Manual:Interior Dialog

The interior dialog allows you to set the properties of the object interior.

Ior - index of refraction
The index of refraction (or refractive index) of a substance is defined as the ratio between the speed of light in vacuum and the speed of light in that substance.

When light crosses a boundary between two media with different index of refraction, it changes its propagation direction according to Snell's law. The change in light direction is caused by the change in its speed and it is is known as light refraction.

Light refraction is easily observed when we look through a semi-transparent object, be it a magnifying glass or a glass or water. In fact, light refraction is so pronounced in everyday objects, that is virtually impossible to model a semi-transparent object with a minimum level of realism without giving it an appropriated index of refraction.

The table below shows the typical values for refractive index of some common substances.
 * {|style="border-collapse: collapse; border-width: 1px; border-style: solid; border-color: #aaaaaa;" cellpadding="5"

!style="border-style: solid; border-width: 1px; background-color:#ddd"| Material !style="border-style: solid; border-width: 1px; background-color:#ddd"| ior
 * style="border-style: solid; border-width: 1px"| Vacuum
 * style="border-style: solid; border-width: 1px"| 1 (per definition)
 * style="border-style: solid; border-width: 1px"| Air @ STP
 * style="border-style: solid; border-width: 1px"| 1.000293
 * style="border-style: solid; border-width: 1px"| Water
 * style="border-style: solid; border-width: 1px"| 1.3330
 * style="border-style: solid; border-width: 1px"| Pyrex
 * style="border-style: solid; border-width: 1px"| 1.470
 * style="border-style: solid; border-width: 1px"| Diamond
 * style="border-style: solid; border-width: 1px"| 2.417
 * }
 * style="border-style: solid; border-width: 1px"| 1.470
 * style="border-style: solid; border-width: 1px"| Diamond
 * style="border-style: solid; border-width: 1px"| 2.417
 * }
 * }



Caustics
Usually, the intensity of light refracted or reflected by a curved surface is not uniform, instead it is concentrated on some areas, creating a very distinct pattern known as caustics. A common situation where caustics are visible is when light shines on a drinking glass. The glass casts a shadow, but also produces a curved region of bright light.

POV-Ray can simulate true caustics using photons, but it is very time-consuming. Fortunately it also can produce fake refraction caustics using a much faster but (physically) inaccurate algorithm. The method makes the object shadow brighter in points where the blocked light ray is nearly parallel to objects normal. The default value for caustics is 0 (no caustics), values close to zero give broad hot-spots while bigger values make them tighter.



Dispersion and Dispersion samples
In nature, the index of refraction of a substance depends of the light wavelength. For most transparent materials, the index of refraction decreases with increasing wavelength.

The dispersion value is the ratio of refractive indices for violet light and red light (the values of refractive indices for other wavelengths are linearly interpolated). The default value is 1 (no dispersion).

POV-Ray does not use wavelengths for ray tracing; in order to simulate dispersion it needs to trace extra rays corresponding to different wavelengths. The number of extra rays is controlled by the dispersion samples value. The default value is 7, but considerably bigger values may be necessary in some cases. 

Fade distance, Fade power and Fade color
Those values control the light absorption by the object.
 * Fade power The fade power controls the form of the attenuation curve. The attenuation is calculated using the formula:
 * $$attenuation=\frac{1}{1+\left(\frac{d}{fade\_distance}\right)^{fade\_power}}$$
 * If fade power is bigger than 1000, then a realistica exponencial attenuation function is used.


 * Fade color The fade color controls the color that light fades to. The assymptotically fully attenuated color is $$light\_color\cdot fade\_color$$.


 * Fade distance The fade distance is the distance light can travel inside de object before it is 50% attenuated.

The default value for fade color is <0, 0, 0>. If a value of <1, 1, 1> is used, there is no attenuation. The default values for fade power and fade distance are both 0.0 (no attenuation).

