Surface Properties

Calculation of Fresnel coefficients during raytrace

If the surface property is of type Fresnel, then the specular reflectance and transmittance coefficients must be calculated during the raytrace.

This is done in the following way:

•Calculate the absorptance, reflectance, and transmittance based on the material property.

•Use the calculated absorptance for the absorptance coefficient.

•Use the calculated reflectance for the total reflectance.

•Adjust the Total Scatter (TS) for the angle of incidence. The TS in the database is for normal incidence.

The adjusted TS for reflectance is

(θ) ( ) Rtotal(θ)

TSrefl = TSrefl 0 --------------------( ) .

Rtotal 0

The adjusted TS for transmittance is

(θ) ( ) Ttotal(θ)

TStrans = TStrans 0 --------------------( ) .

Ttotal 0

• Calculate the specular reflectance and transmittance as the total minus TS,

Rspec(θ) = Rspec(θ) – TSrefl(θ) and

Tspec(θ) = Tspec(θ) – TStrans(θ)

Anisotropic Surface Properties Standard Expert

According the Merriam-Webster Collegiate Dictionary, anisotropic means:

Main Entry: an·iso·trop·ic

Pronunciation: “a-”nI-s&-'trä-pik Function: adjective

Date: 1879

: exhibiting properties with different values when measured in different directions <an anisotropic crystal>

Anisotropic really means “not isotropic,” and isotropic means (again according to the Merriam-Webster Collegiate Dictionary) “exhibiting properties (as velocity of light transmission) with the same values when measured along axes in all directions.” For a surface property, anisotropic behavior means that the coefficients can vary versus the direction of incidence. In a TracePro surface property, the direction of incidence is described by two angles, a polar angle θ and an azimuth angle φ like in spherical coordinates. The polar angle is the angle between the incident ray and the surface normal. With the surface normal as the z axis, the x and y axes lie in a plane tangent to the surface as in Figure 7.7.

TracePro 5.0 User’s Manual

7.23

Technical Reference

FIGURE 7.7 - Polar Coordinates for anisotropic surface property

When you create a surface property in TracePro, you can add as many values of θ and φ as you wish to the property. For each temperature and wavelength, the surface property editor will create a table of coefficients for each pair of θ and φ. For an anisotropic surface property, the BSDF can be either the standard ABg model or an elliptical BSDF model, giving you full control over the behavior of the surface.

When you apply an isotropic surface property to a surface, you must also specify the direction where the azimuth angle is zero. This is the x axis in Figure 7.7.

Anisotropic surface types

Many surfaces exhibit anisotropic behavior. Any surface that “looks different” when light is shined from different directions is exhibiting anisotropic behavior. For example, a brushed or machined surface may reflect a different amount of light depending on the direction of incidence. Many composite materials that have fibers oriented in a particular direction exhibit anisotropy. Some diffusers are designed to exhibit anisotropic behavior.

7.24	TracePro 5.0 User’s Manual