of glass material in Radiance. Because of the color of glass, we consider the wavelengths of lights (red, green, blue) in transmissivity calculations and then in the definition of glass material in Radiance.

Among these three ways, which one is the correct way of modeling photochoromic glass in Radiance?
Your help is highly appreciated in advanced.

Well, it depends on how you measured the transmittance and what you intend to simulate. Assuming that you measured the transmittance using a full-spectrum light source and intend to calculate illuminance, the one below will be correct.

Radiance is a three channel rendering engine where the three channels correspond to the CIE tristimulus values. The average value of transmissivity is calculated based on the Photopic Efficiency function with weightings around the three channels (0.265, 0.670, 0.065 for R,G and B respectively).
In your first case, the value of transmissivity still averages to 0.5232 as 0.265x0.5232+0.670x0.5232+0.065x0.5232 still adds up to 0.5232.
For the second case (void glas Red 0 0 3 0.5322 0 0 ), it will be 0.139 (0.265x0.5232+0+0) and for the third case it will be 0.110.

Regards,
Sarith