Note: Descriptions are shown in the official language in which they were submitted.
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PHI 21200 1 16.6.1984
Cathode far tube,
The invention relates to a cathode I tube
having a faceplate arrangement for suppressing halo, said
arrangement comprising a faceplate consisting essentially
of a transparent material and an internally disposed thin
5 film luminescent screen having an index of refraction ?
larger than that of the faceplate and having opposing
surfaces, one of said surfaces being a light scattering
surface disposed further from the faceplate than the
other
lo Cathode ray tubes can be operated at higher
electron beam currents, and thus it higher brightness
levels if the conventional powdered layer luminescent
screen is replaced with a thin film luminescent screen
capable ox operating at higher temperatures. This improve-
mint in brightness is offset, however, by the adverse effects of multiple reflections within the thin film
screen, Thin film screen cathode ray tubes are especially
useful in projection systems because of the high bright-
news required in these systems.
I Such a cathode ray tube with a thin film screen
is known from Patent 20 00 173 and also from GB-patent
20 24 842.
US. Patent 4,310~783 discloses a cathode ray
tube faceplate construction including a multi layer absorb
bring filter disposed between a faceplate and a lupines-
cent screen for reducing halo by attenuating light rays
multiply-reflected within the jilter, which would other-
wise contribute to halo. This absorbing filter not only
reduces halo, but also usable light. In an alternative
embodiment disclosed in the patent, the absorption filter
is combined with a multi layer layer halo suppressing
inference filter disclosed in US. Patent 4,310,784~ This
interference filter is angle sensitive to provide low
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I I
PHI. 21.200 2
observer side reflectance and high screen side reflect-
awns Such a combination of a multi layer interference
filter on a multi layer absorption filter is overly come
placated.
It is an object of the invention to provide a
simple cathode ray tube faceplate arrangement which effect
lively suppresses halo.
It is another object of the invention to provide
swish faceplate arrangement which suppresses halo without
substantially reducing luminescent light which does not
contribute to halo.
According to the invention the cathode ray tube
comprises an intermediate thin film layer disposed
between the screen and the faceplate, said intermediate
layer having a refractive index smaller than that of the
faceplate.
Said faceplate arrangement may include according
to the invention multi layer interference filter disposed
between the sereen.and the faceplate, the layers of said
interference filter having alternating lower and higher
refractive indices, one of said layers being said inter-
mediate thin film layer.
The present invention will now be described by
way of example with reference to the:aecompanying drawings,
in which:
Figure 1 issue sectional view of one end of a
cathode ray tube faeeplate:and.a lens in a projection
system employing a prior art cathode ray tube;
Figure 2 is a schematic diagram showing the
30 angular distribution of light rays emitted from the
cathode ray tube screen in the system of Figure l;
Figure 3 is:a:seetional view of one end of a
cathode ray tube faceplate.and.a lens Inca projection
system employing first embodiment of a cathode ray
tube in:aeeordanee with the invention; and
Figure 4 issue sectional view of one end of a
cathode ray tube faceplate and a lens in a projection
system employing a second embodiment of a cathode ray
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PHI. 21.200 3
tube in accordance with the invention.
The objects of the invention are accomplished by
providing a faceplate arrangement which not only sub Stan-
tidally prevents -transmission of light rays that would
ordinarily contribute to halo, but which also partially
converts these rays to usable light which does not contra-
byte to halo, thereby increasing image brightness and
improving contrast. The manner in which this is accom-
polished can be best understood by referring to Figure 2
which graphically depicts as a function of emission angle
the distribution of light rays emitted from any excited
point on the luminescent screen. This figure illustrates
only the principal sources of light transmitted through
the faceplate-air interface Andy ignores the relatively
weak rays IIH which are derived from light rays that have
been largely transmitted through interface 23. Further,
the rays I'M do not derive from rays originally emitted
at any particular band of: angles, but from rays duster-
butted over the entire range of angles outside eCFA -ecpF
Andre thus dispersed over a larger of the face-
plate arrangement thereby preventing their collective
contribution Tony localized halo effect. This is not
true of the rays IT, however, which are high intensity
rays deriving from fully reflected rays emitted in the
screen:at.angles within the well defined band of angles
ecFA -ecpF. In accordance with the invention, the light
rays emitted from the screen within this band of angles
ware largely converted to rays IT which are reflected
back toward the scattering surface, which redirects part
of the rays toward the interface at angles within the
useful band of angles Ox - Cole This conversion is
effected by disposing between the faceplate and -the
screen thin film intermediate layer of a material
ha~i.ng:an index of refraction which is sufficiently
smaller than that of the faceplate to decrease the angle
ecpF toga value near that I 9CFAI thereby causing
reflection of rays within band of angles which would
otherwise haze contributed to halo. The refractive
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PHI. 21~200 4
index of the intermediate layer should be smaller than
that of the screen material.
The intermediate layer may be provided as the
sole layer between the faceplate and the screen or in
combination with other layers disposed between the face-
plate and the screen. In one embodiment the intermediate
layer is incorporated as one of the layers of an inter-
furriness filter, which further improves performance of the
faceplate arrangement for a narrow band of wavelengths
near the primary emission wavelength of the luminescent
screen, by converting a large part of both the rays IT and
IT to rays IT which are reflected toward the scattering
surface. This arrangement has the advantage that it can
be designed to convert spurious rays having wavelengths
outside the narrow band to rays IT which totally miss the
lens in a projection system, thereby reducing chromatic
aberration.
The multiple reflections are illustrated in
Figure 1, which depicts part of a cathode ray tube project
lion system including the right end of a cathode ray tube faceplate arrangement 10 spaced from a focusing lens 12,
both shown in cross-section. The lens 12 magnifies an
image formed by light rays received from the faceplate
arrangement lo and projects the image onto a relatively
large reflective or transmissive screen (not shown). The
arrangement 10 includes a faceplate 14 made of a material
having goon thermal conductivity, such as sapphire, and a
thin film luminescent screen 16 deposited onto the face-
plate. Typical thicknesses for the faceplate and the
screen, which are not drawn to scale, are 2 5 millimeters
and 1-3 microns, respectively.
Although Figure 1 is not drawn to scale, it
demonstrates conceptually the effects of multiple reflect
lions within the thin film luminescent screen. Because
the refractive indices of luminescent screen materials are
higher than -those of conventionally used faceplate mater-
tats, a very small percentage of light emitted by the
excited screen succeeds in reaching the lens 12. For
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PEA 21200 5 1~6.1984
example in a projection cathode ray tube having a
sapphire faceplate 14 with a refractive index no = 1.8
and a thin film luminescent screen 16 with a refractive
index no = 2.3, the amount of emitted light actually lea-
vying the faceplate was determined to be less than 5/0. This amount can be doubled by covering the inner surface of
the screen with a highly reflective layer 18 of a material
such as aluminum, thereby reflecting light directed toward
the vacuum of the tube back toward the faceplate. A further
lo increase in the amount of light reaching the lens can be
achieved by roughening the inner surface of the screen 16,
such as by chemically etching this surface before applying
the reflective layer 18. The roughened surface 20 serves to
scatter light emitted within the screen and reflected from a
15 faceplate-screen interface 21 such that some of this light
is redirected toward the interface at angles for which
there is less reflection and more light directed toward
the lens.
The reflective layer 18 and the scattering
20 surface 20 not only increase the amount of useful light
reaching the lens 12, however, they also increase light
contributing to halo surrounding the image of the electron
beam spot focused by the lens.
The manner in which light rays emitted by the
25 luminescent screen are transmitted through the faceplate
arrangement 10 can be best understood by referring to Fig-
no 1 which shows a plurality of light rays emitted at
different angles from a point 22 in a spot excited by an
electron beam 24. All angles are measured relative to a
30 line 26 originating at point 22 and passing perpendicularly
through the faceplate-screen interface 21 and a faceplate-
air interface 28. All light rays emitted toward the inter-
face 21 are at least partly reflected back toward the
scattering surface 20 as rays IBM where they are scattered
35 and redirected toward the interface. Light rays emitted at
angles equal to or greater than the critical angle OCPF for
total internal reflection prom the interface 21 are totally
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PEA 21200 6 16.6.1984
reflected to the scattering surface 20. Part of this
light is redirected toward the interface 21 at an angle less
than ecpF and passes through the interface The lateral
shift between point 22 and the point at which the
reflected rays impinge on the scattering surface 20 are
typically on the order of the sickness of the thin film
screen 16 (erg. I microns) and -thus does not substantially
increase the diameter of the luminescent electron beam spot,
which is typically about 100 microns.
lo The light rays emitted from point 22 which pass s
through the faceplate-screen interface 21 reach the face-
plate-air interface 28. Portions IL of these rays, emitted
from point 22 at angles between 0 and equal pass through
interface 28 and are collected by lens 12. Portions IT
5 emitted from point 22 at angles between Cool and ~CFA (the
critical angle for the face-plate-air interface) totally
miss the lens and are lost within the system. A portion
IT or IT of each ray reaching the interface 28 is reflect
ted, passes through or is reflected by interface Al and
20 eventually returns to and passes through interface 28. The
lateral shifts between the point 22 and the points at which
the rays IT and IT eventually pass through the interface
21 are on the order of the faceplate thickness (e.g. 2-5
millimeters). These laterally-shifted rays form a number
25 of concentric ring-shaped halos around the image of the
electron beam spot, causing a decrease in image contrast
Figure 3 illustrates a first embodiment of a
cathode ray tube faceplate arrangement including a thin
film halo suppression yen in accordance with the invent
30 Sheehan The face plate arrangement 30 includes the same face-
plate 14, thin film screen 169 reflective layer 18 and
scattering surface 20 as the arrangement in Figure 1, but
further includes a thin film layer 32 disposed between the
faceplate and the screen. The layer 32 consists essentially
35 of MgF2 having a refractive index no = 1~38, which
substantially decreases the angle OCPF from that of the
prior art Figure 1 embodiment. This is demonstrated by
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PHI 21200 7 16.6.1984
Table 1 which lists the angles ~CFA and ecpF for the
Figure 1 and Figure 3 embodiments, The smaller band of
angles lying between ~CPF and ~CFA is also apparent from
the rays shown in Figure 3.
TABLE
Fig. 1 (prior art) Fig, 3.
lo ~CFA Sweeney no = 26 sin 1 = 26
~CPF Sweeney F _ 52 sin no 37
lo
The thickness of -the intermediate layer 32 is not
critical, but should be greater than one-half the wave-
length of the light emitted by the screen to prevent
interference effects and should be substantially smaller
than the diameter of the luminescent spot produced by the
electron beam For an intermediate layer thickness of
0.8 microns it has been determined that the exemplary
arrangement shown in Figure 3 will reduce halo intensity
by a factor of three and increase image rightness by a
factor of two.
Figure 4 illustrates a second embodiment of a
cathode ray tube faceplate arrangement in which a thin
film halo suppression layer in accordance with the
invention is incorporated into an interference filter. The
faceplate arrangement 40 includes the same faceplate 14,
thin film screen 16, reflective layer 18 and scattering
surface 20 as the arrangement in Figure 3, but the halo
suppression layer 42 also serves as a low refractive index
layer in the multi layer interference filter 44 which has
alternating low and high retractive indices. The halo
suppression layer 42 need not be disposed on the thin
film screen 16 itself as is shown, jut may serve as any
one of the low refractive index layers in the filter 44.
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PHI 21200 8 1606~198~
A ray diagram is not presented in Figure 4 because of the
difficulty in illustrating the operation of the inter-
furriness filter, but the angles illustrated in Figure 3
would be identical in the I use 4 embodiment.
Roth the thickness and the refractive index of
the halo suppression layer 42 will be determined by the
same criteria as for the Figure 3 embodiment. The
refractive indices of the remaining layers in the
interference filter 44 are not critical, but the differ
lo fence between the refractive indices of any two adjacent
layers should be as large as possible to maximize the
reflection of rays originating from point 22 at angles
between COLE and OF The thicknesses of the layers in
the filter 44 are very important and are determined by use
of conventional techniques such as those described in Born
and Wolf Principles of Optics Pergaman Press, Thea
edition, 1980. The thicknesses of the layers are selected
to provide a pass band centered around the primary wave-
length of luminescent light emitted by the screen 16.
An exemplary 8 layer interference filter has
been designed for use in a face plate arrangement, such
as that of Figure 4, having a primary emission wavelength
of 5440~ and refractive indices and thickness as listed
in Table I The layers are listed in order of successive
distance from the screen 16, with layer A corresponding
to the halo suppression layer lo The materials used for
this filter are MgF2( = 1.38) and Ins (no = 2.3).
TABLE 2
Layer A B C D E F G H
Refractive 1~382.3 1~382.3 1.382.3 1.38 2.3
Index
Thickness
.62 .39~62 .39.62 .79~62 .79
(microns)
_. .
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PHI 21200 9 16,6.1984
It has been determined that in comparison
with the prior art faceplate arrangement of figure 1,
the above described filter will reduce halo by a
factor of I increase image brightness by a factor of 5,
increase image brightness by a factor of 3, and reduce
spurious wavelength emissions by a factor of 3.
0
