IEEE TRANSACTIONS ON ELECTRON DEVICES
1963
63
Two-Color Storage Tube*
L. s. YAGGYt,
MEMBER, IRE,
N. J. KODAI,
MEMBER, IRE, AND
Summary-A direct-view storagetube
has been developed
which is capable of displaying stored information in either of two
colors or in intermediate hues. The operating principle is based on
the fact that flood electrons which pass through the holes in the
storagemaskwhen
the storage-surfacepotential is near cutoff
fall within small areas on the viewing screen opposite the storagemask holes; whereas flood electrons which pass through the holes
when the potential is near its maximum, i.c., flood-gun-cathode
potential, fall on larger overlapping areas on the viewing screen.
The viewing screen consists of a pattern of two phosphors: one
occupies small areas in register with the storage-mask holes, and
the otheroccupies areas surrounding the first.The tube, which has a
10-inch diameter, can be operated with simple circuitry, essentially
the same as thatrequired for conventional half-tone storage tubes.
When the performance of this tube is compared with that of conventional half-tone storage tubes, it can be seen that a moderate
sacrifice in resolution was made in order to replace
the half-tone
monochromatic range with a system of two primary colors and
intermediate hues. This system permits the color coding of stored
information that provides a vivid qualitative or semi-quantitative
representation of a variable in the display. The design and performanceconsiderationsfor the tubes are
discussed and future
investigations for this type of tube are outlined.
I. INTRODUCTION
ECENT INCREASED interest in multicolor storage tubes for use in airborne and search radar,
sonar, and other specialized displays pointed out
the need for further investigation of these devices. Early
developmentalworkonstoredmulticolordisplayswas
summarized by Beintema, et al.’ The multicolor storage
tube developed in that program had several limitations.
Among these were the limitationin the maximum size
of the flat-faced envelope, the low maximum luminance,
and the relatively poor resolution. In addition, the tube
was complicated by the operational requirement that the
deflection and convergence fields be critically aligned.
Thus, a simpler color storage tube that could overcome
these limitations, even at thesacrifice of some of the color
range, was required.
A two-color storage tube meeting the above
requirementhas been developed andtested.Fig.
1 shows a
schematic of the tube in its
present form. It resembles
the 10-inch monochromatic storage tube in physical size
as well as in operating voltages and currents. The change
in writing-gun grid drive produces variation of brightness
c. D. BEINTEMA‘f, MEMBER,
IRE
/-REI
-
I
/
’I
I
FLOOD ELECTRONS--/
REDPHOSPHOR
GREEN PHOSPHOR
Fig. 1-Schematic
of two-color storage tube.
and color in the two-color display; therefore, color coding
of information is a function of video drive. The tube hasa
flatfaceplate, a concentric flood cathode, and an axial
writinggun with electrostatic focus andmagnetic deflection.
11. PRINCIPLE
OF OPERATION
The operation of the two-color storage tube is simple
and basically identical to that of the conventional directview halftone storage tube.2Briefly, in the unwritten state,
the dielectric surface is charged negatively with respect
to the flood-gun cathodeandtherefore
repels the lowenergy flood electrons back toward the collector. Where
the high-velocity writing beam has charged the dielectric
in a positive direction by secondary emission, flood electrons penetrate the storage mask (backing electrode) and
arethen
postaccelerated tothe
viewing screen. The
amount of charge placed on the dielectric by the writing
beam controls the number of flood electrons penetrating
the storage mask and thus the brightness of the screen.
Color shift is effected by the use of a phosphor screen
having an array of phosphor dots corresponding to and
aligned with the holes in the storage mask of the storagetarget assembly. Surrounding each registered phosphor
“dot” is a “donut,” or field of a second phosphor. This
arrangement is shown schematically in Fig. 1. The stored
charge pattern is presented on the viewing screen as in
other
direct-view storage tubes, but the color displayed
* Received August 14,
1962;
revised manuscript received,
depends on how far the storage surface is written in the
October 1, 1962. The work reported here was supported by the
Bureau of Ships, Department of the Navy,underContract
No. positive direction.
Nobsr 81199.
The color-shift principle is better understood by
conVacuum Tube Products Division, Hughes Aircraft Company,
sidering the electron optics at each storage-mask aperture
M. Knoll, H. 0. Hook, and R. P. Stone, “Characteristics of a
transmission control viewing storage tube with halftone display,”
PROC.
IRE, vol. 42, pp. 1496-1504; October, 1954.
64
TRANSACTIONS
IEEE
ON ELECTRON DEVICES
region. Equipotential plots of individual storage-mask
holes indicate that each hole acts as a strong converging
lens as a result of the negatively charged storage surface
in combination with the electric fieldcaused
bythe
positive storage-mask potentialand
positive viewingplate potential. Convergence causes each beam to cross
over near thestorage mask and then diverge as it proceeds
toward the viewing plate. Near cutoff potential VI only
a few electrons pass through the holes; when the storage
dielectric potential is nearflood-gun-cathode potential V,,
more electrons passthrough the holes and cover larger
areas at the viewing screen. Fig. 2 shows this behavior.
The trajectories areforthe
outermost flood electrons
which can penetrate the storage-target aperture for insulator potentials VI, V,, and V3. These trajectories define the flood-beam envelope from each storage-target
aperture for the respective insulator voltages. The floodbeam-spot size at the screen will therefore depend on
the number and divergence of the electrons penetrating
the storage mask, i.e., on the amount that the dielectric
has been charged bythe writingbeam. The emission
spectra of the phosphor composing the L L d ~ist different
”
from that of the phosphor composing the “donut;” thus a
different color is associated with each. Initially, only the
phosphor “dots” are excited, but as the storage dielectric
becomes more positive, the increased number of flood
electrons diverge and excite both phosphors, which then
emit a varying mixture of the two colors. Since the full
color shift occurs a t each storage-mask aperture,the
resolution capability of the tube can be relatively high.
March
BACKING
ELECTRODE
rORAGE
Fig. 2-Flood
electron envelope vs storage surface potential.
111. DESIGNCONSIDERATIONS
A . Color-Shift Range
The range of colors available was one of the primary
considerations in the design of the tube. The color-shift
range depends on the flood-electron optics at the viewing
screen and the phosphor combinations used. These considerations are discussed below.
1) Choice of Red-Green Color System: The International
Commission onIlluminationchromaticitydiagram
provides a logical arrangement of all of the colors visible to
the human eye according to the tristimulus theory. I n a
two-color system, the range of hues available when two
primary colors are mixed is given by the straight line
connecting the two primaries on thechromaticity diagram.
The partial I. C. I. diagram3 in Fig. 3 shows that the
largest number of perceptibly different hues available in
a two-primarysystem are found in the region between
red and green.
Thenature of thetube operation is such that one
primary can be excited singly, but the other primary can
be excited only in combination with the first. If red is
x COORDINATE
Fig. 3-Partial
I. C. I. chromaticity diagram.
chosen for the singly excited primary and green for the
other primary, the extensiveness of the green area on the
I. C. I. diagram andthe high efficiency of the green
phosphor would permit the resulting color, a t full brightness, to reach yellow-green since the green luminance can
override the red luminance a t full brightness.
I n order to achieve the maximum range with the red
and green phosphors, the relative phosphor areas on the
viewing screen are important. The ratio of green to red
phosphor luminance should be a t least 5:2 in orderfor
the full brightness hue to be a t least in the yellow-green
region
of the I. C. I . diagram. If a 5:l relative phosphor
3 K. L. Kelly, “Color designation for lights,” J . Res. IVBS, vol.
31, pp. 271-278, (RP 1565); November, 1943. K. L. Kelly,“Color
efficiency
is assumed, a ratio of green to red phosphor
designation for lights,” J . Opt. SOC.Am., vol. 33, pp. 627-632;
areas of a t least 0.5 is necessary. A ratio greater than this
November, 1943.
1963
Storage
Two-Color
Yaggy, et al.:
Tube
will produce higher brightness in the green at the sacrifice
of brightness in the red. The expected range of hue, then,
is red, orange, yellow and finally, yellow-green.
2 ) Lens Action of the Storage-Mask Hole: As mentioned
in Section II.,the
lens action a t eachstorage-mask
aperture produces the color shift in the two-color storage
tube. Therefore, the maximum size of the beam envelope
from each storage-mask aperture is of interest for colorrangeconsiderations since it willgovern theextent of
color mixing. Otherinvestigators’havestudied
the behavior of beam envelopesize
as different parameters
are varied. These and other data obtained from electron
trajectory plot,s show that the tangent condition for flood
beams from adjacent storage-mask holes is met for the
usual storage-target parameters. I n fact, an overlapping
beam condition is encountered in most direct-view storage
tubes a t full brightness; thus full coverage of the componentphosphorscan
be achievedwithout
difficulty.
Thetechnique of phosphorprinting
bythe sensitized
polyvinyl alcohol process and the degree of registration
attainable between the storage mask and phosphor screen
pattern are two factors which limit the number of holes
per inch in the storage mask.
3) TargetXpacing E$ects: The storage-target-electrode
spacings (storage-mask-to-collector-mesh spacing and
storage-mask-to-viewing-screen spacing) are of interest
because of their effect on color shift. Studies’ show that
variation of spot size with collecting field issmallfor
electric gradients greater than about 1 kv/inch. The collector field in the two-color type is approximately 1 kv/
inch, indicating that small changes of 10 per cent in the
collecting field dueto
spacingvariation
would have
negligible effect on color shift.
The previous data also give an empirical relation which
shows that flood-beamenvelope size from each storagemaskaperture varieswith viewing-screen potential according to
D, X KV;;l4,
(1)
where
DB = beam diameter
K = constant
V s 8= viewing-screen potential.
Substituting time of flight and the power approximation
for tangential velocity (v, cc ET;,,where E,, is the viewingscreen field) when otherparametersare
held const’ant
gives
65
B. Color-ShiftUniformity
A consideration which is equally important as therange
of hues available in the tube is the uniformity of color
shiftover the usable area of the display. For uniform
color shift, assuming aconstant charge on the storage
dielectric, it is necessary that the flood-electron beams
fromthe individualstorage-mask holes be centeredon
their respectively aligned phosphor “dots” and spill onto
the second phosphor to the same extent for equalcharging
a t all areas of the display. Thus, uniformity of color shift
over the target area depends on several factors: 1) mechanicalregistrationbetween
the phosphor(‘dots” and
the storage-mask holes, 2 ) protection from external magnetic fields, 3) uniformity of flood-beam collimation, and
4) uniformity of storage characteristics.
1) Registration: The registration between the phosphor
dots and the storage-mask holes has been controlled by
good mechanical design of the storage-targetassembly.
Misregistration between the storage mask and the phosphor screen pattern also can occur if thereisparallax
between the exposure light rays (during phosphor screen
printing) andtheactual
flood-beam trajectory. In the
beginning, a point-source exposing light was used during
phosphorprinting. I n orderto minimize the parallax,
the storagemaskwas
moved to within 13 mils of the
viewing screen during printing with the light
source 24
inches away. Even with this distance
between the light
source and the storagemask, however, the problem of
parallax caused a 2-mil misregistration. A new “lighthouse” assembly, shown in Fig. 4, was then used which
had an off-axis spherical mirror to collimate the exposure
light for normalincidence. Calculations (see the Appendix)
show that the expected minimum misregistration should
be considerably reduced with this arrangement.
I n addition to eliminating this misregistration, another
important advantage wasgained by theuse of a collimated
light source. Formerly the storage-mask-to-viewing-screen
distance was readjustedfrom
13 mils tothe
normal
operating distance after the printing operation;
now the
phosphorprintingoperationis
accomplished withthe
storage mask at the normal operating distance from the
viewing screen. This is important becausemechanical
readjustment, which is potentially harmful to registration,
is minimized.
2) Flood-Beam Collimation: Flood-beam collimation affectsthe uniformity of the storagecharacteristics, Le.,
the cutoff potential,and therefore the color-shift uniformity. An expression for thevariation
of storagetarget cutoff voltage AV,, with the flood-beamangle of
incidence at the storage target is given by4
where a isaconstantand
S is the storage-mask-toviewing-screen spacing. This relation shows that if X is
doubled, the viewing-screen potential must be increased
N. J. Koda, N. H. Lehrer, and R. D. Ketchpel, “Twenty-one
by a factor of eight in order to obtain the
samefloodinch direct-view storage tube,” 1957 IRE WESCON CONVENTION
beam diameter at the viewing screen.
RECORD,
pt. 7, pp. 78-86.
-.
66
-
TRANSACTIONS
IEEE
i-
ON ELECTRON DEVICES
March
larger for a given 0 variation than that in the axial case.
3) Flood Gun: Tests on the first tube showed that the
individualfilamentarycathodes
produced a nonunifom
luminance patternon the screen. This nonuniform pattern
was caused bythe voltagedrop across the individual
filaments and by the fact that electrons from each filamentarycathode do notdistributeoverthefulltarget
area.Unipotentialcathodes
whose electrons cover the
whole target area are desirable for this reason. Consequently, a new arrangement of multipleconventional
point-source flood guns with unipotential cathodes placed
symmetrically in a ringwas constructed. Such an arrangement was developed for use in the conventional 10-inch
Tonotron‘ tubes.
IT.PERFORMANCE
Fig. &Photograph
of collimated “lighthouse”.
where Vcmis the collector potential and e is the floodelectron angle of incidence from the normal. Theangle of
incidence is usuallydeterminedbyelectrontrajectory
plots obtained from the electrolytic tank.
For deflection symmetry, it is desirable to havethe
writinggunon
the tube axis and the flood-gun source
displaced off the axis. However, this imposes a critical
requirement on theflood gun and collimation lens design.
With an axial flood-gun source, a well-designed lens can
collimate the flood beam so that all electron trajectories
terminate on the target within averysmalldeviation
from normal incidence. Spherical aberration of the lens
cannot be avoided since almost the full diameter of the
lens is being utilized, andsuchaberrations
cause the
trajectories to depart slightly from the normal incidence
and affect the uniformity of the storagecharacteristics
in a circularly symmetric fashion.In theoff-axis flood-gun
case, the same lens can provide small variations in angle
of incidence over the target, but the average direction of
approachisnot
that of normal incidence. Aberration
affects the collimation in an asymmetricfashion; therefore,
the asymmetry must be smoothed out with multiple offaxis sources arranged in a ring. While circular symmetry
is restored by thismeans, collimation errors have a greater
effect on uniformity of storage characteristic than in the
axial flood-gun case. The flood electrons now arrive at an
average angle e from the normal even at perfect c o l l i r n ~
tion, andthequantity
inside the parenthesisin (3) is
The over-alltransfercharacteristics
of the two-color
storage tube are illustrated in Fig. 5. Here, the relative
contributions to brightness made by the two phosphors
are plotted against the writing-gun grid drive. The input
tothetube
consists of voltage excursions from cutoff
potential on the writing-gun control grid, and the output
consists of light of various hues, indicated here by the
separate luminance curves for the twophosphors.
The procedure by which Fig. 5 was obtained began with
the determination of color shift as a function of storagesurface potential. The direct measurementof chromaticity
requires extensive equipment which was not available for
this work. However, knowing the I. C. I. coordinates for
the two phosphors, it is possible to obtain the desired
curves withrelatively
simple equipment: a brightness
meter of good sensitivity, linearity, and spectral response
approximating that of thehuman eye, and twofilters,
one for each phosphor. The red filter needs only to pass
ameasurablepart
of the light from the red phosphor
while absorbingvirtuallyallthelightfromthe
green
phosphor and thegreen filter, to do the converse. Spectral
curves for phosphors and filters wereused to select the
filters. Using thisequipment,the
techniques consists
simply of taking three curves of brightness vs storagesurface potential: without filter, with red filter, and with
green filter. If these curves are taken with care, such that
values are repeatable, then they haveexactly the required
shapes, and all that is lacking is the true scale for the
ordinates of the curves taken through filters. Theeffective
filter factors and hence the true scales of brightness can
be obtainedby solving simultaneously any two of an
infinitenumber
of equations which are obtainedby
recognizing that the true ordinates of the green curve
plus the true ordinates of the red curve must equal the
ordinates of thecurvetakenwithout
filtersfor corresponding abscissa values:
+ nB,(‘VJ
=
B(VJ
mBu(VJ -I-a ( V A
=
B(VJ,
mB,(VJ
6
“Tonotron” is a trademark of the Hughes Aircraft Company.
Yaggy, et al.: Two-Color Storage Tube
Fig.6-Photograph
67
of theexperimental tube-color storagetube.
Physical size:
Screen size:
Deflection system:
Color range:
GRID DRIVE, VOLTS FROM CUTOFF
Fig. 5-Over-all
transfer characteristics.
where
E, = brightness measured through green filter,
23, = brightness measured through red filter,
B = brightness measured without filter,
m
5
n
ZE
V
E
effectivefilter
factor for green-filter-phosphor
combination,
effective filter factor for red-filter-phosphor combination,
storage-surface potential.
The answers obtained from different pairs of equations
were in good agreement.Toconvertthe
abscissa units
from storage-surface potential to volts of grid drive, data
were taken independently which relate the two variables
at the specified writing speed.
I n Fig. 3, this same information has been translated6
into the language of the I. C. I. chromaticity diagram.
The numbers shown on the figure are grid-drive voltages.
I n interpreting these over-all characteristics,several
limitations should be recognized:
They apply directly only to the indicated scanning
rate (writing speed) and focus adjustment. For approximate purposes, they can be adapted to other
scanning rates by applying the same factor to both
scanning rate andgrid drive,
Thesediagramsapply
totheadjacent,
parallel
scanning lines or individual, separate lines. If any
overlap of lines takes place, the required grid drive
drops accordingly. Defocusing of the writing beam
requires greater drive for a given hue.
model
photograph of thecompletedexperimental
isshown in Fig. 6. Typicalmechanicalandelectrical
characteristics of thetubearesummarizedas
follows:
6 The line connecting the qoordipate? for the two phosphor? is
divided for eachvalue of gnddnveinto
twosegmentshavlng
lengths in the ratio of the separate phosphor brightness shown m
Fig. 5.
Resolution:
Luminance:
Persistence:
Writing speed:
Viewing screen:
Storage mask:
Collector mesh:
Collimating can:
Collimating voltages:
body dag7 No. 1
No. 2
No. 3
Anode (both guns) :
Cathode, flood gun:
Cathode, write gun:
Heater, write gun:
H.eater, flood gun:
10 4 inches maximum diameter;
18 inches maximum length
7 inches minimumdiameter
Magnetic
Red, orange, yellow,
yellowgreen
Approximately 60 lines per inch,
(relative luminance 80 per cent)
Approximately 100 ft. L.
Approximately 15 min
30,000 to 40,000 inches/sec
The writing speed can be interchanged for more storage time
if this is desirable.
10 kv
20 volts, dc
50 volts, dc
25 volts, dc
+
30 volts, dc
8 volts, dc
15 volt’s,dc
40 volts, dc
0 volts, dc
-3.5 kv, dc
6.3 volts, ac
6.3 volts, ac
Controlled persistence, Le., electronically controlled slow
decay of storedinformation,often
used in direct-view
storage tubes, is not possible on this tube without a shift
of color as the information is erased. For storage without
color shift, the maximum viewing time is less than 100
per cent duty cycle since all information is erased by a
single pulse within a fraction of a second and the complete
writing of one frame usually takes several seconds in a
radar-type display.Shouldnearly
100 percent viewing
time be imperative, a solution to this problem exists, in
principle, by the method of selective erasure,‘ where the
storedinformation is immediately erased justahead of
7 Envelope dag electrodes we numbered in sequence from the
tube neck (Fig. 1).
8 N. .H. Lehrer, “Selective erasure and
nonstorage writing in
IRE, vol. 49, pp. 567-573;
direct-mew halftone storage tubes,”PROC.
March, 1961, also published as Research Rept. No. 146, Hughes
Research Laboratories, Mahbu, Calxf.
March
writing ofnew information. Selective erasure in directviewmonocolor
storage tubesis
considered practical,
and the combinat’ion oftwo-color storageand selective
erasure wouldbe
a desirable objective for future dcvelopment.
V. CONCLUSION
The principle of color shift in the two-color direct-view
storage tube is based on the change in the flood-beam
envelopecoming
fromeach
storage-mask hole as the
writing beam charges the storage dielectric in the positive
direction. The successful development of the tube from
the concept stage is the result of careful attention tofloodelectron t,rajectories in the tube. Perhaps the
most difficult
objective to meet was that of uniformity ofcolor shift
over the usable area of the tube. The factorswhich helped
achieve this objective were the following: I) The filamentary-cathode ring Aood gun was replaced by multiple
unipotential cathode flood guns to eliminate the pattern
caused by the potential drop across each filament. 2) The
“lighthouse” was redesigned to substantially improve the
registration between the phosphor pattern and storagemask holes. 3) The flood-electron collimation system was
analyzed
both
theoretically
and experimentally for
optimum flood-electron collimation. 4) The storage-target
assembly was carefully designed and assembled for accuracy andstability under tube processing inorder to
maintain registration.
Perhapsthe most important limitation to thetube
performance is the shift of colors with controlled erasure.
A selective erasure capability in the tube mould be highly
desirable to overcome this limitation.
1 7 -
Fig. 7-Spherical collimating
CENTER OF CURVATURE
mirror error calculation.
inch diameter useful area of the mirror with the center
5 2 inches fromtJhe axis, it can beseen that the component of error perpendicular to the plane of tilt is small
for any point in the area by virtue of either the small
distancefrom the point to the axis or the small angle
between the plane through the axis containing the point
and the plane of the tilt.
The maximum error can be reduced further by moving
the point source slightly toward the mirror and toward
APPEXDIX
the target such that rays reflected from all parts of the
DESIGN
OF “LIGHTHOUSE” FOR PRIKTING PHOSPHOE
mirror no longerconverge
butpart
diverge andthe
If a point source of light is located a t a distance of one remainder converge over the remainder of the mirror.
half the radius of curvature from a spherical mirror, the With the proper source position and target tilt, normal
rays reflected from the mirror willbe
approximately incidence can be achieved a t two widely spaced points
parallel to an axis of symmetry which consists of the on the target, and the largest error at any point can be
mirrorradius containing the point source. The angular made small indeed.
I n practice, two adjustments were made: 1) tilt of the
deviation 0 of the reflected ray canbe expressed as a
function of the angle which the incident ray makes with mirror in the plane through the point source and target
axis, and 2) distance from the point source to the mirror,
the axis, as shown in Fig. 7.
For a mirror with a 60-inch radius of curvature, a ray motion being along a line through the center of the 8 3reflected from a point on the mirror 10 inches from the inch diameter area of the mirror under the target. With
axis makes an angle of 0’14’ with the axis. A ray passing thesetwo adjustments, any of the condit’ionsdescribed
above can be obtained.
through a storage-mask hole a t this angle willbemisregistered with respect to a normal projection of the mesh
ACKNOWLEDGMENT
on the viewing screen, which is a distance of D away, by
Theauthors gratefully acknowledge the counsel and
d = D tan 0’14’.
advice given by H. M. Smith throughout the developBy tilting the target assembly approximately 0°07‘, the mental program. R. 5. Morrison was primarily responsible
aboveerrorcanbedistributed
over thetarget, giving for the fabrication of the experimentaltubes, and his
approximately one half the maximum error at theextreme contributions to over-all mechanical design of the tube
edges. This tilting can onlyreduce the component of error were substantial. The support and encouragement given
parallel to the plane defined by system-axis and light- throughout the contract by the Naval Electronics LaboratoryandtheBureau
of Ships are mostappreciated.
source; however, by considering a plan view of the 8
*-