In Radiance we can define different material properties for the various objects of our models. These different materials have different ways of manipulating the rays of light that interact with them, based on the physics of light. We have reflection, transmission, and/or refraction depending on the type of material.
In this version of the <Virtual Environment> we have limited the material types to the most commonly occurring materials found in buildings. In future versions of the <Virtual Environment> additional material types may be made available.
Plastic
Plastic has a colour associated with diffusely reflected light, but the specular component is uncoloured, most materials fall into this category. Define the R, G, B reflectance values and the specularity and roughness. [The name “plastic” should not be interpreted as referring to plastic objects]. The reflectance values have the range 0.0 to 1.0 (although 0.0 and 1.0 do not occur in nature) [sometimes for a given surface a single reflectance value is given – this probably refers to the average “hemispherical” reflectance, where you are only interested in illuminance this single value can be used for each of the R, G and B values]. Specularity also has the range 0.0 to 1.0, 0.0 for a perfectly diffuse surface and 1.0 for a perfect mirror. In reality plastic materials are generally not very reflective and the specularity value is usually in the range 0.0 – 0.07. Roughness, with the same limits, refers to how the surface scatters what light is reflected, 0.0 meaning perfectly smooth. Plastic materials generally have a roughness in the range 0.0 – 0.02.
Metal
Metal is the same as plastic except that the specular component is coloured by the material. Define the R, G, B reflectance values and the specularity and roughness. Specularity and roughness have the same theoretical limits as given above. However, metal materials are reflective and the usual range for specularity is 0.5 – 1.0, and for roughness a range of 0.0 – 0.5.
Glass
Glass is used to model transparent materials. Define the R, G, B transmissivity values. [glass is a special case of dielectric with a refractive index fixed at 1.52 and all that needs to be defined is the transmission at normal incidence ]. The properties of glass are commonly defined in terms of the transmittance (by glazing manufacturers), to convert to transmissivity use the following equation -
transmissivity = (sqrt(a+4*sq(b*Tn))-c)/(d*Tn)
where Tn = transmittance,
a = 0.8402528435,
b = 0.042579995,
c = 0.916653006,
d = 0.00362611194
We have also found it impossible to get RGB data from glazing manufacturers, who will only quote a single transmittance value (even for tinted glass). We suggest for illuminance images this single value is used for each of the R, G and B values. For luminance images make minor adjustments to the relevant colour e.g. for a green glass increase the G value and decrease the R and B.
The following materials have recently been added and will be discussed more fully in a separate document.
Trans
Trans is used to model a translucent surface. It takes the same parameters as plastic plus the transmission factor and a transmitted specularity value.
Dielectric
Dielectric is a transparent material that refracts and reflects light (such as water or crystal). Define the R, G, B transmissivity values, the refraction index and the Hartmann constant.
