Lighting computations are generally set up to compute the light intensity based on the normal vector for the polygon. If the polygon normal faces away from the eye, the lighting model will likely determine that the light intensity for that polygon is zero. When the polygonal mesh being rendered is a closed surface, this is not a problem because the back-facing polygons will always be obscured. However, when the polygon mesh represents an open shape (for which back-facing polygons may be visible), the dark appearance of these polygons may hinder the user's perception of the overall shape. In such a case, two-sided lighting can be useful. Two-sided lighting is the process of reversing the normals for all back-facing polygons before computing the light intensities for that polygon.
In IDL's Object Graphics, two-sided lighting is enabled by default. When the additional lighting calculation is not required, one-sided lighting can be used to improve rendering performance. On an IDLgrModel object, set the LIGHTING property to a value of 1 to enable one-sided lighting.