Refractive Material Types

dielectric, interface and glass are materials that transmit and reflect light specularly, and also handle refraction. The first two are used for boundaries of volumes that are meant to represent solid objects of the respective material. The last one is an optimization that allows to model thin panes of glass or similar material to be modelled with a single surface.

Dielectric

A Dielectric material is transparent, and it refracts light as well as reflecting it. This material should only be used for surfaces that are part of a closed volume, because Radiance needs to know the distance the light travels within this volume to determine the total absorbtance. If the back side of such a volume is not defined, an infinite distance is assumed, which absorbs all light and results in a black appearance of the front surfaces.

The dielectric material requires that the surface normals are oriented out of the virtual volume, as defined by the right hand rule. The back of a surface with this material does not interact with rays in a simulation, except when a ray has previously traversed another dielectric surface from the front. As a consequence, volumes with wrongly oriented surfaces may suffer from "black holes".

[dialog]

Color Transmissivity:
The color values specify the transmissivity per unit length.

[0.0 0.0 0.0]: black (full absorption)
[1.0 1.0 1.0]: white (no absorption)

Index of Refraction:
The index of refraction determines how strongly a ray will change it's direction when traversing the boundary of a volume of dielectric material, depending on the incident angle. Real world values are typically between 1.3 and 1.9, common glass has an index of refraction of about 1.5.

Hartman Constant:
The hartman constant describes how the index of refraction changes as a function of wavelength. It is usually zero.

Interface

An interface is a boundary between two dielectrics. There are seperate sets of color transmissivity values and index of transmission, one for the inside (the volume at the back of the surface) and one for the outside (to the front of the surface).

There are similar restrictions for interface as ther are for dielectric about the orientation of the surfaces. In most cases, the affected volumes will be composed of surfaces of both material types, dielectric for the outer, and interface for the interior boundaries.

A dielectric material is equivalent to an interface with outside values all 1 (vacuum).

[dialog]

Inside Color Transmissivity:
Outside Color Transmissivity:
The color values specify the transmissivity per unit length on each side of the surface.

Inside Index of Refraction:
Outside Index of Refraction:
The index of refraction determines how strongly a ray will change it's direction when traversing the boundary in one or the other direction.

Glass

The glass material is similar to dielectric, but it is optimized for thin glass surfaces with an index of refraction of 1.52. By using a single surface in place of two, the computation of internal reflections is avoided. One transmitted ray and one reflected ray is produced.

The surface orientation is irrevelant for glass.

[dialog]

Color Transmissivity:
The transmissivity for glass is the fraction of light not absorbed in one traversal of the material at normal incidence.

This value is different from the transmittance, that is normally measured. Transmittance is the total light transmitted through the pane including multiple reflections.

To compute transmissivity (tn) from transmittance (Tn) use:

[equation]

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