Layer Parameters

Diffuse: Diffuse reflection is when light is scattered at several angles on a surface. You can select a texture, color, or procedural.
Layers used in: Basic

Reflectance: Reflectance is the texture for the specular component when you view the surface from directly above. Reflectance at the grazing angle (90 degrees) is also implicitly defined. So, the specular reflectance is calculated as a combination between user Reflectance and Reflectance 90 (white by default), depending on the viewing angle.
Layers used in: Basic, Metal, Glass, SSS, and Coating

Anisotropy: Stretch and blur highlights against the grain of the material, which is particularly useful for metals. For no anitrosopy, set the value to 0%. For full anitrosopy, set the value to 100% (the material is a perfect reflector/refractor in one direction and completely rough in the other direction).
Layers used in: Basic, Metal, Glass, SSS, and Coating

Rotation (deg): To rotate the stretched and blurred highlights created using Anisotropy, enter a value from 0 to 360 degrees or select a texture.
Layers used in: Basic, Metal, Glass, SSS, and Coating

Roughness: Adds texture to the material at a microscopic level, affecting specular reflectance and transmittance. 0% creates a perfect mirror reflection. Lower values produce crisper and brighter reflections. Increasing the roughness will spread and distribute reflections and create a more matte surface. Higher values produce bigger highlights and reflections that are more blurry and dim. At values approaching 100%, light becomes so scattered that the reflections are barely visible, if at all.
Layers used in: Basic, Metal, Glass, SSS, and Coating

Bump: Adds texture to the material at a macroscopic level. A bump map gives the illusion of texture without physically distorting the geometry, minimizing rendering time. Each layer of material can have its own bump map. The grayscale map tells Thea Render how to change the surface normals as if the surface has been displaced; the modified normals are used in lighting calculations. A bump map looks like the inverse of what you might expect: black represents the highest extreme and white represents the flatest extreme, while shades of gray represent grades in between.
Layers used in: All

Normal: This is a more detailed version of bump mapping, where you select a RGB color image instead of a grayscale image. While standard bump mapping uses grayscale values to describe the surface's hills and valleys in terms of height, normal mapping translates red, green, and blue values to x, y, and z coordinates. This creates texture in terms of normal vectors up the hill or down the valley. Specifically, the red, green, and blue values (0 to 255) are
translated to x (–1 to 1), y (–1 to 1), and z (0 to 1) coordinates respectively.
Layers used in: All

Index of Refraction (n): When you place something behind a transparent object, it becomes distorted. The level of distortion is determined by the Index of Refraction, which defines how much light is bent and reflected when it comes into contact with a transparent surface. For example, air's value, 1.0, causes no distortion of background objects. A value of 1.5 means that the background objects become considerably distorted (e.g., a glass marble). A value just
below 1.0 causes the object to reflect along its edges (e.g., a bubble seen from under water).
Layers used in: All

Extinction Coefficient (k): This refers to light that is likely to be lost (i.e., to absorption and scaττering). The higher the extinction coefficient, the more opaque the material. You can use a value of zero or above.
Layers used in: Basic, Metal, Glass, and Coating

Translucent: Make the material semi-transparent by clicking on a texture. If no texture is selected, than no translucency will be rendered.
Layers used in: Basic

Micro Roughness: Adjust the sharpness of reflections, as the viewing distance goes from near to far. When looking at a surface that is completely rough, planes that are closer appear rougher (because you can see them more clearly) while surfaces that are further away appear smoother (because you can't seem them as clearly). You can adjust the Height and Width to define the average height and width (μm) of the bumps on the surface.
Layers used in: Basic, Metal, Glass, SSS, and Coating

Thickness (μm): This refers to the coating thickness. The thickness and extinction coefficient are used to calculate the amount of light absorbed by any layers underneath the coating.
Layers used in: Coating

IOR File: You can create a physically accurate material by using an Index of Refraction file, which provides the exact index or refraction and extinction coefficient for each wavelength of a material. The file extension is .ior or .nk.
Layers used in: Glass and Metal

Transmittance: The amount of light that passes through a material.
Layers used in: Glass and Thin Glass

Absorption (1/m): Change the absorption density and color. You can use a value of zero or higher. The higher the density, the more absorptive the material.
Layers used in: Basic, Glass, SSS, and Coating

Note:
For a basic material, you need to select a color or a texture for translucency first. For glossy materials, you need to select a color or a texture for transmittance first.

Abbe Number: Can be used to create a rainbow effect in the interior of an object, such as a gemstone. Without this rainbow effect, gemstones would look like glass. Lower values correspond to a stronger rainbow effect. Increase the value for a more subtle effect. You can look up the values for specific materials online. The Abbe number describes the variation of the index of refraction with respect to the wavelength.
Layers used in: Glass

Interference: Makes a surface iridescent, simulating a phenomenon called thin film interference. You may have seen this in soap bubbles, oil slicks on water, or peacock feathers. When light waves come into contact with a thin film, some waves are reflected from the top surface while others penetrate the film, hit the bottom surface, and are reflected. When these light waves interact, momentary streaks of color result. The iridescent colors change when the viewing angle is changed. Adjust the Thickness to change the irridescence level. 200‐1000 is a good general range for making a visible change.
Layers used in: Thin Glass

Scattering (1/m): Changes the scatter density and color for a subsurface scattering material. You can use a value of zero or higher. Note that the higher the value, the longer it will take to render the material. The scatter color is used for both in‐scattering and out‐scattering light interaction.
Layers used in: SSS

Asymmetry: Controls the asymmetric coefficient of a subsurface scattering medium, which follows the Henyey‐Greenstein phase function. You can use a value between –1 and +1, where –1 corresponds to a perfect back scattering media, +1 to a perfect front scattering media, and 0 to a isotropic media.
Layers used in: SSS