New Screening Process
for Manufacturing Color
Picture Tubes
Istvan Gorog
Thomson Consumer Electronics
A new process uses electrophotographic techniques in manufacturing
color picture tubes. It is less expensive, reduces energy and water consumption, and produces little waste.
regions of the world, the need for
CRTs increases.
CRT plants are typically organized as
“lines.” A plant’s production line receives incoming components, principally glass, metal parts, electron
guns, and deflection yokes, and produces finished tubes. Phosphor
screening is a key operation in the
line process.
Practically all picture tube manufacturers worldwide use a slurry-based
process, wherein mixtures of the
phosphor powders and a photosensitive material are applied as thin layers
onto the glass, are optically exposed,
and subsequently developed by washing away the unexposed regions.
Glass
+Ill>
Phosphors or
Rlack Matrix
DEVELOP
ElectroPhotographic Screening Process
Introduction
More than 150 million color television picture tubes, also called color
cathode ray tubes (CRTs), are produced worldwide annually. Color
tubes are the principal display components in virtually all TV sets and in
most personal computers. Manufacturing CRTs is an industry that is still
growing, particularly as consumer
demand rises in the developing economies of South America, China, and
the former Soviet Union. Also, as
new information age applications
emerge in the more developed
Concept Description
Thomson Consumer Electronics has
developed an alternative screening
process to the slurry-based process,
ElectroPhotographic Screening
(EPS). EPS uses electrophotographic
techniques requiring less capital
investment, reducing energy and
water consumption, and producing
practically no waste.
The EPS process sequentially deposits three kinds of cathodoluniinescent materials, phosphors, in precisely controlled, interleaved patterns
onto the inside surface of the front
glass or faceplate of the picture tube.
In the first step, the inside of the faceplate is coated with a conductive
layer. In the next step, a photoconductive layer is coated over the conductive layer (see figure la). The
surface of the photoconductor is subsequently charged uniformly with a
corona charger, and the charged surface is then exposed. The shadow
mask (shown in figure 1 a) serves as
the optical exposure mask, the same
as in the conventional slurry process.
In a finished CRT, the shadow mask
serves to selectively constrain the
scanning electron beams so that they
strike only the appropriate phosphor
elements.
During phosphor screening, the
shadow mask is placed in its precisely predetermined position with
respect to the glass faceplate and is
appropriately illuminated. The optical system is arranged so that the
illuminating light rays accurately
mimic the electron trajectories of the
scanning beams during normal tube
operation. Optical elements provide
a projected light source arrangement
that produces the required illumination (see figure la). As a result of
the exposure, the photoconductor discharges the surface in the illuminated
regions, while it retains the surface
charge in regions shadowed by the
mask.
The next step is phosphor development (see figure lb). Here, the developer housing and developer grid
comprise a suitable process developer
unit, on top of which the exposed
glass plate is placed face down, i.e.,
with its charged surface facing inside
the developer unit. In the developer
unit, electrically charged phosphor is
blown against the charged plate. As
shown in figure lb, the surface
charge is positive, causing positively
charged phosphor particles to be
repelled by the surface charge to
accumulate only in the previously
illuminated and discharged regions.
The charge, expose, and develop
steps are repeated in sequence three
times, with the light source arrangement in the exposure step appropriately moved each time to produce the
desired interleaved red, green, and
blue phosphor patterns.
In the final step, the phosphor particles are fixed in position by exposure
to intense infrared radiation that
softens the photoconductor. Fixing
ensures that the phosphor particles do
not move during subsequent phases
of tube manufacturing.
Economics and Market
Potential
Studies have shown an estimated
energy savings of 4 kWh per tube
produced with the EPS process over
the conventional slurry phosphor
screening of CRTs. Further, phosphor use is expected to be significantly reduced. For example, in the
conventional process about 75% of
the phosphor applied to the faceplate
is removed during development of
the CRTs. Some of the phosphor
wasted during application and development is lost; some of it is captured and recovered in a complex
chemical process that has its own
waste stream. In the EPS process all
phosphor that is applied and not
attached to the faceplate during development is captured and can be
reused after a simple sieving
operati on.
The potential market for the EPS
phosphor screening process is substantial with more than 150 million
color tubes produced worldwide each
year. If the useful life of a screening
machine is 15 years, then even in the
absence of market growth, 10 million
tubes must be replaced each year.
Key Experimental
Results
All process segments and required
materials for the EPS process have
now been completely developed.
Some patents have been obtained and
others are pending. In the laboratory,
many commercial-quality phosphor
screens have been produced and
fabricated into functioning picture
tubes. A pilot manufacturing facility
designed to produce one screened
faceplate per minute is nearing completion at Thomson's plant in
Marion, Indiana. This pilot facility
will establish process capabilities in a
higher-volume manufacturing environment. To evaluate the benefits of
the EPS process, accurate quantitative monitoring of all materials and
energy consumption will be completed as part of the pilot project.
Future Development
Needs
The advantages of EPS have been
proven in the laboratory, but it now
needs to be refined and proven in
production. An area with obvious
economic and environmental importance that requires additional research
and development is solvent recovery.
While the EPS process requires only
minor amounts of solvents to coat a
faceplate, in mass production this
adds up to substantial volumes.
Current plans are to incinerate the
exhausted solvents. Given the available technologies today, the EPS solvent stream is too dilute and too
small for economical recovery.
Nevertheless, developing a suitable
recovery technology could produce
economic benefits and reduce waste
heat. With solvent recovery EPS
could become a completely closed
system, where all input materials are
either shipped out as part of the
finished product, or are recycled
intemall y.
For more information, contact
Istvan Gorog
Thomson Consumer Electronics
1002 New Holland Avenue
Lancaster, PA 17601
Phone: (7 17) 295-6938
Fax: (7 17) 295-6489
Compiled by
Pacific Northwest Laboratory for the U.S.
Department of Energy, Innovative Concepts
Program 590240. This flier was printed on