The fundamental properties of a flow cytometer that govern its capacity to detect and resolve dim fluorescent particles are 1) the efficiency with which it can convert a photon emitted by a fluorochrome into a photoelectron at the photocathode of the photomultiplier tube and 2) the amount of optical background noise that is collected along with the fluorescence emission from the particle. Either of these properties alone is insufficient to predict an instrument's performance. Thus, to characterize a flow cytometer, both of these properties need to be determined. To determine these properties, one must not only consider the optical characteristics of the instruments but also understand the impact of the signal processing on the data collected. After the collection efficiency and the optical background noise level are determined, it is possible to predict the instrument's performance. A specific flow cytometer performance level is not unique to only one pair of these properties. To achieve a specific level of instrument performance, whole families of pairs of these properties are possible.