Bias shield and method of developing a latent charge image

The invention includes an apparatus 40 for developing a latent charge image formed on a photoreceptor 36 disposed on an interior surface of a faceplate panel 12. The apparatus 40 comprises a developer tank 42 having a sidewall 44 closed at one end by a bottom portion 46 and at the other end by a panel support 48 having an opening 50 therethrough to provide access to the faceplate panel 12. A back electrode 52 has a potential applied thereto to establish an electrostatic drift field between the back electrode and the photoreceptor 36, which is grounded. Triboelectrically-charged, dry-powdered, light emitting phosphor material, having a charge of the same polarity as the potential applied to the back electrode 52, is sprayed into the developer tank 42, between the back electrode 52 and the faceplate panel 12. The triboelectrically charged phosphor material is directed toward the photoreceptor 36 on the faceplate panel 12 by the applied electrostatic drift field. A bias shield 65 comprising two pairs of insulative shield members 66 and 68 disposed around a peripheral sidewall 18 of the faceplate panel 12. At least one conductive strip 72 is provided on one of the major surfaces of the shield members to repel the triboelectrically charged phosphor material from the panel sidewall 18 and to influence the deposition of the phosphor material on the photoreceptor, at the edge thereof. A method of developing the latent charge image utilizing the bias shield also is described.

The invention relates to an apparatus and method of developing a latent
 charge image on a photoreceptor which is disposed on an interior surface
 of a faceplate of a cathode-ray tube (CRT), and, more particularly, to an
 apparatus having a bias shield, and a method of operating a developing
 apparatus with the bias shield.
 BACKGROUND OF THE INVENTION
 U.S. patent application, Ser. No. 09/131,022, filed on Aug. 7, 1998 now
 U.S. Pat No. 6,007,952, and entitled, APATUS AND METHOD OF DEVELOPING A
 LATENT CHARGE IMAGE, by D. P. Ciampa et al., describes an apparatus for
 developing an electrostatic latent charge image on a photoreceptor that is
 disposed on an interior surface of a faceplate panel of a cathode-ray tube
 (CRT). The developing apparatus includes a developer tank having a back
 electrode and two pairs of panel skirt sidewall shields. The back
 electrode has a potential applied thereto that establishes an
 electrostatic drift field between the back electrode and the photoreceptor
 on the faceplate panel. Triboelectrically charged phosphor materials are
 introduced into the developer tank and directed toward the photoreceptor
 on the faceplate panel by the electrostatic drift field shown
 schematically in FIG. 1. The panel skirt sidewall shields are disposed
 around the peripheral sidewall of the faceplate panel to prevent the
 triboelectrically charged phosphor materials from reaching the sidewall of
 the faceplate panel. The panel skirt sidewall shields are formed of a
 suitable insulative material, such as ultra high molecular weight (UHMW)
 polyethylene. As shown in FIG. 2, to prevent the accumulation of phosphor
 particles on the shields, the shields are primed with positive charges
 that cancel the normal component of the electric field at the shields, so
 that the shields will not attract and accumulate the positively charged
 phosphor particles. While priming with positive charges reduces the
 accumulation of phosphor particles, it does not provide a means for
 controlling the amount of phosphor material deposition at the edge of the
 photoreceptor or of ensuring that the weight of the phosphor materials
 deposited in the peripheral areas of the photoreceptor is the same as that
 deposited in the central portion thereof A need therefore exists for a
 developing apparatus having means to provide uniform phosphor deposition
 while preventing an accumulation of phosphor materials on the shields.
 SUMMARY OF THE INVENTION
 In accordance with the present invention, an apparatus and method are
 disclosed for developing an electrostatic latent charge image which is
 formed on a photoreceptor that is disposed on an interior surface of a
 faceplate panel of a CRT. The apparatus comprises a developer tank having
 a sidewall closed at one end by a bottom portion and at the other end by a
 panel support having an opening therethrough to provide access to the
 panel. A back electrode is disposed within the developer tank and spaced
 from, but substantially parallel to, the interior surface of the faceplate
 panel. The back electrode has a first potential applied thereto to
 establish an electrostatic drift field between the back electrode and the
 photoreceptor that is grounded. Triboelectrically-charged, dry-powdered,
 light emitting phosphor materials, having a charge of the same polarity as
 the first potential applied to the back electrode, are introduced into the
 developer tank, between the back electrode and the faceplate panel. The
 triboelectrically charged phosphor materials are directed toward the
 photoreceptor on the faceplate panel by the applied electrostatic drift
 field. A bias shield is disposed around a peripheral sidewall of the
 faceplate panel. The bias shield comprises two pairs of insulative members
 having oppositely disposed major surfaces with at least one conductive
 strip provided on one of the major surfaces thereof. A suitable potential
 is provided to the conductive strip to create a surface electric field
 that directs the triboelectrically charged phosphor materials uniformly
 towards the photoreceptor and prevents the accumu on of phosphor materials
 on the bias shield.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 3 shows a color CRT 10 having a glass envelope 11 comprising a
 rectangular faceplate panel 12 and a tubular neck 14 connected by a
 rectangular funnel 15. The funnel 15 has an internal conductive coating
 (not shown) that contacts an anode button 16 and extends into the neck 14.
 Preferably, the internal conductive coating consists essentially of iron
 oxide and graphite, as is known in the art. The panel 12 comprises a
 viewing faceplate 17 and a peripheral flange or sidewall 18, which is
 sealed to the funnel 15 by a glass frit 19. As shown in FIG. 4, a
 relatively thin, light absorbing matrix 20, having a plurality of openings
 21, is provided on an interior surface of the viewing faceplate 17. A
 luminescent three-color phosphor screen 22 is carried on the interior
 surface of the faceplate 17 and overlies the matrix 20. The screen 22,
 shown in FIG. 5, preferably, is a line screen which includes a
 multiplicity of screen elements comprised of red-, blue-, and
 green-emitting phosphor strips, R, B, and G, centered in different ones of
 the matrix openings 21 and arranged in color groups or picture elements of
 three strips or triads, in a cyclic order. The strips extend in a
 direction, which is generally normal to the plane in which the electron
 beams are generated. In the normal viewing position of the embodiment, the
 phosphor strips extend in the vertical direction. Preferably, portions of
 the phosphor strips overlap at least a portion of the light absorpting
 matrix 20 surrounding the openings 21. Alternatively, a dot screen also
 may be utilized. A thin conductive layer 24, preferably of aluminum,
 overlies the screen 22 and provides means for applying a uniform potential
 to the screen, as well as for reflecting light, emitted from the phosphor
 elements, through the faceplate 17. The screen 22 and the overlying
 aluminum layer 24 comprise a screen assembly. Again with reference to FIG.
 3, a multi-apertured color selection electrode, such as a shadow mask, a
 tension mask or a focus mask, 25 is removably mounted, by conventional
 means, in predetermined spaced relation to the screen assembly. The color
 selection electrode 25 is detachably attached to a plurality of studs 26
 embedded in the sidewall 18 of the panel 12, in a manner known in the art.
 An electron gun 27, shown schematically by the dashed lines, is centrally
 mounted within the neck 14, to generate and direct three electron beams 28
 along convergent paths, through the apertures in the color selection
 electrode 25, to the screen 22. The electron gun is conventional and may
 be any suitable gun known in the art.
 The tube 10 is designed to be used with an external magnetic deflection
 yoke, such as yoke 30, located in the region of the funnel-to-neck
 junction. When activated, the yoke 30 subjects the three beams 28 to
 magnetic fields, which cause the beams to scan horizontally and
 vertically, in a rectangular raster, over the screen 22. The initial plane
 of deflection (at zero deflection) is shown by the line P--P in FIG. 3, at
 about the middle of the yoke 30. For simplicity, the actual curvatures of
 the deflection beam paths, in the deflection zone, are not shown.
 The screen 22 is manufactured by an electrophotographic screening (EPS)
 process that is described in U.S. Pat. No. 4,921,767 issued to Datta et
 al. on May 1, 1990. Initially, the panel 12 is cleaned by washing it with
 a caustic solution, rinsing it in water, etching it with buffered
 hydrofluoric acid and rinsing it again with water, as is known in the art.
 The interior surface of the viewing faceplate 17 is then provided with the
 light absorbing matrix 20, preferably, using the conventional wet matrix
 process described in U.S. Pat. No. 3,558,310 issued to Mayaud on Jan. 26,
 1971. In the wet matrix process, a suitable photoresist solution is
 applied to the interior surface, e.g., by spin coating, and the solution
 is dried to form a photoresist layer. Then, the color selection electrode
 25 is inserted into the panel 12 and the panel is placed onto a
 three-in-one lighthouse (not shown) which exposes the photoresist layer to
 actinic radiation from a light source which projects light through the
 openings in the color selection electrode. The exposure is repeated two
 more times with the light source located to simulate the paths of the
 electron beams from the three electron guns. The light selectively alters
 the solubility of the exposed areas of the photoresist layer. After the
 third exposure, the panel is removed from the lighthouse and the color
 selection electrode is removed from the panel. The photoresist layer is
 developed, using water, to remove the more soluble areas thereof, thereby
 exposing the underlying interior surface of the viewing faceplate, and
 leaving the less soluble, exposed areas of the photoresist layer intact.
 Then, a suitable solution of light-absorbing material is uniformly
 provided onto the interior surface of the faceplate panel to cover the
 exposed portion of the viewing faceplate and the retained, less soluble,
 areas of the photoresist layer. The layer of light-absorbing material is
 dried and developed using a suitable solution which will dissolve and
 remove the retained portion of the photoresist layer and the overlying
 light-absorbing material, forming openings 21 in the matrix 20 which is
 adhered to the interior surface of the viewing faceplate. For a panel 12
 having a diagonal dimension of 51 cm (20 inches), the openings 21 formed
 in the matrix 20 have a width of about 0.13 to 0.18 mm, and the opaque
 matrix lines have a width of about 0.1 to 0.15 mm. The interior surface of
 the viewing faceplate 17, having the matrix 20 thereon, is then coated
 with a suitable layer of a volatilizable, organic conductive (OC)
 material, not shown, which provides an electrode for an overlying
 volatilizable, organic photoconductive (OPC) layer, also not shown. The OC
 layer and the OPC layer, in combination, comprise a photoreceptor 36,
 shown in FIG. 6.
 Suitable materials for the OC layer include certain quaternary ammonium
 polyelectrolytes described in U.S. Pat. No. 5,370,952 issued to P. Datta
 et al. on Dec. 6, 1994. Preferably, the OPC layer is formed by coating the
 OC layer with a solution containing polystyrene; an electron donor
 material, such as 1,4-di(2,4-methyl phenyl)-1,4 diphenylbutatriene
 (2,4-DMPBT); electron acceptor materials, such as
 2,4,7-trinitro-9-fluorenone (TNF) and 2-ethylanthroquinone (2-EAQ); and a
 suitable solvent, such as toluene, xylene, or a mixture of toluene and
 xylene. A surfactant, such as silicone U-7602 and a plasticizer, such as
 dioctyl phthalate (DOP), also may be added to the solution. The surfactant
 U-7602 is available from Union Carbide, Danbury, CT. The photoreceptor 36
 is uniformly electrostatically charged using a corona discharge device
 (not shown), but described in U.S. Pat. No. 5,519,217, issued on May 21,
 1996, to Wilbur et al., which charges the photoreceptor 36 to a voltage
 within the range of approximately +200 to +700 volts. The color selection
 electrode is then inserted into the panel 12, which is placed onto a
 lighthouse (also not shown) and the positively charged OPC layer of the
 photoreceptor 36 is exposed, through the color selection electrode 25, to
 light from a xenon flash lamp, or other light source of sufficient
 intensity, such as a mercury arc, disposed within the lighthouse. The
 light which passes through the apertures in the color selection electrode
 25, at an angle identical to that of one of the electron beams from the
 electron gun of the tube, discharges the illuminated areas on the
 photoreceptor 36 and forms a latent charge image (not shown). The color
 selection electrode 25 is removed from the panel 12 and the panel is
 placed onto a first phosphor developer 40, such as that shown in FIG. 7.
 The phosphor developer 40 comprises a developer tank 42 having a sidewall
 44 closed at one end by a bottom portion 46 and at the top end by a panel
 support 48, preferably made of PLEXIGLAS.TM. or another insulative
 material, having an opening 50 therethrough to provide access to the
 interior of the faceplate panel 12. The sidewall 44 and bottom portion 46
 of the developer tank 42 are made of an insulator, such as PLEXIGLAS.TM.,
 externally surrounded by a ground shield made of metal. A back electrode
 52 is disposed within the developer tank 42 and is spaced about 25 to 30
 cm beneath the center of the interior surface of the faceplate panel 12
 and is substantially parallel thereto. A positive potential of about 25 to
 35 kV is applied to the back electrode 52 and the organic conductor of the
 photoreceptor 36 is grounded. With a spacing of 30 cm between the back
 electrode 52 and the faceplate panel 12, a drift field of 1 kV/cm or
 10.sup.5 V/m is established.
 Phosphor material, in the form of dry powder particles, of the desired
 light-emitting color is dispersed from a phosphor feeder 54, for example
 by means of an auger, not shown, into an air stream which passes through a
 tube 56 into a venturi 58 where it is mixed with the phosphor particles.
 The air-phosphor mixture is channeled into a tube 60, which imparts a
 triboelectric charge to the phosphor powder due to contact between the
 phosphor particles and the interior surface of the tube 60. For example,
 to positively charge the phosphor material a polyethylene tube is used.
 The highly charged phosphor-air mixture passes through a sealed manifold
 62 of PVC tubing which terminates in a pair of commercially available
 nozzle heads 64. The manifold 62 rotates above the back electrode 52 while
 the phosphor-air mixture is sprayed into the developer tank 42 above the
 back electrode. The electrostatic force, arising from the combination of
 the back electrode 52 being held at a high positive potential and the
 photoreceptor 36, which is disposed on the interior viewing surface of the
 rectangular panel 12, being held at ground potential, drives the phosphor
 onto the photoreceptor. To prevent the deposition of phosphor material on
 the inner sidewall of the rectangular panel 12, a bias shield 65,
 comprising two pairs of panel skirt sidewall shields 66 and 68, is
 utilized. Each of the shields 66 and 68 has two oppositely disposed major
 surfaces. The shields 66 are spaced from the short sides of the panel
 sidewall while the shields 68 are spaced from the long sides of the panel
 sidewall. The shields 66 and 68 are formed of an insulative material, such
 as UHMW polyethylene, and have a thickness of about 9.5 mm and a height of
 about 10 cm for a faceplate panel having a diagonal dimension of about 51
 cm. The pairs of shields 66 and 68 have a dielectric constant that is
 twice that of vacuum. A ground plate 70, shown in FIG. 8, is disposed on
 one of the major surfaces of the shields 66 and 68.
 To prevent the accumulation of phosphor particles on the shields 66 and 68
 and to influence the deposition of the phosphor materials, the shields,
 shown in FIG. 8, are provided with a conductive strip 72 to which a
 suitable bias potential, V, is applied. The resultant electric field is
 now established by the combination of the bias potential, V, and by the
 field induced by the potential applied to the back electrode 52. If the
 height of the conductive strip 72 is approximately 5 mm, and a potential
 of 25 kV is applied to the back electrode 52, located 25 cm from the
 photoreceptor 36 on the interior surface of the faceplate panel 12, then
 the voltage drop across a 5 mm gap, corresponding to the height of the
 strip 72, would be about 500 volts. With the OPC of the photoreceptor 36
 charged to about +300 volts, and with a bias voltage in the range of 0 to
 +4.5 kV applied to the strip 72 the bias voltage could be utilized to
 influence the deposition of the phosphor materials at the periphery of the
 photoreceptor to tailor the amount of phosphor deposited at the edge of
 the screen by providing an electric field different from that which would
 occur without the conductive strip 72. The effect of a biased conductive
 strip is summarized in the TABLE below. This TABLE contains the data of a
 series of experiments that were conducted with a shield 66 only
 constructed for the 9 o'clock edge of the screen and completely overlaid
 on its interior (opposite to the panel skirt) side with a conductive
 electrode to which a bias voltage, V, was applied. The height of the
 conductive strip 72 was approximately 5 cm and the closest edge of the
 conductive strip was approximately 0.5 cm from the photoreceptor 36, with
 the closest edge of the conductive strip substantially parallel to the
 local contour of the panel surface supporting the photoreceptor 36. As the
 bias voltage, V, was adjusted in the range of zero to 4.5 kV, and the
 developer was operated with about 25 kV applied to the back electrode 52,
 substantial bias voltage-dependent changes were observed in the phosphor
 deposit on the shield 66 as well as in the peripheral regions of the
 phosphor screen. Specifically, with zero voltage applied to the shield 66,
 i.e., with the shield grounded, the entire shield was covered with a heavy
 deposit and the peripheral screen regions were covered with a thin layer
 of phosphor. With a bias voltage in the range of 0.5 to 2.5 kV, the
 phosphor layer on the peripheral regions of the active screen reached the
 same approximate thickness as that in the center of the screen, and a
 progressively increasing phosphor-free clear zone was observed on the
 shield in the vicinity of the shield edge closest to the photoreceptor 36.
 As the bias voltage, V, was further increased, the above-described clear
 zone increased further (see TABLE) and the phosphor coverage of the
 peripheral regions of the active screen became progressively thinner.
 TABLE
 Bias Voltage (kV) Clear Zone (in) Clear Zone (cm)
 0.5 0.25 0.635
 1.5 0.69 1.753
 2.5 0.75 1.905
 3.5 1.1 2.794
 4.5 1.25 3.175
 In a second embodiment of the invention, shown in FIG. 9, the pairs of
 shields 66 and 68 have the ground plate 70 disposed on the major surface
 facing the faceplate sidewall 18. On the oppositely disposed major surface
 a plurality of conductive strips 74, 76, 78, 80, 82 and 84 are provided.
 Each of the conductive strips has a different voltage applied thereto.
 While six conductive strips are shown, it is within the scope of the
 invention to use either a greater or a lesser number of strips. In this
 embodiment, V.sub.1 =3775 volts, V.sub.N =8925 volts and the intermediate
 voltages are proportionally established to approximate the local electric
 potential that is created by the parallel plate combination of back
 electrode 52 and the photoreceptor 36.
 FIG. 10 shows the dashed equipotential lines 85 for a plurality of
 conductive strips with voltages V.sub.1, V.sub.2, V.sub.N-1 and V.sub.N
 applied thereto. The equipotential lines 85 are substantially parallel to
 the conductive strips. A high voltage, HV, within the range of 25 to 35 kV
 is applied to the back electrode 52. The resultant electric field lines 87
 are substantially normal to the direction of the equipotential lines 85.
 These electric field lines uniformly direct the phosphor materials, in
 straight lines, toward the photoreceptor 36.
 FIG. 11 shows another embodiment of the invention. In this embodiment, two
 conductive strips 94 and 96 are disposed on the major surface of the
 insulative members 66 and 68 facing the faceplate sidewall 18. A high
 resistance coating 98, made from a mixture of carbon black and a suitable
 binder, is deposited on the sidewall-facing surfaces of the insulative
 members 66 and 68, between and in contact with the conductive strips 94
 and 96. As shown in FIG. 11, the resistive coating 98 forms a resistor
 R.sub.2, in a voltage divider that further includes variable resistors
 R.sub.1 and R.sub.3. One side of variable resistor R.sub.1 is connected to
 the high voltage power supply, HV that provides the voltage to back plate
 52, shown in FIG. 7. The other side of variable resistor R.sub.1 is
 connected to the conductive strip 96. Variable resistor R.sub.3 is
 connected between ground and conductive strip 94. Variable resistors
 R.sub.1 and R.sub.3 are adjusted to provide a low potential on strip 94
 and a high potential on strip 96. The potential on strip 94 is set close
 to, but somewhat higher than, the potential on photoreceptor 36, such that
 it closely matches the local potential that would be created by a parallel
 plate combination of the photoreceptor 36 and the back electrode 52.
 Likewise, the potential on coating 98 is set to be approximately equal to
 that corresponding to the local potential what would be created by a
 parallel plate combination of the photoreceptor 36 and the back electrode
 52. The resultant potential across R.sub.2 and the shields 66 and 68 is
 adjustable to provide the desired continuous potential gradient on the
 shields to prevent the deposition of phosphor materials thereon and to
 influence the deposition of phosphor materials at the edge of the
 photoreceptor 36. The actual values of R.sub.1 and R.sub.3 are empirically
 selected. Other materials that may be used to form the high resistance
 coating 98 include resistive inks, chrome oxide, and cermet. Cermet is a
 sputter-deposited material that is described in U.S. Pat. No. 4,010,312
 issued to Pinch et al. An alternate high voltage supply, not shown, can be
 connected at point 100 of the voltage divider, to permit dynamic control
 of the electric field.