Metallic MATERIAL

This material type is used to create more realistic metal. Metallic Material is clearly distinguished from the Glossy material both in terms of specular and IOR. In the real world, the reflection properties of metals are very high; and as we have already mentioned, it is determined according to the characteristics of the light reflected back because it absorbs certain wavelengths of the light. So the specular highlight color is usually colorful on copper and gold metals. But where do these colors come from? Why is gold metal yellow-orange and silver white?

There is a very complicated and technical description of the situation, but we will make a short description.


When light (electromagnetic radiation) hits the surface of a metal, it gets absorbed by electrons orbiting the metal atoms, and re-emitted as the electrons fall back to a more stable configuration. The electrons are free to move throughout the metal, which explains its high electrical and heat conductor. Certain wavelengths of the rays absorbed by the metal surfaces cause energy exchange between the electrons. The configuration of these electrons differs according to the type of metal. In metals, only copper and gold show certain color in visible light. The color of gold and copper is related to its electron structure. In the case of gold, the energy difference between electrons in gold is about 400-492nm and this strong absorption cut out the blue light from the reflection, creating the yellow-orange color of gold. Copper has also similar effect for its electron structure but with lower absorption energy (Blue/Blue-Violet), thus we see orange color. For example, silver metal absorbs more or less all wavelengths of light, we see bright white color. The table below shows the colors we see after absorbing certain wavelengths.



So if you want to make a realistic metal using metallic material, it is the right thing to set the right color from specular.


We also highly recommend using the new BRDF models when using metallic materials. Because the new BRDF models use much more advanced Fresnel formulas than the default Octane BRDF. As a result your render output will be much more realistic.  


Now let's look at Octane's Metallic material options. We will not explain the parameters that we have already explained in Diffuse/Glossy or Specular section. When you select Metallic, some new parameters will appear. We will only explain these new parameters. Diffuse, specular, specular map and index parameters are important in creating Metallic Material, so we will include these explanations.



DIFFUSE

This parameter does not work alone in case of metallic material but works with specular map. "Metals have their own color" and "Give color to the metal" are different things. One is the color we see reflected after the metal surface absorbs a certain wavelength of the light. This is a situation that needs to be done from the specular option or from the IOR section.

"Coloring a metal" is like painting a natural metal element. Can also called "Coating". For example, what is called "Car Paint" is actually this. If your goal is to create a metal color that does not offer physical realism but looks nice to the eye, you can use this option with specular map. For example, if you want to create a purple colored car, you can use this option. Since there is no metal variety called purple metal, you can put your imagination into action. Diffuse parameter will be explained in more detail in our specular map explanations.


SPECULAR

According to the IOR type you choose, you can set the color and amount of reflection of the metallic material using this option. You can change the reflection amount with the float value or set the both color and reflection amount with the HSV values. If you use HSV value, you can adjust the amount of reflection with "V" parameter. Thus you can also make your metal material "Dielectric" or "Conductor". There are quite enough resources on the web for these two concepts. We recommend you look.

Let's say if your goal is to make a real Gold, you will enter the color here or you will use "RGB Ior" (which you will get much more realistic results). We will explain more detailed description about the connection of IOR with specular in the "Index" section.

Info: In reality, the reflection quantities of real metals types have their own reflectance and do not change. Keep this information in mind. So if your goal is to create a full realistic metal, do not play with the reflection amount. If you do not intend to make realistic metal, you can ignore this warning



SPECULAR MAp

This parameter is used to mix the RGB and texture values of Diffuse and Specular. There are various ways to use it. If you want to input an RGB diffuse value to the metal material, you can use this parameter to see the diffuse color and also you can change the amount of reflection as mentioned above. The texture types you assign here can be greyscale / RGB or procedural. The following pictures show two different uses.




INDEX

This controls the Complex IOR settings of the metallic material. By default metals use the Schlick approximation for the Fresnel effect. For a more precise falloff, a complex IOR can be entered (commonly known as n and k values). When a complex IOR is set up, the metallic color will get scaled so the brightness matches the Fresnel falloff for that IOR.

This complex IOR is very complicated topic to explain here. Let's try to simplify the situation: The "n" and "k" are the "values", and we can say that the sum of the reflection is equal to the reflected part and the attenuation effect. The "n" value is the Index of Refraction value. "k" is a phenomenon called absorption loss (or extinction coefficient) and indicates how much light is weaken after entering a surface. The "k" value you see here points to this. In fact, "k" indicates the amount of absorption loss when the electromagnetic wave propagates through the medium. But in octane it is just for more accurate fresnel effect.  

Let's explain this section in more detail:


Metallic Reflection Mode

This changes how the reflectivity is calculated according to three types: "Artistic", "Ior + Color" and "RGB Ior".

ARTISTIC

You can only use Specular color in this mode. The values you will enter into the "Index of refraction" below have no impact. It is simple and ideal for non-realistic results.


IOR + color

In this mode you can also use "n and k" IOR values as "Index of Refraction" along with the Specular color. "n" and "k" are actual index values. For these values go to refractiveindex.info site and enter real IOR values according to material types you selected. Use the numeric field for "n" on the left and "k" on the right. As it's color, you can still specify any color from specular channel.  



RGB IOR

Using this mode you can get the most accurate result. Here you can apply the theory that we mentioned at the beginning in a practical sense. Using the wavelengths of the visible spectrum, you can specify both color and index (Specular use disabled). There are three main spectrum we will use for this: Red, Green and Blue. The complex IORs of each wavelength are different ("n" and "k"). As we mentioned at the beginning of this chapter, it is a situation that changes according to the electron structure of materials. Take Gold, for example, and see how it works in Octane. Gold absorbs a strong absorption at 450nm wavelength. So we do not see a blue color in the gold material. At other wavelengths, absorption is not strong.

Now you can use RGB IOR with this information. For this you need to know which wavelengths for all three channels correspond to: 650nm for red, 550nm for green and 450nm for blue.



Now go to the site Refractiveindex.info and select "Gold" in the "selected data for 3D artist" section. Once you have chosen this, enter 0.65 (ie 650nm) for Red in the Wavelength section and use the values of "n" and "k" immediately below for the red light in the Octane Index (the first row). Do this in the other wavelengths and enter the values one by one in the respective locations. If you have done right, the realistic result of Gold metal will be as followin picture. Of course fundementally, everything else can be very different with other cosmetic factors and texture use. But that's not the point.