Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ELECTROLUMINESCENT D SPLAY DEVIL S
_C'KG~OUI~I) OF TIE INVENTION
conventional thin-film electroluminescent displays
are multi layer devices consisting of a glass substrate, a
transparent conductor, a phosphor layer between two ins-
feting layers, and an opaque back conductor generally alum-
numb Light is produced in the phosphor layer by the applique-
lion of an AC voltage to the two conductive layers, and
emission occurs as a result of the excitation of activator
ions by injected electrons.
A major concern in using any type of display, for
example in an aircraft instrument panel, is the ability of
the viewer to read the display under conditions of high
ambient illumination, e.g. direct sunlight. Under such
conditions, the opaque back conductor which has high metallic
reflectance reflects more light than the electroluminescent
element emits, and so the display is rendered ineffective.
There have been many attempts to provide a phosphor-
emitting display device that operates with a high contrast
ratio under high ambient illumination conditions with a mint
mum sacrifice of emission brightness.
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Solutions to the problem include the incorporation of a
dark material into the glass or of a dark dye into the phosphor
dielectric layer; these methods proved unsatisfactory because they
result in the reduction of the intensity of the emitted light as
well as of the reflected light. An overlay of a perforated opaque
layer on the viewing side of the glass substrate of the device has
been proposed, but this is unsatisfactory because it restricts the
viewing angle.
The incorporation of a light-absorbing layer on the
viewing side of the back conductor was suggested in US. Patent
No. 3,560,784. The absorbing materials have an index of
refraction substantially equal to that of the phosphor layer and
provide a contiguous and continuous transition at the interface of
thy phosphor layer and the l.. ight-absorbing layer, thus minimizing
the reflection of light on this interface. The light absorbing
layer is substantially transparent at the interface and a
gradually increasing concentration of light-absorbing material is
introduced within the dark layer toward the back layer. This
method has the disadvantages of requiring elaborate material and
process control as well as complex apparatus to create this
tapered concentration of absorptive materials within a thin film
layer.
SUMMARY OF THE INVENTION
It has now been found that the problem of reflection
when the display is viewed under conditions of high ambient
illumination can be eliminated by employing as the black layer a
material that is capable of combining the interference effect with
the ability to absorb the light transversing through, that is,
induced absorption.
According to the present invention there is provided in
a thin-film electroluminescent display device comprising a glass
viewing surface, a transparent conductor, a light-emitting
phosphor layer between two insulators, and an opaque back
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conductor, the improvement which comprises including in said
device a layer of light absorptive material having optical
properties such that incident light is refracted into said layer
and absorbed therein upon multiple internal reflection being the
surfaces thereof so as to minimize the reflection of incident
light and thereby improve contrast on the viewing surface.
BRIEF DESCRIPTION OF THE DRAWINGS
-
Fig. l is a simplified diagrammatic view showing the
passage of incident light between an insulating layer and the back
conductor in an electroluminescent display device.
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Fig. 2 is an enlarged cross-sectional view of a
conventional electroluminescent display device.
Fig. 3 is an enlarged cross-sectional view of the
preferred embodiment of this invention.
Fig. 4 is an enlarged cross-sectional view of an-
other embodiment of this invention.
Fig. 5 is an enlarged cross-sectional view of still
another embodiment of this invention.
'it. is an enlarged cross-sectional view of an-
lo other embodiment of this invention.
D~TAI IT DfiJSC~IPTION OF THY INVENTION
Fig. shows a conventional display that consists
of a Klaus substrate 1, a transparent conductor 2, a trays-
parent insulator 3, a phosphor layer 4, an insulating layer
5, and an aluminum conductor 6. The aluminum layer has a
high metallic reflection which, when used under high ambient
lighting conditions such as direct sunlight, reflects more
light toward the observer than the electroluminescent eye-
mint emits and thus renders the display ineffective.
Fig. 3 illustrates the preferred embodiment of the
present invention. The first layer 12 deposited directly on
the glass substrate 11 is a transparent, electrically conduct
live coating. This first layer is generally a composition of
tin oxide and indium oxide. Glass substrates with this coat-
in are available commercially, for example Nesatron glass.
The second layer 13 and the fourth layer 14 are
insulators required to prevent electrical breakdown. Such
materials as yttrium oxide (YO-YO), alumni oxide (Allah
tantalum oxide (Tao), or the like are suitable for these
layers.
The third layer 15 is the phosphor which is the
source of light. Suitable materials include, but are not
limited to, ZnS:Mn(orange), CaS:Ce(green), SrS:Ce(blue),
ZnS:Te,Mn(red), and the like.
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The fifth layer 16 is the black layer to improve
contrast. It is a hard film of a semi-conducting highly
dispersive material. It has a suitable optical dispersion
in the visible spectrum to create the desired induced Abe
sorption between the dielectric layer 14 and the back metal
electrode layer 17 of the thin-film electroluminescent disk
play. Examples of suitable materials include, but are not
limited to!, lanthanum hexaboride (Lab), chromium oxide
(Cry), titanium oxide (Shea, vanadium oxide (VOW), and
lo boron carbide (BC4). These materials are by nature refract
tory ceramics and as such have an intrinsic stability against
short circuit and blow-out.
The final layer 17 is the back conductor, Usually
aluminum.
As illustrated in Fig. 1, the incoming light 18 is
made to reflect between the two boundaries of the black
layer 16 so that each time it traverses through the black
layer a portion is absorbed; in other words, the light is
trapped by the black layer. In addition, very little light
is reflected from the black layer, so it is an effective
antireYlection material.
The black layer luff of this invention it preferably
incorporated in front of he back electrode 17, as in Fig.
3. It is, however, within the scope of this invention to
place it in other locations in an electroluminescent display
device, for example as a replacement for insulation layer 14
(Fig. 4), between insulation layer 13 and the phosphor 15
(Fig. 5), or as an electrode to replace the back electrode
17 (Fig. 6).
- 30 Although the thicknesses of the layers are not
critical, in general the coating 12 on the glass is about
200 to AYE, and preferably about AYE; the phosphor layer
15 is about 1,000 to AYE, and preferably about AYE,
between two films of electric insulation 13 and 14, each
about 500 to AYE, and preferably about AYE each; the
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aluminum electrode 17 is about 200 to Lowe, and prefer-
ably about Lowe; and thickness of tube black layer of this
invention 16 depends upon the material selected. When a
layer of Lab is used, it may be about 200 to AYE, and
preferably about AYE; a layer of Two is generally about 400
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to Lowe thick, and preferably about AYE; a layer of V203
is generally about 300 to Lowe thick, and preferably about
AYE; a layer of Cry is generally about 200 to Lowe
thick, and preferably about AYE; and a layer of BC4 is
lo generally about 50 to AYE, and preferably about loo. The
small amount of' residual reflection may appear green, magenta
or told, depending upon the optical thickness of the black
layer or the position of the quarter wave peaks in the visit
bye spectrum. Thus the color can be varied by changing the
thickness of the layer.
Experiments were carried out to illustrate the
effectiveness of various materials as the black layer in
electroluminescent (EL) display devices. When used with an
aluminum electrode, the material was deposited between it
and the insulating layer 14. The displays were tested with
a Sylvania SG-77 Sun Gun, and the results are tabulated below:
Material Two Lab V203 Cry BC4
O O O O O
Vacua Hartz AYE AYE AYE AYE Lola
monitor Thickness
Vital Appearance metallic metallic metallic metallic metallic
in Reflection yellow blue purple silver
Visual Appearance absorbing absorbing absorbing absorbing brownish
in Transmission grew grew grew brown grew
Electric 100 to 100 to 3,000 >20 >100 >100
Resistivity<3,000 ohms Amy. megaohm/sq. megaohm/sq.
ohms
El Test without * >100 ft.- - - -
Al eastward (a) Bert
El Test with $ >100 ft.-
Al Electrode (a) lam Bert (a) _ (a)
(a) = works satisfactorily
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From the above it can be seen that, if the nests-
tivity is sufficiently low as in the case of Two and Lab,
the material can be used directly s the back surface elect
trove.
S In each case the black layer improved the contrast
to the point where the gun could be placed six inches in
front of the display (a third of the distance used to Sims-
late sunlight and, therefore, nine times the brightness)
without obliterating the glow of the display.
I The use of this light-interfering black layer no-
dupes to a minimum (about one per cent) the reflectance from
both interfaces, the one between the black layer and the
adjacent layer toward the viewer and the one between the
black layer and the back metal conductor. In edition, there
is no sacrifice of emission brightness, brightness levels of
more than one thousand foot-lamberts being obtained, in
contrast with about one hundred foot-lamberts for typical
television Cuts. Moreover, lifetimes in excess of guy
hours were achieved.
The products of this invention can be made by any
known and convenient means. Preferably, however, such thin-
film electroluminescent display devices are produced by
vacuum deposition, such as electron beam evaporation tech-
piques, resulting in the production of large area substrates
with high resolutions. Generally the entire structure is
sealed to prevent contamination from the external environ-
mint.
The display devices of this invention are suitable
for use as electroluminescent panels, e.g., numerical disk
plays or other types of information display panels such as
are used in aircraft instrument panels, computer terminals,
word processors, and the like, in the form of vertical scale
displays, round dial displays, illuminated retitles, matrix
displays, and so forth.
