The Physical Optics material is used for rendering wave optics surface effects as a diffraction, thin-film interference, and birefringent iridescence. Light is scattered into diffraction orders, which can appear as sharp spectral peaks or broader diffuse halos depending on the surface parameters and coherence settings. The material also supports thin-film interference for layered dielectric coatings, allowing effects similar to soap bubbles, oil films, coated optics, and other wavelength-dependent iridescent surfaces (figure 1). Birefringent uniaxial coatings are supported as well. 


Physical Optics Examples

Figure 1: Physical Optics material examples


Physical Optics Material Parameters

Base Layer

Albedo - The material's base color. Equivalent to diffuse, with no shading, light direction, or ambient occlusion. RGB Spectrum, Gaussian Spectrum, float, or image textures can all be used as input for albedo.

Transmission - Controls the light passing through the surface of the material (via refraction).

Opacity - Acts as a binary visibility control. 

Metallic - Controls the metallic nature of the surface blending between dielectric (0) and metallic (1).

Metallic Edge Tint - The color of the edges of the surface when set to metallic.

Surface Properties

Roughness - Sets the roughness values for the Specular reflection and Transmission channel.

Light Coherence (um) - Controls the coherence of light, measured in micrometers. Lower values will blur reflections similar to high roughness values. 

Grating Weight - Provides artistic control to reduce the intensity of the diffraction grating lobes.

Grating Height (um) - Grating profile height in micrometers. A value of 0 will disable diffraction gratings.

Grating Profile - Determines the shape of the periodic surface profile for the grating. The profiles determine how energy is split among the diffraction lobes.

Grating Lobe Count - Controls the number of simulated lobes. A value of 0 will disable diffraction gratings.

Grating Pitch X (um) - Distance from peak to peak of the grating profile over the surface's x axis in micrometers. A value of 0 will disable this axis. Higher values will bring the lobes closer together. 

Grating Pitch Y (um) - Distance from peak to peak of the grating profile over the surface's y axis in micrometers. A value of 0 will disable this axis. Higher values will bring the lobes closer together. 

Grating Roation - Rotation of the grating profile around the surface normal. A value of 1 represents 360 degrees of rotation.

IOR

Index of Refraction - Determines the reflection strength on the surface based on Fresnel's law. With a value greater than 1, reflection is strongest on the surface parts that turn away from the viewer's angle (grazing angles), while the reflection appears weaker on the surface parts perpendicular to the viewing angle. This results in a more realistic-looking surface. With a value lower than 1, the Fresnel effect is disabled, and the reflection color appears as a uniform color across the highlight. The Specular channel's color determines the reflective highlight's color.

Dispersion Coefficient - 

Dispersion Mode - Determines how the IOR and dispersion coefficients are interpreted. 

  • Abbe Number - Measures a transparent material's dispersion. A higher value indicating less dispersion and clearer color perception. 
  • Cauchy Formula - Calculates the refractive index of an optical material at any specific wavelength within the visible spectrum. 

Extraordinary IOR Delta - The difference between the IOR and the extraordinary IOR. The extraordinary IOR will cause double refractions. 

OA Polar Angle (deg) - Optical axis polar angle in world space, controlling the direction of the extraordinary refraction. 

OA Azimuth Angle (deg) - Optical axis azimuth in world space, controlling the direction of the extraordinary refraction. 

Allow Caustics - If enabled, the photon tracing kernel will create caustics for light reflecting or transmitting through the object.

Thin Film Layer

Film Thickness (um) - The film coating's thickness, mainly used to simulate to look of a thin layer of additional surface material. Values between 0 and 1 will cause thin film interference. Values higher than 1 will cause birefringence interference when the film has an extraordinary IOR.

Film Roughness - Determines the reflective roughness of the film.

Film IOR - The film coating's index of refraction.

Film Extraordinary IOR Delta - The difference between the IOR and the extraordinary IOR of the thin film coating. The extraordinary IOR on the coating will cause birefringence interference, currently suported only when the extraordinary IOR delta in the IOR group is set to 0. 

Film OA Polar Angle (deg) - Controls the film optical axis polar angle.

Film OA Azimuth Angle (deg) - Controls the film optical axis azimuth angle. 

Geometry Properties

Bump - Simulates a relief using a Grayscale texture interpreted as a Height map.

Bump Height - Determines the height represented by a normalized value of 1.0 in the bump texture. A vaule of 0 disables the bump map and a negative value will invert the bump map.

Normal - Distorts normals based on an RGB image.

Displacement - Adjusts the height of a surface's vertices at render time using a texture map. Displacement maps differ from Bump or Normal maps in that the geometry is altered by the texture, as opposed to just creating the appearance of detail. Displacement mapping is more complex than using a Bump or Normal map, but the results are more realistic, especially along a surface's silhouette. Displacement works with the texture nodes, and the displaced mesh must have UV Texture coordinates. Procedural texture nodes, such as Turbulence or Marble, need to be converted using the Baking Texture node. For more information, see the Displacement topic in this manual.

Priority - Used to resolve the ambiguity in overlapping surfaces, the surface priority control allows artists to control the order of preference for surfaces. A higher number suggests a higher priority for the surface material, which means it is preferred over a lower priority surface material if a ray enters a higher priority surface and then intersects a lower priority surface while inside the higher priority surface medium.

Smooth - Enables or disables normal interpolation. If normal interpolation is disabled, triangle meshes appear faceted.

Smooth Shadow Terminator - If enabled, self-intersecting shadows are smoothed according to the polygon's curvature.

Round Edges - Rounds the geometry edges by using a shading effect rather than creating additional geometry. For more information, see the Round Edges topic in this manual.

Custom AOV - Writes a mask to the specified custom AOV.

Custom AOV Channel - Determines whether the custom AOV is written to a specific color channel (R, G, or B) or to all the color channels.

Material Layer - Adds a Material layer above the base material. See the Material Layers topic in this manual for more details.