The transmission allows light to scatter through the surface, for materials such as glass or water.
Note that you must also disable Opaque for the mesh that has been assigned the Standard Surface shader.
If you can only see black where there should be transparency, you may not have a high enough refraction ray depth value (found in the Ray Depth section of the Render Settings). The default value is two.
This filters the refraction according to the distance traveled by the refracted ray. The longer light travels inside a mesh, the more it is affected by the transmission_color. Therefore green glass gets a deeper green as rays travel through thicker parts. The effect is exponential and computed with Beer's Law. It is recommended to use light, subtle color values.
Transmission Color: Red (Depth 1 to 10)
If you use a fully saturated color like (1, 0, 0), the way that is interpreted is that all red light is allowed to pass through, and no green and blue light. transmission_color values near 0 make the interior of the mesh very dense to block all light, and then set the depth multiplier to a small value like 0.001 might not make much of a difference because the depth is large anyway.
Fully saturated colors for transmission_color are not recommended
If this value has a color and shadows tinted with that color are required, then disable opaque for the mesh that has been assigned the standard_surface shader. In the example below, you can see that with opaque enabled the rays cannot pass through the sphere. Whereas with opaque disabled, the rays can pass through the sphere and absorb the color set by the transmission_color, thereby creating the effect of colored shadows.
Note that transmission_color will not work for single-sided geometry unless thin_walled is enabled.
Controls the depth into the volume at which the transmission color is realized. Increasing this value makes the volume thinner, which means less absorption and scattering. It is a scale factor so that you can set a transmission_color and then tweak the depth to be appropriate for the size of your object.
The effect of increasing transmission_depth can be seen in the animation below. Note that a transmission_scatter color has also been used in this case.
Depth is scene scale-dependent and can have a dramatic effect on its appearance. The transmission_color and depth control transmittance/absorption, and that depends on the object scale. So for a small object to see anything you might need to set a quite low depth, or for a big object a high one. If you cannot see the effect of depth, then you may need to check the size of your scene.
When the scene scale is too small, the transmission_color (orange) appears incorrect (left image) with a transmission_depth of 1. Lowering the transmission_depth fixes it (right image). It is recommended to model to real-world scale to avoid these situations.
Transmission_scatter is suitable for any liquid that is fairly thick or where there is enough of it for scattering to be visible, such as a deep body of water or honey. If you have a glass of water, there is not that much scattering, however, for an ocean, it is required. Other examples include materials like ice, opalescent glass or milky glass.
For scattering to work, disable opaque for the mesh that has been assigned the standard_surface shader.
The directional bias, or anisotropy, of the scattering. The default value of zero gives isotropic scattering so that light is scattered evenly in all directions. Positive values bias the scattering effect forwards, in the direction of the light, while negative values bias the scattering backward, toward the light.
Specifies the abbe number of the material, which describes how much the index of refraction varies across wavelengths. For glass and diamonds, this is typically in the range of 10 to 70, with lower numbers giving more dispersion. The default value is 0, which turns off dispersion. The chromatic noise can be reduced by either increasing the global Camera (AA) samples or the Refraction samples.
Transmission_dispersion is ideal for gemstone materials like a diamond.
Adds some additional blurriness of refraction computed with an isotropic microfacet BTDF. The range goes from -1 to 1, where 0 means no roughness. It is computed as
transmission_roughness = specular_roughness + transmission_extra_roughness.
Negative values result in a lower roughness for transmission than for reflection.
When enabled, transmission will pass through AOVs. If the background is transparent, then the transmissive surface will become transparent so that it can be composited over another background. Light path expression AOVs will be passed through so that for example a diffuse surface seen through a transmissive surface will end up in the
diffuse AOV. Other AOVs can also be passed straight through (without any opacity blending), which can be used for creating masks for example.
Alpha masks (rollover for beauty)