Reference:Interior Texture

Syntax:

object {
  texture { TEXTURE_ITEMS... }
  interior_texture { TEXTURE_ITEMS...}
  }

All surfaces have an exterior and interior surface. The interior_texture simply allows to specify a separate texture for the interior surface of the object. For objects with no well defined inside/outside (bicubic_patch, triangle, ...) the interior_texture is applied to the backside of the surface. Interior surface textures use exactly the same syntax and should work in exactly the same way as regular surface textures, except that they use the keyword interior_texture instead of texture.

Note: Do not confuse interior_texture {} with interior {}: the first one specifies surface properties, the second one specifies volume properties.