The Phong material is a generalisation of the Diffuse material type, adding a glossy coating on top of the diffuse base (or "substrate"). The influence of this coating is controlled by its index of refraction (IOR), with higher values corresponding to a stronger specular reflection. (the default IOR is 1.5)

It is commonly used for materials such as polished wooden floors, car paints (when multiple Phong materials are blended together) and metals.

Metals in particular can be represented using either the "specular reflectivity" option (which allows a specular colour to be defined for the material), or via measured material data (referred to as NK data).

IOR (Index Of Refraction): Controls the influence of the glossy coating; higher values produce a stronger specular reflection. The IOR should be set to the real-world value for the material, if available.
For example, the IOR of plastics is around 1.5 to 1.6.
Oil paint binder has an IOR of around 1.5.

Phong material with IOR 1.2

Phong material with IOR 1.5

Phong material with IOR 2.5

NK data: The Indigo distribution comes with a set of lab-measured data for various metals. If one of these data sets is selected, the diffuse colour and specular reflectivity colour attributes are ignored.

Specular reflectivity colour: When not using NK data, this allows you to set a basic colour for the metal; this is useful for uncommonly coloured metals such as Christmas decorations.

Green specular colour Au (gold) NK dataset Al (aluminium) NK dataset

Base Emission