Tests were performed on Field Emission Picture Element Technology, Inc, carbon thin film cathodes to characterize the effects of phosphors on the emission lifetime. The cathodes were tested in sealed, glass tubes in a diode configuration: The phosphors in the tubes were ZnS:AgAl (blue), ZnS:CuAl (green), and YVO4:Eu (red). After lifetesting, before and after comparisons of the cathode current versus voltage characteristics were made. Due to the large variance in the data, the results did not clearly separate all of the different phosphors' effects on lifetime. However, there was some indication that the tubes with blue phosphor had a longer lifetime than the green. The tests with the red phosphor produced mixed results with some of the devices having a lifetime similar to the blue but others having a much longer lifetime. The results with the red phosphor point to the role of phosphor screen preparation in extending the life of the cathode.