Radar System Engineering

480 THE RECEIVING SYSTEM—INDICATORS [SEC. 13.2
nary oscilloscope tubes and known as type P-1 (phosphor number
1) are almost universally used in such applications. This material
has an exponential decay with a time constant of a few milliseconds;
it is extremely efficient in terms of light intensity.
2. Applications in which persistence is needed to smooth out the
effects of flicker. The best Ameri-
,.5. can screen for this purpose, designated
P-1 2, utilizes a zinc-magnesium
104 k. fluoride phosphor which has an
exponential decay of time constant
g10’
\ \ between 60 and 100 msec (Fig. 13.4).
Because the response of the eye is
g
?7
m102
logarithmic, this screen is satisfactory
for scanning rates as slow as a
10 \l\< P14
few per second. On the other hand,
P12 the exponential rate of decrease prevents
any long-time tailing-out and
LLWl 0.01 0.1 1 10 lW
Time m sec consequent blurring of a changing
Fm. 13.4.—CRTscreencharacteristics. picture. The efficiency is somewhat
less than that of the P-1 screen.
3. Applications in which the scanning is so slow that considerable
persistence is needed to afford viewing time and to provide a composite
picture. This classification covers scanning rates from a
few per second up to one or two per minute; it includes the bulk
of present-day radars with the exception of fire-control equipment.
Long-persistence phosphors have better buildup and decay properties
under weak excitation than under strong excitation. LTnfortunately the
extremely thin layer penetrated by electrons of cathode-ray tube energies
must be strongly excited to provide enough total light. On the other
hand, if excitation by light could be used the excitation density could be
kept very low because the screen would be excited throughout. Thk is
accomplished by the simple but exceedingly clever expedient of covering
the persistent screen with a second layer of a blue-emitting phosphor
which undergoes primary excitation by the electrons. The blue light
from this layer in turn excites the persistent screen. This process
results in considerably lower over-all efficiencies than those of the singlelayer
screens.
Two varieties of such “cascade” screens are commercially available
in this country. The P-14 is suitable for frame times up to a very few
seconds, and the P-7 has a much longer persistence (Fig. 13.4). The
persistent phosphors, which emit a predominantly yellow or orange light,
are composed of copper-activated zinc-cadmium sulphides, the zinc-