Optical fibers are drawn from thick glass preforms. The preform is heated to a high temperature using resistive or inductive heating. At the high temperature the viscosity of the glass decreases so that the preform necks down and thins eventually to the desired thickness of the fiber. By controlling the temperature and draw speed the disered thickness may be maintained.

In the model for the drawing process the radiation plays a major role. Therefore the view factors for radiation are recomputed when the shape of the preform changes. Assuming 1D velocity field the shape may be efficiently computed from a simplified equation.

The picture shows the temperature field resulting from a simple furnace geometry.