Determination of low bulk absorption coefficients.

For producing low-loss glass fibers it is essential to know the absorption losses of the bulk material from which the fibers are to be drawn. It has been proposed to determine the absorbed radiation power L(a) from the rise in temperature DeltaT(R) at the surface of the bulk material. The relation between the absorption coefficient alpha and DeltaT(R) is strictly derived and is used to calculate the dimensions that the sample and the wires of the thermocouple must have to make the T(4)-radiation of the sample and from the wires negligibly small. It is essential to eliminate radiation because the emissivities of the wires and of the sample are liable to change considerably with time. In conclusion it is shown that the proposed method is superior by a few orders of magnitude over the conventional methods which are based on measuring the difference between the powers at the input and output of the sample.