Cathodoluminescence (CL) characterization in a demountable vacuum chamber is an important benchmarking tool for flat- panel display phosphors and screens. The proper way to perform these measurement is to minimize the effects of secondary electrons, excite the phosphor/screen with a uniform beam profile, and maintain a clean vacuum environment. CL measurements are important for preliminary evaluation and lifetesting of phosphor powders and screens prior to incorporation into the FPD. A survey of many CL characterization systems currently in use revealed the myriad of spectroradiometers, colorimeters, electron guns, vacuum pumps, mass spectrometers, etc. that introduce many avenues for error that are often difficult to isolate. A preliminary round-robin experiment was coordinated by Sandia and involved five other research groups. The purpose of this experiment was to obtain an indication of equipment capabilities and instrument variations, as well as reliability and consistency of results. Each group was asked to measure the luminance and chromaticity coordinates of a Y3Al2Ga3O12:Tb pellet and calculate the luminous efficiency. Pellets were chosen in order to reduce errors associated with processing and handling of powders or screens. Some of the data reported in this experiment were in good agreement, while others differed significantly. Determining sources of error in CL measurements is an ongoing effort. By performing this experiment, we were able to identify some of the causes of error and develop a characterization protocol for display phosphors.