Electric lamp and discharge devices – Photosensitive – Having phosphor screen
Reexamination Certificate
1999-06-15
2003-10-21
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Photosensitive
Having phosphor screen
C313S504000, C313S506000, C313S507000
Reexamination Certificate
active
06635990
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to light-emissive devices, especially devices of the photocathode type which can be used as, for example, display devices
One type of photocathode display is described in ‘Photocathode Displays’, Information Display, Vol. 13, No. 8, 1997, Society for Information Display. In this type of display a monochrome thick-film flat-panel inorganic electroluminescent (EL) dot-matrix display
1
(see
FIG. 1
) is used to project an image on to a diode intensifier
2
. The diode intensifier comprises a pair of parallel glass plates
3
,
4
with a high vacuum in a thin chamber
5
defined between them. The rear glass plate
3
has a metal coating
6
on its inner surface (the surface facing the vacuum) to act as a photocathode. The front plate has a phosphor coating
7
on its inner surface. A high positive voltage (e.g. 10 kV) is applied to the phosphor
7
relative to the photocathode
6
. Where light from the display
1
hits the photocathode
6
it causes the emission of electrons, which are accelerated by the high voltage towards, the phosphor
7
. The electrons strike the phosphor and excite it to emit light (see arrow
8
) towards a viewer
9
. To make a full colour display the front plate can be coated with a pattern of red, green and blue (RGB) phosphor dots which can each be excited by means of the display
1
.
This display has some advantages over other display technologies. However, it also has significant problems. One of the key problems is that the operation of the thick-film inorganic EL device requires very high AC drive voltage signals, often in excess of 100 V. This means that the display drive electronics are expensive and difficult to implement because they must be able to process and output display image data at high frequency and high voltage. In addition, the thick-film inorganic EL displays are also relatively inefficient.
There is a therefore a need for a device in which these problems may be at least partially overcome.
SUMMARY OF THE INVENTION
According to the present invention there is provided a display device comprising a primary light-emissive region, a light-sensitive region and a secondary light-emissive region, wherein: the primary light-emissive region comprises an organic light-emissive material and a pair of electrodes arranged to apply an electric field across the light-emissive material to cause it to emit light; the light-sensitive region comprises a photocathode responsive to light from the primary light-emissive region to release charged particles towards the secondary light-emissive region; and the secondary light-emissive region comprises a phosphorescent material excitable by the charged particles from the light-sensitive region to emit light.
Preferably the light-sensitive region and the secondary light-emissive region are operable to intensify light emitted from the primary light-emissive region. This may be achieved by the application of a suitable voltage between the photocathode and the phosphorescent material. The voltage is suitably such as to negatively bias the photocathode relative to the phosphorescent material. The charged particles are suitably negatively charged particles such as electrons.
The display device preferably comprises a plurality of primary light-emissive regions arranged in a planar array. These preferably either all lie behind the light-sensitive region and secondary light-emissive region, both of which then are preferably planar, or lie behind a respective light-sensitive region and a respective secondary light-emissive region, in which case those secondary light-emissive regions may provide different emission colours—most preferably red, green and blue. Each primary light-emissive region is preferably independently controllable by means of its electrodes. On one side of the planar array the electrodes of the primary light-emissive regions are preferably electrically connected in a first, row direction (and electrically separate in a second, column direction). On the other side of the planar array the primary light-emissive regions are preferably electrically connected in the column direction (and electrically separate in the row direction). The row and column directions are preferably (but not necessarily) orthogonal or substantially orthogonal. The row and column directions suitably correspond to the directions of rows and columns of pixels in the display device. Each primary light-emissive region may correspond to a pixel (suitably in a monochrome device) or to one colour sub-unit of a pixel (suitably in a multi-colour device). The electrodes of the primary light-emissive regions are preferably in a passive matrix arrangement, although an active matrix or another arrangement may be used. An active matrix device may use thin-film transistor (TFT) circuitry integrated with the light-emissive region. The TFT circuitry may be embodied in, e.g., poly-crystalline or amorphous silicon.
The secondary light-emissive region may comprise a plurality of phosphorescent areas. Each of these areas may comprise one of a number of phosphorescent materials. Preferably, each area comprises either a red- or green- or blue-light-emissive material. Where there is more than one primary light-emissive region each phosphorescent area preferably overlies a single one of the primary light-emissive regions.
The thickness of the light-emissive material is preferably such as to spatially redirect light to be emitted from the or each primary light-emissive region, preferably to narrow the emission cone and most preferably to redirect it directly towards the light-sensitive region and/or perpendicular to the plane (if any) of the primary light-emissive region(s). The or each primary light-emissive region may comprise either an interference or a microcavity structure for spatially redirecting light to be emitted from the region, preferably narrowing the light cone.
The or each primary light-emissive region may comprise means for altering the colour of the light emitted from that region, preferably to narrow the emission spectrum in order to narrow the energy spread of photoelectrons emitted from the light-sensitive region. One means for achieving this is a colour filter. A preferred alternative is an interference or a microcavity structure for spectrally redistributing (suitably selecting and/or narrowing) the light to be emitted from the region.
Preferably at least one of the electrodes is light-transmissive. That is preferably the electrode closer to the light-sensitive region. It is preferably the anode, but it could alternatively be the cathode.
The device preferably comprises a display driver for applying a voltage between the electrodes of each primary light-emissive region to cause it to emit light. The voltage is preferably less than 20V and most preferably less than 10V. The display driver is preferably a passive matrix display driver and is preferably connected to row and column electrodes of the device for applying a passive matrix drive scheme. The display driver may drive each primary light-emissive region to emit only at (for example) 5 Cd/m
2
or less.
To achieve grey scales the display driver may drive the or each primary light-emissive region in a pulse-width modulated format and/or a time-division multiplexed format and/or an amplitude modulated format.
The display device may be a monochrome, bi-chrome or multi-colour display, as determined by the output colours of the secondary light-emissive region(s). The display device may be used as the display of an electronics product, and a second aspect of the present invention provides an electronics product comprising such a display device. Other uses include colour converters (for converting the light colour emitted by the primary light-emissive region(s)) or image intensifiers.
The device is preferably arranged so that light from the primary light-emissive region does not (or substantially does not) escape the display device (other than indirectly, via conversion to charged particle release by means of the light-sensitive region)
Cambridge Display Technologies
Finnegan Henderson Farabow Garrett & Dunner LLP
Patel Vip
Quarterman Kevin
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