FIELD OF THE INVENTION
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The present invention relates to plasma display panels (PDPs) and more
particularly to a dynamic color temperature and color deviation calibration method for
improving image quality shown on PDP.
BACKGROUND OF THE INVENTION
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A manufacturing process of a conventional alternating current discharge type
plasma display panel (PDP) 10 is shown in FIG. 1. First, two different activation layers
are formed on glass substrates 11 and 12 respectively. Then seal the peripheries of
the glass substrates together. A mixed gas consisting of helium (He), neon (Ne), and
xenon (Xe) (or argon (Ar)) having a predetermined mixing volume ratio is stored in a
discharge space formed in between the glass substrates. A front plate 11 is defined as
one that facing viewers. A plurality of parallel spaced transparent electrodes 111, a
plurality of parallel spaced bus electrodes 112, a dielectric layer 113, and a protective
layer 114 are formed from the front plate 11 inwardly. From a corresponding rear plate
12 inwardly, a plurality of parallel spaced data electrodes 121, a dielectric layer 124, a
plurality of parallel spaced ribs 122, and a uniform phosphor layer 123 are formed.
When a voltage is applied on electrodes 111, 112, and 121, dielectric layers 113 and
124 will discharge in discharge cell 13 formed by adjacent spaced ribs 122. As a result,
a ray having a desired color is emitted from phosphor layer 123.
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The emissivity of a phosphor layer 123 is varied as panel temperature or
operating frequency of PDP changes. Accordingly, as referring to FIG.s 2, 3 and 4
color temperature change and color deviation are occurred on panel of PDP 10,
resulting in a poor image quality shown on panel. Above operating frequency is
defined as discharge number per unit time occurred on a discharge cell 13. The higher
the discharge number the higher the operating frequency will be. As shown in FIG. 2,
the higher the operating frequency the lower the emissivity of phosphor layer 123 will
be. This condition is even worse in a green phosphor layer 123. Hence, undesired
color temperature change and color deviation are occurred on the conventional PDP
as panel temperature and operating frequency increase. This in turn renders an
unacceptable image quality.
SUMMARY OF THE INVENTION
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It is thus an object of the present invention to provide a dynamic calibration
method implemented on a plasma display panel (PDP), the method comprising the
steps of: (a) utilizing laws of color matching for calculating an emissivity change of a
pixel of the PDP in response to a brightness change of one of red, green, and blue
lights emitted by a corresponding one of red, green and blue discharge cells of the
PDP through a numeric operation; (b) dynamically adjusting brightness of one of the
emitted red, green, and blue lights by increasing or decreasing strength of input video
signal of each of the discharge cells; and (c) eliminating a color temperature and a
color deviation of the PDP due to an emissivity change. The invention can render an
image having an optimum color purity and color temperature by eliminating adverse
effects on PDP due to emissivity change.
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The above and other objects, features and advantages of the present invention
will become apparent from the following detailed description taken with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
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- FIG. 1 is a sectional view of a conventional plasma display panel (PDP);
- FIG. 2 is a graph showing a relationship of emissivity of various phosphor layers
versus operating frequency measured in the FIG. 1 PDP;
- FIG. 3 is a graph showing a relationship of color temperature versus operating
frequency of a preferred embodiment of dynamic color temperature and color
deviation calibration method according to the invention;
- FIG. 4 is a graph showing a relationship of color deviation versus operating
frequency obtained by the method according to the invention; and
- FIG. 5 is a flow chart illustrating the method according to the invention.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
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Typically, an image shown on a well known PDP consists of a plurality of pixels.
The number of pixels is determined by the resolution of PDP. A pixel consists of three
discharge cells capable of emitting red, green, and blue lights respectively. Hence, the
color of a pixel of image shown on PDP is a combination of red, green and blue lights
emitted by respective discharge cell. For example, a, b, and c are gray scales of red,
green and blue lights emitted by respective discharge cell of each pixel of PDP. Also,
Ro, Go, and Bo are brightness emitted by unit gray scale of phosphor layer in red,
green and blue discharge cells of each pixel of PDP. Hence, brightness of red, green,
and blue discharge cells may be expressed by equations 1, 2 and 3 below:
brightness of red discharge cell = a x Ro
brightness of green discharge cell = b x Go
and
brightness of blue discharge cell = c x Bo
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Also, brightness of pixel may be expressed by the following equation 4:
brightness of pixel = brightness of red discharge cell + brightness of green
discharge cell + brightness of blue discharge cell = a x Ro + b x Go + c x Bo
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Further, ratio among brightness of red, green and blue discharge cells may be
expressed by the following equation 5:
brightness of red discharge cell : brightness of green discharge cell : brightness
of blue discharge cell = a x Ro: b x Go: c x Bo
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One aspect of the invention is to eliminate the adverse effect such as color
temperature change and color deviation of PDP caused by such emissivity change.
Thus, laws of color matching proposed by Grassman is utilized in which the
brightness of color emitted by each of red, green and blue discharge cells may be
calculated through a numeric operation as illustrated in the flow chart of FIG. 5.
Further, it is possible to adjust the brightness of thus emitted red, green or blue lights
by increasing or decreasing the strength of input video signal (or input voltage) of
each discharge cell. Hence, the adverse effects such as color temperature change
and color deviation of PDP caused by emissivity change as experienced in prior art
may be eliminated. As a result, an image having an optimum color purity and color
temperature is rendered.
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In one embodiment of the invention, reduced brightness per gray scale of each of
red, green, and blue discharge cells of PDP due to the panel temperature increase is
represented by TR, TG, and TB respectively. Hence, when panel temperature is
increased brightness of each of red, green, and blue discharge cells, and pixel may be
expressed in the following equations 6, 7, 8, and 9 respectively:
brightness of red discharge cell in elevated temperature environment
= a(RO-TR)
brightness of green discharge cell in elevated temperature environment
= b(GO-TG)
brightness of blue discharge cell in elevated temperature environment
= c(BO-TB)
and
brightness of pixel in an elevated temperature environment = brightness of red
discharge cell in an elevated temperature environment + brightness of green
discharge cell in an elevated temperature environment + brightness of blue discharge
cell in an elevated temperature environment
= aRO+bGO+cBO-aTR-bTG-cTB.
where aTR, bTG, and cTB are reduced brightness of each of red, green, and blue
discharge cells of pixel due to panel temperature increase respectively. Such aTR, bTG,
and cTB are the main factors for causing color temperature change and color deviation
of PDP.
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As stated above one aspect of the invention is to improve the reduced emissivity
per gray scale of each discharge cell due to the panel temperature increase and
eliminate the adverse effects such as color temperature change and color deviation of
PDP caused by such reduced emissivity. Hence, red, green and blue phosphor layers
coated on the corresponding discharge cell are used in an experiment as detailed in
FIG. 5. First analyze the reduced emissivity per gray scale on phosphor layer of each
discharge cell due to panel temperature increase. Then a temperature function is
used to calculate the reduced brightness (i.e., TRi, TGi, and TBi) per gray scale on each
of red, green, and blue phosphor layers of discharge cells and obtain expressions to
represent their relationship with respect to panel temperature of each discharge cell
as below:
ti < T < ti+1 → TRi
ti < T < ti+1 → TGi
ti < T < ti+1 → TBi
where ti and ti+1 are upper and lower temperature limits of respective portion of panel
and T is panel temperature or operating frequency. Reliable references, obtained after
repeated experiments, are used to establish a comparison table. Hence, a control
circuit of PDP may be enabled to select one of TRi, TGi and TBi from the comparison
table based on measured panel temperature T of the detected element for
dynamically calibrating strength of input video signal of respective discharge cell.
Then each of red, green and blue lights is emitted from the respective discharge cell.
Such lights in turn are used to compensate (i.e., increase) the reduced emissivity per
gray scale of each of discharge cells due to panel temperature increase and eliminate
the adverse effects such as color temperature change and color deviation of PDP
caused by such reduced emissivity. As a result, an image having an optimum color
purity and color temperature is rendered. Most importantly, the image quality of a
conventional PDP may be greatly improved by implementing the method of the
invention.
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Moreover, TRi, TGi, and TBi, i.e., reduced brightness per gray scale on respective
discharge cell due to panel temperature increase, may be expressed in the following
equations 10, 11 and 12:
TRi = kRiRo
TGi = kGiGo
and
TBi = kBiBo
where kRi, kGi and kBi are brightness compensation coefficients obtained by
experiments. akRi, bkGi, and ckBi are increased brightness on red, green, and blue
discharge cells respectively. Thus, brightness of compensated discharge cells and
pixel may be expressed in the following equations 13, 14, 15 and 16 respectively:
brightness of red discharge cell after compensation = a(1 +kRi)Ro-aTRi
brightness of green discharge cell after compensation = b(1+kGi)Go-bTGi
brightness of blue discharge cell after compensation = c(1+kBi)Bo-cTBi
and
brightness of pixel after compensation = brightness of red discharge cell after
compensation + brightness of green discharge cell after compensation + brightness of
blue discharge cell after compensation
= a x Ro + b x Go + c x Bo
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In comparison of equations 16 and 4, it is found that the reduced emissivity on
phosphor layers of discharge cells due to panel temperature increase may be
completely eliminated by the compensated red, green and blue discharge cells. In the
case that when phosphor layers are at maximum critical gray scales, discharge
number is at a maximum value. Hence, TRi, TGi, and TBi, i.e., reduced brightness per
gray scale on respective discharge cell due to the panel temperature increase, can
not be further increased by the increased discharge number, thereby an effective
compensation on the reduced brightness TRi, TGi, and TBi is made impossible. In the
case that when phosphor layers are at maximum critical gray scales, the reduced
brightness per gray scale on respective discharge cells are kRRO, kGGO, and kBBO
where kR<1, kG<1, and kB<1. kR, kG, and kB are compensation coefficients at maximum
critical gray scales of phosphor layers. After experimented, at the maximum critical
gray scales of phosphor layers brightness of red, green, and blue discharge cells may
be expressed by equations 17, 18 and 19 below:
brightness of red discharge cell at maximum critical gray scale = akRRo ;
brightness of green discharge cell at maximum critical gray scale = bkGGo
and
brightness of blue discharge cell at maximum critical gray scale = ckBBo
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Also, brightness of pixel may be expressed by the following equation 20:
brightness of pixel at maximum critical gray scale = brightness of red discharge
cell at maximum critical gray scale + brightness of green discharge cell at maximum
critical gray scale + brightness of blue discharge cell at maximum critical gray scale =
akRRO + bkGGO + ckBBo (20)
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Further, ratio among brightness of red, green and blue discharge cells may be
expressed by the following equation 21:
brightness of red discharge cell : brightness of green discharge cell : brightness
of blue discharge cell = akRRo : bkGGo : ckBBo
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In comparison of equations 21 and 5, it is found that above ratio in equation 21
has changed when panel temperature rises. Such change is the main factor that
causes color deviation of PDP and lowers image quality thereof. Hence, in another
preferred embodiment of the invention a technique is proposed to solve above
problem, that is, reduced brightness per gray scale on respective discharge cell due to
the panel temperature increase TRi, TGi, and TBi, can not be compensated at maximum
critical gray scales. In detail, at maximum critical gray scale of PDP, a control circuit of
PDP is enabled to select correct gains αi, βi, and γi from the comparison table
based on measured panel temperature T of the detected element for dynamically
calibrating strength of input video signal of respective discharge cell. As a result, the
reduced emissivity per gray scale of each of discharge cells due to panel temperature
rise is increased. Above gains αi, βi, and γi may be expressed in the following
equations 22 and 23:
αi : βi : γi = kGkB : kRkB : kRkG
max{αi, βi, γi} ≦ 1
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Further, brightness of compensated discharge cells and pixel may be expressed
in the following equations 24, 25, 26 and 27 respectively:
brightness of red discharge cell after compensation = (a kR αi)Ro
brightness of green discharge cell after compensation = (bkGβi)Go
brightness of blue discharge cell after compensation = (ckBγi )Bo
and
brightness of pixel after compensation = brightness of red discharge cell after
compensation + brightness of green discharge cell after compensation + brightness of
blue discharge cell after compensation
= (akRαi)R0 + (bkGβi)G0 + (ckBγi)Bo
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In comparison of equations 24 and 26, it is found that ratio among brightness of
red, green and blue discharge cells may be expressed by the following equation 28:
brightness of red discharge cell after compensation : brightness of green
discharge cell after compensation : brightness of blue discharge cell after
compensation = (akR αi)R0 : (bkGβi)G0 : (ckBγi)B0= aR0 : bG0 : cB0
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In comparison of equations 28 and 5, it is found that above ratio among
brightness of red, green and blue discharge cells has returned to a true ratio, resulting
in a total elimination of color deviation caused when phosphor layers are in the
maximum critical gray scales.
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As understood that a linear relationship exists between panel temperature of PDP
and operating frequency. Hence, the invention employs a control circuit of PDP based
on measured operating frequency of the detected element for dynamically calibrating
strength of input video signal of respective discharge cell. Then red, green and blue
lights are emitted which in turn are used to compensate (i.e., increase) the reduced
brightness due to the change of operating frequency. As a result, an image having an
optimum color purity and color temperature is rendered even when PDP is operated in
various operating frequencies.
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While the invention has been described by means of specific embodiments,
numerous modifications and variations could be made thereto by those skilled in the
art without departing from the scope and spirit of the invention set forth in the claims.
