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Nd
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Nd
Simply put, Nd should be regarded as the IOR (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.
Before explaining how Nd influences the look of a material, it is important to understand the “Fresnel effect”. 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.
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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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The Nd represents the refractive index, which is the well-know concept of Index of Refraction we often use.
The K is the extinction coefficient: the amount of absorption loss when an electromagnetic wave propagates through a material. This is usually confused with the Abbe, but it is not related to this. K is related to the extinction of the wave.
The use of the K value is optional. In most situations it is enough to just use the Nd value. Only in specific situations where the extinction effect is important, is it necessary to use the K parameter to get a more precise result.
The values of the extinction coefficients are obtained from measurements in laboratory, and are also included in the IOR files.