A storage tube also enables a storage scope to capture and
store an isolated transient event for display and subsequent
study at leisure. To a very limited extent, this can also be
achieved by an oscilloscope tube of the conventional type but
equipped with a long-persistence phosphor. This is a phosphor
which in addition to the flash or fluorescence has also an
afterglow or phosphorescence. Sometimes this is a compound
phosphor; the flash and afterglow may even be different
colours. Because the trace 'stored' in the afterglow is viewed
directly, the long-persistence tube belongs to the family of
'direct-view' tubes. There are also indirect-view storage tubes
and these are dealt with briefly at the end of this chapter.
All phosphors continue to glow for a brief period after
bombardment by electrons, see Appendix 1, but so-called long-
persistence phosphors exhibit an afterglow of a few or many
seconds, according to type. Thus an oscilloscope fitted with a
long-persistence tube displays ('acquires') a signal in real time,
but continues to display the signal after it has ceased to exist.
This is the reason that long-persistence and storage oscilloscopes
are shown in Figure 2.2 as midway between real-time and non-
real-time scopes. The persistence or length of the afterglow of
a long-persistence tube is fixed once and for all during manu-
facture, being determined by the type of long-persistence
phosphor used. (This is virutally true for all practical purposes.
However, it is reported that some ingenious but impecunious
home constructors, wishing to use a long-persistence oscillo-
scope tube for a TV display, succeeded in disabling the afterglow
by exposing the screen of the tube for a long period to bright
sunlight. Apparently the ultraviolet light gradually 'burnt out'
the component of the phosphor responsible for the yellow
afterglow, without at the same time killing the short-persistence
blue phosphor.) An oscilloscope with a degree of persistence
which could be varied at will would be a very useful machine,
and just such a capability is furnished by one of the types of
storage tubes to which we now turn. But we must start at the
beginning and look at the two basic types of storage tube, and
the fundamental principle - which is the same for both- upon
which they work.