In its simplest form, it is used to create transparent materials such as water or glass. The most difficult, but most enjoyable type of material to study. The difficulty is due to the interaction of light with the surface. As we have already mentioned, when light hits a surface, it is reflected or absorbed or refracted. Usually this three works together but when the light hit, it changes its behavior when it enters from one medium (for example air) into another medium (for example glass). These changes depend on the optical and topological properties of the surface. In Specular Transmission, when light enters another medium, it reduces speed, changes direction and bends. That's why we can see the result of the light movement on surfaces such as glass and water. Let's also emphasize the importance of the index of refraction (IOR) factor here and try to explain it better.
First, let's assume that the light in the air (or under vacuum) enters the water. In such case, a large part of the light enters the water and continue to its travel, and some of it is reflected by the water. Inside the water, the light continues to bend.
Now suppose that the same light enters the glass this time. As you can see the light entering the surface, changes its angle and continues to bend.
So why does the light behave this way? The reason for this bend is due to the Refractive Index. In this case, the Transmission case should not be considered without the Refractive Index. The refractive index (or index of refraction) of a medium is a measure for how much the speed of light is reduced inside the medium. In other words, the refractive index is the ratio of light velocity in air (or vacuum) to its velocity in the transmitting medium. For example, a typical glass has a refractive index of 1.5, which means that light travels at 1.5 times slower than in the air or vacuum. As this value increases, the rate of refraction increases. As the refraction rate increases, the velocity of the light in the medium decreases.
As you can see, IOR value is a very important parameter. Therefore, knowing the true IORs of transparent materials will be a good step in creating realistic material. You can find the actual IOR information of the relevant material from this site which we mentioned earlier.
Now let's look at Octane's Specular material options. We will not explain the parameters that we have already explained in Diffuse or Glossy section. When you select Specular, some new parameters will appear. We will only explain these new parameters.
By using this parameter you can control the reflection strength on your specular surface. Most specular transmission surfaces have a reflection depending on the surface properties. In other words, these surfaces shows both reflective and transmission properties. Here you can enter the reflection value from this parameter. Be careful not to enter high values like the reflection strength in the glossy material. In such case, the photons will hit the surface and reflect back and will not be able to penetrate the medium as much as necessary. You will get results for sure, but it may not seem like it should be. You can enter RGB, Float value and Texture for the amount of reflection. In the following image, the float value is used in the reflection. Reflect source is the environment HDR.
Dispersion is that the index of refraction for most materials are actually a function of the wavelength of the light. When you send in white light composed of all visible wavelengths, the colors will disperse and get separated from each other. That's called "separation of light dispersion". Each visible wavelength has a slightly different index of refraction. For example, In the below picture the index of refraction of red light for clear glass is about 1.50 but the index of refraction of blue light is closer to about 1.51. In fact, for most materials the smaller the wavelength of the light the larger the index of refraction which means smaller wavelength light will bend more than larger wavelength light. Look at the prism below.
We will not repeat the Index, which we have previously described in the Glossy section and at the beginning of this section. Only with the information above, we would like to show how slow the light when you increase the IOR values. We would also add that this value works effectively with Transmission. You can read more detailed information in the Transmission section.
This parameter controls how the light passes from the surface (more precisely, expressed as the ratio of the speed of light to the refractive index). They are tightly linked with Index and work together. We have explained in previous articles that when light enters a medium it moves slower than it's in the air (or vacuum). Here you can check this by using Texture / Greyscale or RGB value in Transmission. As you can see in the picture below, the movement of the light inside the bottle is shown using greyscale values. When you look at the bottle on the far left, the light actually moves 1.1 times slower than it is in the air. So we almost barely see the bottle. In other bottles, you can see the differences in the passage of light through the bottle by playing with both transmission and IOR values.
In the picture below, an RGB value is used. When you look at the bottle on the far left, the light is still 1.1 times slower than it's in the air, but the RGB value makes the bottle look a bit more visible. This is because some wavelengths of the light have more bend as the angle changes. For example, since the orange color is relatively low wavelength, the light is more distorted in this wavelength than the previous one. In other bottles, you can see the differences in the passage of light through the bottle by playing with both transmission RGB and IOR values.
Fake Shadows is a Boolean value that activates architectural glass option for all meshes sharing that material. When enabled, the specular material exhibits the characteristics of Architectural glass with its transparent feature allowing light to illuminate enclosed spaces or frame an exterior view. If you want a realistic shadow for your specular material, you can turn this option off. If you turn it off, your render may be noise based on your scene structure. In such case, use this option carefully as you will have to enter extra samples to reduce the noise. For example, you can turn it on this option for distant objects, but you can turn it off for close and detailed specular material.