Radar System Engineering

SEC.13.2] CATHODE-RAY TUBE SCREENS 481
cadium ratio governing the decay characteristics. ‘l’he blue layer is
silver-activated zinc sulphide. Since its decay is very rapid, an orange
filter is customarily used to remove its “flash,” particularly at the faster
scanning rates.
The decay of these cascade screens is an inverse power rather than an
exponential function of the time, so that the disappearance of old signals
is less clean-cut than with the P-1 and P-12 types. Unfortunately no
phosphors with exponential decays of more than about 100-msec time
constant have as yet been developed.
It is difficult to obtain sufficient light from intensity-modulated displays
during scanning, since each point on the tube is excited only intermittently.
The problem is especially acute on the slower scans, partly
because of the long time “between excitations and partly because the
cascade screens are less efficient than those with less persistence.
In intensity-modulated displays the characteristics of the screen have
important effects on the signal-to-noise discernibilityy. As has been
pointed out in Chap. 2, the energy per pulse necessary for an echo to be
just discernible is inversely proportional to the square root of the number
of pulses included in the observation. From this standpoint the screen
should enable the operator to integrate or average over the maximum
number of pulse cycles consistent with other requirements; the intensity
of each spot should represent the average of all the excitations received
over a very long time in the past. The limits within which this can be
accomplished are set by the achievable properties of the screen, and by
the degree to which past information can be retained without causing
confusion as the picture changes.
The screen properties of importance in this connection are the type
of decay and the manner in which the light intensity “builds up” under
successive excitations. To examine their effects, consider first a scan
which is so slow that either the limitations of achievable persistence or
the requirements of freedom from display confusion due to target motion
prevent appreciable storage of information from one scan to the next.
In such a case, the averaging must be done over a single pulse group.
With modern narrow antenna beams and customary scanning rates the
time occupied by this group is always short compared with the total
scanning time and achievable decay times. The screen chosen should
have sufficient persistence so that there is no appreciable decay’ during
the process of scanning across the target and the entire echo arc is observable
at one time. In order that the average intensity of this arc shall
represent all of the data, it is essential that the screen integrate the effects
of all the pulses that overlap on a given focal spot—that is, it should not
1Except for that involved in the disappearance of the secalled “flash,” which
occursfor a very short time interval during and immediately after each excitation.