U.S. patent number 3,622,996 [Application Number 04/864,076] was granted by the patent office on 1971-11-23 for electroluminescent display.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Navy. Invention is credited to James L. Bath, Donald C. McIntyre.
United States Patent |
3,622,996 |
Bath , et al. |
November 23, 1971 |
ELECTROLUMINESCENT DISPLAY
Abstract
An electroluminescent display panel controlled by AC flooding
and exciting oltages of different frequencies for obtaining rapid
response times.
Inventors: |
Bath; James L. (New Brighton,
MI), McIntyre; Donald C. (Roseville, MI) |
Assignee: |
The United States of America as
represented by the Secretary of the Navy (N/A)
|
Family
ID: |
25342474 |
Appl.
No.: |
04/864,076 |
Filed: |
October 6, 1969 |
Current U.S.
Class: |
345/78 |
Current CPC
Class: |
G08B
5/36 (20130101); G09G 3/30 (20130101) |
Current International
Class: |
G09G
3/30 (20060101); G08B 5/22 (20060101); G08B
5/36 (20060101); G08b 005/36 () |
Field of
Search: |
;340/166,336 ;313/38B
;315/169 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yusko; Donald J.
Claims
What is claimed is:
1. A thin film, electroluminescent display apparatus having a fast
turn-on time, a relatively low-current drain and a low
deterioration of the light emitting material utilized,
comprising
a sheet of photoluminescent material;
an electrical insulating layer contacting each side of said
sheet;
rows of transparent electrical conductors disposed on one
insulating layer and columns of electrical conductors disposed on
the other insulating layer to define a rectilinear array of light
emitting areas;
means for continuously applying across said rows and columns of
electrical conductors a uniform AC flooding voltage of an amplitude
just below that necessary to excite said photoluminescent material
into fluorescence;
means for applying across preselected conductors of said rows and
said columns an AC excitation voltage of a predetermined pulse
repetition rate and of a frequency that is different from said AC
flooding voltage to produce an optical display; and
means for minimizing the light flicker due to the changing phase
relationship between said AC flooding voltage and said AC
excitation voltage.
Description
The invention described herein may be manufactured and used by or
for the Government of the United States of America for governmental
purposes without the payment of any royalties thereon or
therefor.
This invention relates to electroluminescent display devices and,
more particularly, to a method for obtaining rapid response times
from the individual elements in a multielement, thin-film,
electroluminescent display panel.
One of the main problems with a digitally addressed, thin-film,
electroluminescent-matrixed display is the relatively long turn-on
time of the phosphor elements. This turn-on time has precluded the
use of these thin-film panels for animated displays or for the
display of fast-changing parameters. The concept which, when
employed, permits rapid response times for the elements is called
"flooding." Flooding is the process by which each of the elements
are continuously excited to a point immediately below illumination
level. When an individual element is to be activated, a small
additional voltage is applied across this element. The resultant
turn-on time is reduced to the order of a few hundred microseconds.
Except for some design advantages, a DC, DC; a DC, AC; or an AC, AC
excitation-flooding voltage may be used to decrease the turn-on
times of the thin-film elements. In order to minimize element
deterioration and flooding power requirements, AC excitation and
flooding voltages having different frequencies are used in the
preferred embodiment.
It is therefore an object of this invention to provide a method and
apparatus for increasing the response times of electroluminescent
devices by providing them with a flooding voltage at a point just
below their excitation levels.
It is a further object of this invention to provide thin-film
electroluminescent devices with an AC source of excitation voltage
and an AC source of flooding voltage.
Other objects, advantages and novel features of the invention will
become apparent from the following detailed description of the
invention when considered in conjunction with the accompanying
drawings.
FIG. 1 is a representation of the electroluminescent display panel
showing the application of flooding and excitation voltages
thereto;
FIG. 2 is a drawing indicating the construction of a typical
thin-film electroluminescent display panel;
FIG. 3 is a schematic diagram of one system for applying flooding
and excitation voltages to the individual elements in a thin-film
electroluminescent display panel; and
FIG. 4 is a circuit diagram of the AC switch shown in FIG. 3.
Referring to FIG. 1, a typical electroluminescent display panel 1
with a symbolic digital display is shown. At the edges of this
display are shown row conductors 2 and column conductors 3. These
conductors pass through the panel from one side to the other so
that connections to the conductor may be made at either end. The
aforementioned flooding voltage, shown diagrammatically at 4, is
applied at 5 and 6 to all of the column conductors and all of the
row conductors, respectively, by the system shown in FIG. 3. This
effectively applies the flooding voltage across each individual
electroluminescent element by creating a potential across the
points that each of the row conductors overlies each of the column
conductors. In the panel and as will be described subsequently, the
row conductors are separated from the column conductors by two
insulating layers and a layer of electroluminescent material
sandwiched therebetween. When the flooding voltage is applied in
the manner described, a potential is created across the
electroluminescent material in much the same way as a potential is
developed across the plates of a conventional capacitor. The
flooding voltage is set such that the potential thus created is
only slightly less than that necessary to cause the
electroluminescent material to fluoresce. The excitation voltage,
shown at 7, is applied across those row and column conductors
selected by logic 8 to activate the electroluminescent material
sandwiched between the points of intersection of the selected row
and column conductors. The potential at these points of
intersection or overlapping is thus raised above that necessary to
cause fluorescence. The materials sandwiched between overlapping
conductors along with the overlapping portions of the conductors
are referred to hereinafter as thin-film electroluminescent (TFEL)
elements. A typical TFEL display panel is fabricated as shown in
FIG. 2.
Vacuum-deposited, electroluminescent thin-films have been employed
to provide a uniform brightness over large area display surfaces
used for visual readout. One such display panel currently available
is provided with the above crossed electrode or conductor
configuration. This is a 258.times. 258 array of 66,564 individual
elements in a 7 -inch square active area to provide a linear
resolution of 36 elements per inch. Its active element is a
phosphor film which, because of its lack of granularity, provides
high resolution. The intersection area of a pair of crossed
electrodes or conductors accurately defines and locates the area of
a specific light-emitting element. There can be no displacement or
positional flicker of this solid-state element since the position
of the intersection does not change. The topmost electrode or
conductor is transparent to enable the luminescence to be visible.
Wide angle viewing is inherent in this type of display since all
elements are in one plane near the top or front surface of the
panel.
FIG. 2 shows the laminar construction of the panel in cross
section. The panel is constructed on a glass substrate 10 which
forms the front surface of the display. On top of the substrate are
deposited a series of transparent conductors in parallel rows. One
such conductor is shown in cross section at 11. In one embodiment,
these conductors were tin oxide layers etched into the substrate. A
layer of dielectric or insulating material 12 is then deposited
over the transparent conductors and onto the substrate. This
dielectric film is also transparent to allow the luminescence to be
seen. Over the dielectric film is deposited a film of
electroluminescent material 13 which in one embodiment is a
phosphor. Another insulating layer or dielectric film 14 is then
deposited over the phosphor. This film has an index of refraction
equal to that of the electroluminescent film and is nearly opaque
to absorb any ambient light which may enter through the substrate.
If all of this light is absorbed, the contrast between the light
from the display and the background will be at a maximum.
On top of this last insulating layer are deposited a series of
metal electrodes 15 which are layed out so that they are in
parallel rows perpendicular to the parallel rows of transparent
conductors. When excitation voltage 7 is applied across the
phosphor, it luminesces or fluoresces as shown at 16, emitting
light 17 through the substrate towards the viewer.
It will be appreciated that with the appropriate insulating
materials and phosphors crosstalk between the elements can be
reduced. Crosstalk occurs when more than one element fluoresces
when only one element is activated. A portion of the circuitry
shown in FIG. 3 not only allows for rapid turn-on of each element
but also further reduces crosstalk.
While the flooding and excitation voltages may both be DC, AC
driving and flooding voltages have the advantage of slower element
deterioration and less power drain over DC--DC or DC--AC
combinations. In the above-mentioned panel, AC voltages of 740
volts peak to peak have been used to drive the display depending on
the frequencies of the voltages employed.
It is a finding of this invention that flooding the panel just
described decreases the time necessary to turn on an element to a
few milliseconds from turn-on times on the order of seconds. A
further finding is that with AC flooding an excitation voltage of
substantially different frequencies not only is turn-on time
greatly decreased over other flooding excitation arrangements but
current drain is diminished and element deterioration is greatly
reduced. This result was unexpected because previous tests showed
that as flooding voltage was increased, turn-on time also
increased. However, as the flooding voltage was increased past a
certain point, turn-on time decreased. If the flooding voltage is
set so that each element is dimly lit, the turn-on time is faster
than the response time of most photomultipliers. Even with the
elements dimly lit, contrast between the activated and inactivated
elements is good. There is, however, a trade-off between fast
turn-on time and light output from the panel. As the turn-on time
is made to increase, light output decreases. With AC--AC
flooding-excitation systems in which two frequencies of AC voltage
are used, this trade-off is less critical. In the embodiment shown
in FIG. 3, a 400 Hz. flooding voltage was used with an excitation
voltage. This excitation voltage has a carrier frequency of 20 kHz.
with a pulse repetition rate of 40 Hz. Flooding voltage was 480
volts peak to peak with an excitation voltage of approximately 740
volts peak to peak in the above embodiment.
FIG. 3 shows a portion of the circuit used to drive the TFEL
display. In this embodiment, the flooding voltage is generated at
supply 20 with a frequency of 400 Hz. The voltage amplitude is
regulated by a variac 21 to a point at which the elements shown
diagrammatically at 22 are dimly lit. This voltage is coupled
across each element through transformer 23 and resistors 24, 25, 26
and 27, which are crosstalk suppression resistors. Capacitors 30
and 31 are provided for additional crosstalk suppression. Without
this portion of the circuit, there is no path from suppression
resistor to ground for the excitation voltage. This is due to the
inductance of transformer 23. To complete a low-impedance path for
the excitation voltage, capacitors 30 and 31 are added across each
side of the flooding supply transformer to center tap. The value of
these capacitors is chosen so that impedance to the 400 Hz.
flooding supply is high compared to the impedance to the excitation
voltage derived from oscillator 32 which drives power amplifier 33
at 20 kHz. with a pulse repetition rate modulation of 40 Hz. This
excitation voltage is coupled across elements 22 through
transformer 34 and AC switches 35, 36, 37 and 38. It will be
appreciated that switches 36 and 37 must be activated to turn on
the shaded element 22. A switch capable of switching the AC
excitation voltage to this element is shown in FIG. 4 and discussed
hereinafter. It was discovered that the slippage in phase
relationship of the 400 Hz. flooding voltage and the 40 Hz. pulse
repetition rate resulted in modulation of the light output of the
elements. The elements would become bright or dim as the frequency
changed phases with respect to each other. A 10 millihenry coil 39
is shown shunted across the column driver load to minimize the
amplitude of this modulation. Also, by adjustment of the frequency
to obtain a maximum slippage rate of one-half the repetition rate,
the flicker of the light modulation can be made unnoticeable.
FIG. 4 is a schematic diagram of an AC switch capable of connecting
one side of the excitation voltage to a particular
electroluminescent element. It is composed of three transistors 46,
47 and 48 and five diodes 49, 50, 51, 52 and 53. Transistors 46 and
47 are cascaded to form a bias control for transistor 48. Diode 49
is connected in parallel with the base to emitter of transistor 47
so as to ensure that any spurious noise or transient voltages that
might tend to reverse bias and destroy the transistor 47 emitter to
base junction will be conducted by diode 49 and thereby limit
reverse emitter to base voltage to approximately 0.6 volts. When
transistor 48 is biased into conduction, the first half cycle of
the excitation voltage flows from common driver 54 through diode
51, transistor 48 and diode 53 to load output 55, having been
blocked at diodes 52 and 50. The second half cycle flows from load
input 55 through diode 52, transistor 48 and diode 50 when
transistor 48 is biased into conduction. The second half cycle is
likewise blocked at diodes 51 and 53. This diode arrangement serves
to shunt the two half cycles of the excitation voltage through
transistor 48 and, as such, completes the AC switching circuit.
* * * * *