A method is described for measuring the absolute light output from a gas or liquid in a cylindrical tube. The apparatus used is simple to build and use. The technique results in measurements of unusually high accuracy and sensitivity. The volume integration, often done with the aid of a mathematical model, is replaced by the physical measurement made by moving a calibrated luminous disk along the axis of the empty cylindrical tube.
P. H. Lee, G. A. Woolsey, and W. D. Slafer, "Integration over cylindrical luminous volumes using a calibrated piston source," Appl. Opt. 15, 2825-2829 (1976)