POV-Ray : Documentation : Conserve Energy for Reflection
  POV-Ray 3.6 Documentation Online View Specular Reflection Iridescence Conserve Energy for Reflection

One of the features in POV-Ray is variable reflection, including realistic Fresnel reflection (see section "Variable Reflection "). Unfortunately, when this is coupled with constant transmittance, the texture can look unrealistic. This unrealism is caused by the scene breaking the law of conservation of energy. As the amount of light reflected changes, the amount of light transmitted should also change (in a give-and-take relationship).

This can be achieved by adding the conserve_energy keyword to the object's finish {}.
When conserve_energy is enabled, POV-Ray will multiply the amount filtered and transmitted by what is left over from reflection (for example, if reflection is 80%, filter/transmit will be multiplied by 20%). Specular Reflection Iridescence

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