An excited CRT raster is designed to give the impression of a continuously excited display without any flicker or scan line structure. High fiequeixj, field refresh rates and closely spaced scan lines obscure from the human observer the way in which each phosphor particle receives its excitation. These factors also determine how fast the writing speed of the electron beam is across a given point on the phosphor screen, and therefore how long a dwell time and how large a charge impulse each particle receives. A writing speed of one cm per microsecond would produce at each particle a charge impulse 100 nanoseconds in width (for a one mm beam spot diameter), a time much shorter than the rise or fall time of most CRT phosphors. Consequently the product of the beam current times the dwell time over a particle during each impulse may be considered to be in effect an instantaneous charge. The phosphor in this sense is an integrator.
On the opposite extreme are the refresh times between consecutive pulses, or the time consumed in writing one complete field. This is many orders of magnitude larger than the pulse width dwell times and at least one order of magnitude larger than the luminescent decay time of most phosphors used in CRT displays. Here we make the reasonable assumption that the phosphor responds independently to the separate impulses, with no accumulative effect among them, so that the charge per single pulse determines the phosphor linearity behavior. For these reasons we characterize the excitation density in terms of charge dosage per unit area per pulse rather than in terms of the per unit area beam current density.