A sample chamber for use in measuring the absorption of radiation as it passes through a gas within the chamber includes a block having an extended serpentine passage through it. The walls of the extended passage are coated with a highly reflective material so that the extended passage acts as a light pipe for transmitting the radiation. A number of smaller passages permit gases in the space surrounding the sample chamber to diffuse into the extended passage through which the radiation is conducted. The sampling chamber is made by joining two halves, each of which has a planar face in which an elongated groove is produced. The halves are molded of plastic and in quantity the chamber is quite inexpensive.