Nd
As mentioned in the Maxwell Materials page, the Nd is the index of refraction of the material. The name Nd is used because it is the common way to denote an index of refraction that has been measured at the wavelength of 583 nm for a certain material. You can find a list of common IOR values here. The Nd is important not only for transparent materials but for all materials as it defines both surface reflections and the refraction of light inside the object.
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Before explaining how Nd influences the look of a material, it is important to understand the “Fresnel effect”, named after the french physicist who first described it. This effect states that the strength of reflections on a surface is dependent on the viewing angle. For example, if you look straight on at your monitor screen, you will see very weak reflections, but if you look at the screen at an angle, the reflections will appear stronger. It is named after the french physicist who described this . With a higher Nd number the surface will start to look equally reflective, no matter the viewing angle. So in effect, a higher Nd “weakens” the Fresnel effect.
The same plastic material above with Nd
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1.
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1 (left) and
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Nd 1.6 (right). The resulting "fresnel curve" flattens out as the ND increases, making the surface equally reflective from all viewing angles.
The relationship between Nd and the Fresnel effect is that with a higher Nd number the surface will start to look equally reflective, no matter the viewing angle. So in effect, a higher Nd “weakens” the Fresnel effect. that So changes in the Nd number affect the overall reflectivity of an object. For example, you can have the Reflectance 90° color set to full white, but with an Nd of 1 the object will have very weak reflections. Setting the Nd to a high value like 40 and keeping the Reflectance 90° to that same full white color will result in a mirror-like surface. The following images demonstrate this:
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You should turn on Force Fresnel when you are working with untextured refl. 0° and refl.90° channels to create more realistic metals or other shiny materials. Force Fresnel will then ensure a correct reflectance across the entire surface, even if you set a very dark refl. 0° color.
K parameter
From a physical point of view, the index of refraction is not just a plain number. In fact it is a number derived from a complex calculation to define the refractive index at one particular wavelength. This is the calculation:
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