Glass - Concepts

Glass and Daylight Exterior Glass model transparent glass and plastic surfaces for both interior and daylight exterior environments (AGi32 surface type 14)

These surface types must not be used as a substitute for Daylight Transition surfaces to radiatively couple exterior and interior environments for daylighting calculations ( instead of AGi32 surface types 20, 21 and 22). In other words, they should not be used as windows.

Recommended Uses – Office partitions, tabletops, shower doors, exterior glass enclosures (e.g., bus stop waiting area).

Technical Details

Visibility in Radiosity and Ray-Traced Images - Glass surfaces are correctly rendered as transparent surfaces in both radiosity and in ray-traced images. Note: Be sure to set specularity (recommended minimum = 0.05) for the glass to look realistic in the Ray Trace images.

Meshing – Glass surfaces are meshed into an array of patches as specified in the meshing parameters for glass/transition surfaces (Tools-System Settings-Advanced Settings). Glass surfaces are not further subdivided during adaptive subdivision, as they contain no elements.

Radiosity Calculations - Glass surfaces are considered to be double-sided transmissive surfaces for the purposes of radiosity calculations. They behave as Lambertian emitters; that is to say they diffuse both direct and interreflected light to surface elements on the other side of the glass. However, unlike double-sided transmissive surfaces (AGi32 surface type 11 with Transmittance assigned),they appear transparent in the renderings and their reflectance value is zero.

Glass will appear in the RGB Color and Grayscale Color Renderings as a shaded surface but not in the Pseudocolor renderings (the glass surface shades other surfaces as seen through the glass, the glass surface itself has no luminance/brightness). There is no practical way to change the Pseudocolor appearance of surfaces as seen through the absorbing glass, so in order to display these surfaces as they are changing in gradient the glass surface cannot be displayed.

Fresnel Equations - The Fresnel Equations describe the behavior of light when moving between media of differing refractive indices. The reflection of light that the equations predict is known as Fresnel reflection. (ref. Wikipedia, Fresnel equations.) In Radiosity calculations, Glass surfaces do not currently support Fresnel reflectance (they do not reflect light at all). However, they do support Fresnel transmittance losses, as daylight transition surfaces do for direct sources. The transmission of light through transparent materials is dependent on the angle of incidence. When light strikes the surface perpendicularly, nearly all of the light is transmitted (approximately 96% of the specified Transparency value). However, when light strikes the surface at a glancing angle, a very large percentage of the light may be reflected instead. In Radiosity calculations, AGi32 is able to accurately consider the transmission of light through a transparent surface based on incident angle. It is not, however, able to consider the reflection of light off of the transparent surface. (Ray Trace calculations do consider Fresnel reflection and transmission.)

Refraction - Glass surfaces do not support refraction, as they are primarily intended to model planar surfaces. All light not transmitted through is absorbed.

Exitance Meter and Virtual Meter response to Glass surfaces - The Exitance Meter values corresponds with the radiosity calculations - point by point values reflect a Lambertian effect from glass transmittance. The Virtual Meter is applied after the radiosity calculations and therefore sees the glass differently. The Virtual Meter sees the glass as an image preserving material instead of a diffusing material. In general, luminaires and other direct sources (such as daylight) are attenuated correctly through the glass material. Interreflected light (patches seen through the glass) is not attenuated using Fresnel transmission.

We suggest using the Exitance meter instead of the Virtual meter for calculations through Glass surfaces. The following general disparities were noticed between the Exitance and Virtual meters:

Raytracing Calculations - Glass behaves as a Fresnel transmitter in the ray tracing calculations (it does not diffuse the light; instead it behaves as an image-preserving surface).