Patent Application
20100097301
1. Field of the Invention
The present invention relates to a light emitting apparatus capable of reducing nonuniformity (unevenness) in a displayed image, which is caused by temperature distribution.
2. Description of the Related Art
Heating of light emitting apparatus itself and heating of a driving apparatus, etc., which are generated when the light emitting apparatus is driven, may generate temperature distribution in the light emitting apparatus. Recently, a flat panel display used as a light emitting apparatus has been required to have a larger size, and temperature distribution generated in the light emitting apparatus has become more significant with an increase in the panel size.
The temperature distribution thus generated may be observed as nonuniformity in a displayed image depending on temperature characteristics of various members constituting the light emitting apparatus. Accordingly, it is required to compensate for temperature changes and temperature distribution.
Japanese Patent Laid-Open No. 2001-282179 discloses a cold cathode display apparatus including a resistance layer made of an amorphous silicon material, which is disposed between a cold cathode and a cathode electrode. The amorphous silicon material has a negative resistance-temperature characteristic. Thus, a resistance value of the resistance layer reduces as the environmental temperature rises, whereby luminous brightness varies. By providing a temperature sensor for detecting the temperature of the resistance layer, therefore, an amount of electrons emitted from the cold cathode can be controlled in accordance with an output of the temperature sensor.
However, the technique of controlling a signal in accordance with the output of the temperature sensor, as proposed in Japanese Patent Laid-Open No. 2001-282179, has the problems that the display apparatus is complicated in itself because of the provision of the temperature sensor, an additional circuit is required to perform sophisticated signal control, and hence the cost is increased.
An exemplary embodiment of the present invention provides a light emitting apparatus which can compensate for temperature changes and temperature distribution without making the apparatus structure more complicated.
According to one exemplary embodiment of the present invention, a light emitting apparatus includes a plurality of light emitting devices including luminous bodies, and a plurality of resistors having a negative resistance-temperature characteristic, the plurality of resistors being connected respectively in series to the plurality of light emitting devices, the plurality of resistors exhibiting different temperatures from each other during driving, wherein the plurality of resistors are made of the same material, and when the plurality of resisters are at the same temperature, one or ones among the plurality of resistors, which exhibit higher temperatures during the driving, have larger resistance value at the same temperature than other one or ones among the plurality of resistors, which exhibit lower temperatures during the driving.
With the exemplary embodiments of the present invention, variations among the resistance values of the resistors due to temperature distribution caused during driving and nonuniformity in brightness among the light emitting devices can be reduced without making the light emitting apparatus more complicated.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
An exemplary embodiment of the present invention will be described below. As illustrated in
The light emitting device 1 is just required to emit light upon application of a voltage or a current. The light emitting device 1 is, e.g., an incandescent lamp. A preferred example of the light emitting device 1 is a photoluminescence device including a phosphor, such as a plasma cell or a cold cathode/hot cathode fluorescent tube. Another preferred example of the light emitting device 1 is an electroluminescence device, such as an organic EL device, an inorganic EL device, or a light emitting diode. A more preferred light emitting device is a cathode luminescence device in which a phosphor is excited so as to emit light upon irradiation of an electron beam emitted from an electron emitting device. Any of the above-described self-luminous devices can be suitably employed as the light emitting device in the exemplary embodiment of the present invention.
When the light emitting device 1 is one of the above-described luminescence devices, it includes not only a luminous body (e.g., a phosphor (fluorescent body), a phosphorescent body, or a semiconductor junction) that emits light by itself, but also an excitation unit for exciting the luminous body so as to emit light. The excitation unit includes, for example, gas conversable to a plasma state, a discharge device for generating plasma, an electron/hole injection layer, and an electron emitting device for emitting electrons. In the case of the luminescence device, the luminous body is excited so as to emit light upon application of a voltage to the excitation unit for the light emitting device 1. When one light emitting device 1 includes a plurality of excitation units, a plurality of resistors 2 may be connected in series to each of the excitation units.
When the light emitting apparatus is constituted as a color light emitting apparatus, other light emitting devices emitting different colors from that of the light emitted from the light emitting device 1 are additionally provided. For example, when the light emitting device 1 emits red light, other light emitting devices emitting green and blue lights are additionally provided. In that case, one light emitting device 1 typically constitutes one sub-pixel (sub-picture element). A plurality of sub-pixels providing respectively different colors cooperatively constitute one pixel (picture element). As a matter of course, the exemplary embodiment of the present invention can also be applied to each of the light emitting devices emitting lights in colors differing from that of the light emitted from the light emitting device 1 by connecting a resistor in series as with the light emitting device 1. When a plurality of fluorescent tubes or light emitting diodes are used for backlight of a liquid crystal shutter device, the plurality of fluorescent tubes or light emitting diodes are each regarded as the light emitting device 1.
A configuration including the light emitting device 1 and the resistor 2 arranged adjacent to each other is here called a “light emitting unit 5”. A structure obtained by providing the light emitting device 1 and the resistor 2 in each area corresponding to one sub-pixel can be regarded as the light emitting unit 5 in which the light emitting device 1 and the resistor 2 are arranged adjacent to each other. In other words, a plurality of light emitting units 5 constitute the light emitting apparatus. In the case of a color light emitting apparatus, the light emitting units providing lights in different colors constitute one pixel, and a plurality of pixels constitute the light emitting apparatus.
The resistor 2 will be described in more detail below.
The term “negative resistance-temperature characteristic” implies a characteristic that a resistance value reduces as temperature rises. Such a resistance-temperature characteristic can be typically approximated by the following exponential function:
R=R
0exp{(Ea/kb)×(1/T−1/T0)} (1)
In the formula (1), R is a resistance value (Ω) at a temperature T(K), R0 is a resistance value (Ω) at a temperature T0(K), Ea is activation energy (eV) of a material, and kb is the Boltzmann's constant (8.617×10−5 (eV/K)). The activation energy Ea represents the magnitude of the resistance-temperature characteristic. The smaller a value of activation energy, the smaller is a change of the resistance value depending on temperature. Generally, the activation energy has a value of about 0.05 to 1 eV. Incidentally, Ea/kb is also called the “B constant”.
Further, the resistance value R0 is represented by R0=ρ0tw/1 using a volume resistivity p0 (Ωm), a sectional thickness t (m), a sectional width w (m), and a length l (m) in the direction in which a current flows, at the temperature T0 of the resistance material.
Generally, many materials having large volume resistivities are semiconductors and exhibit negative resistance-temperature characteristics in many cases. In particular, when trying to obtain a larger resistance value by reducing t and w, the volume resistivity ρ0 is increased. Also, the material having a larger volume resistivity generally has a higher level of activation energy and causes a larger reduction in volume resistivity depending on temperature changes.
A manner of applying a voltage to the light emitting device 1 and the resistor 2 is not limited to particular one. As illustrated in
An image display apparatus is obtained by providing a scanning circuit to select the light emitting device 1 to be driven, and by energizing the driving unit 4, which is connected to the wirings 3, for each of the light emitting devices 1. A modulation circuit for modulating the voltage applied to the driving unit 4 may be provided for the purpose of gradient light emission. Thus, the driving unit 4 may include the scanning circuit and the modulation circuit.
In the exemplary embodiment of the present invention, the plurality of the resistors 2 each having a negative resistance-temperature characteristic may have the other function than that of reducing an extent of the resistance distribution. Examples of the effect of the resistor 2 will be described below.
The resistor 2 has the function of dropping a voltage and limiting a current flowing through the light emitting device 1 so as to prevent the problem that an excessive current flows through the light emitting device 1 and damages the light emitting device 1. In the case of the active matrix wiring illustrated in
Further, the individual light emitting devices 1 have a variation in their characteristics for the reasons attributable to the manufacturing process, etc. Therefore, the resistors 2 also have the function of reducing the variation in characteristics of the individual light emitting devices 1 when the resistors 2 are connected in series to the light emitting devices 1. The resistors 2 having larger resistance values are more effective in reducing the characteristic variation. However, the resistor 2 having a larger resistance value simultaneously provides a larger voltage drop by itself and requires a larger voltage to drive the light emitting device 1. Accordingly, the resistance value of the resistor 2 is determined in consideration of a balance between a degree of the characteristic variation and an allowable value of the voltage drop. Another factor determining the resistance value of the resistor 2 is the relationship between the voltage applied through the wirings 3 and the brightness of the light emitting device 1. When the variation is relatively large due to the problem with the manufacturing process and the problem specific to the device, the resistor 2 is required to have a larger resistance value.
In the light emitting apparatus, temperature distribution generates due to heating of the light emitting devices 1, the resistors 2, and the wirings 3, and other heat sources, such as electric circuits including the driving unit 4. The temperature distribution is determined depending on not only structural conditions, such as the structure of the light emitting apparatus, arrangement of the electric circuits including the driving unit 4, fans for cooling the electric circuits, etc., and the shape of a chassis to accommodate the light emitting apparatus, but also operating conditions, such as an installation environment, an installation method, and a display pattern.
However, main factors generating the temperature distribution are the structural conditions, and the temperature distribution exhibits substantially the same tendency without depending on the operating conditions. For example, when the light emitting apparatus has a rectangular shape, a temperature rise is relatively small in a peripheral portion where heat is more apt to radiate, and it is relatively large in a central portion where heat is less apt to radiate. Further, when the heat sources, such as the electric circuits, and heat exhausting elements, such as the fans, are localized, the temperature distribution generates depending on arrangement of the heat sources and the heat exhausting elements. When the heat sources are positioned in a central portion of a rear surface (i.e., a surface opposed to a display surface) of the light emitting apparatus, a temperature rise is relatively large in the central portion due to the heating of the heat sources. It is hence possible to estimate the tendency of temperature distribution that generates when the light emitting apparatus is driven.
In the exemplary embodiment of the present invention, therefore, the temperature distribution in the light emitting apparatus is previously determined which is estimated to generate when the light emitting apparatus is driven. The estimated temperature distribution is one set based on the temperature distribution which is obtained when the light emitting apparatus is driven under predetermined conditions, and which is called a “reference temperature distribution”.
The predetermined conditions for setting the reference temperature distribution are such that the light emitting apparatus is driven at a predetermined environmental temperature and a predetermined gradation (or brightness) for a predetermined time.
The predetermined environmental temperature is within the range of the operating environmental temperature defined for the light emitting apparatus. In practice, the predetermined environmental temperature is desirably room temperature (e.g., 300K).
The predetermined time can be optionally determined. For example, the predetermined time may be a time during which users of the light emitting apparatus continuously operate the apparatus in average. The reference temperature distribution may be determined based on average values of temperatures at various points during the average operating time. Generally, the temperature distribution caused in the light emitting apparatus is saturated at an equilibrium state between heat generating factors and heat radiating factors. Therefore, the predetermined time is desirably set to a time necessary for a temperature change to saturate or substantially saturate. Although the time necessary for a temperature change to saturate depends on the size of the light emitting apparatus and a heat diffusion characteristic thereof, it is usually about 5 to 10 minutes, or about 30 to 180 minutes in the case of a long saturation time, counting from the start of the driving. The reference temperature distribution can be determined with higher accuracy by averaging temperature changes for a certain time after the saturated state has been reached.
The display pattern during the driving should be set to a pattern in which all the light emitting devices are turned on at a predetermined gradation described below. When there are light emitting devices emitting lights in plural colors, those light emitting devices should be all turned on. The predetermined gradation is desirably set to a gradation at which the difference between a maximum (highest) temperature and a minimum (lowest) temperature in the temperature distribution is maximized. The reference temperature distribution may be set through the steps of measuring temperature distributions at several gradations changed one by one, determining the gradation at which an average temperature distribution is obtained, and selecting the average temperature distribution as the reference temperature distribution. More specifically, the gradation may be set to be not more than 100% and not less than 20%, desirably not more than 50% and not less than 20%. When the display apparatus is a television, a gradation of 20% is desirable, but a gradation of 50% is sufficiently satisfactory from the practical point of view.
In view of the above-described points, the reference temperature distribution can be typically set based on the temperature distribution that is obtained when the light emitting apparatus is driven at the environmental temperature of 300K and the brightness of 50% for 60 minutes.
The temperature distribution in the light emitting apparatus when it is driven under the thus-determined predetermined conditions can be measured by attaching a plurality of temperature sensors, e.g., thermocouples, to the light emitting apparatus. As an alternative, that temperature distribution may be observed by the infrared thermography. When the temperature sensors are used, it is not necessary to measure temperatures at all points in the light emitting apparatus. In other words, the number of measurement points may be set to such a value as allowing the temperature distribution over an entire light emitting area to be sufficiently estimated. The temperature measurement is desirably performed in an environmental test room.
In fact, because of a difficulty in continuously defining the temperature distribution, the reference temperature distribution is defined by measuring the temperature in each of a plurality of divided regions and regarding the resistors included in each of the divided regions to be at the same temperature. Stated another way, the reference temperature distribution may be set as a temperature distribution representing the temperatures of the resistors, which are grouped into a plurality of different regions. A level of accuracy in compensating for the temperature distribution is increased by increasing the number of divided regions such that a temperature range in each region is narrowed. For the sake of simpler explanation, the above-described temperature distribution is assumed here to be the reference temperature distribution.
As a reference mode for the exemplary embodiment of the present invention, the following description is made about the case that, when the temperatures of the resistors 2 in the light emitting apparatus 10 are the same, i.e., T0 (the temperature of the individual resistors at that time is called here the “same (equal) temperature”), the resistances of the resistors 2 have the same value R0EQ. The state of the same temperature is obtained, for example, when the light emitting apparatus 10 is statically installed in a space at an environmental temperature of T0 without driving the light emitting apparatus and the temperatures of the resistors 2 are all T0. When the light emitting apparatus is driven and the temperature distribution represented by TMIN, TLOW, THIGH, and TMAX generates as described above, changes in resistance values of the resistors are as per illustrated in
In the exemplary embodiment of the present invention, the expression “resistance value is equal” implies that a percentage of the difference between two values with respect to an arithmetic mean of those two values (hereinafter referred to as a “middle value”) is less than 1%. Also, the expression “variation (distribution or nonuniformity) in the resistance value is reduced” or “variation (distribution or nonuniformity) is small” implies that a percentage of the difference between two values with respect to a middle value of those two values is desirably less than 10%. When evaluation is made on three or more values, a percentage is calculated as percents of the difference between a maximum value and a minimum value with respect to a middle value of the maximum value and the minimum value or with respect to an arithmetic mean of those three or more values (hereinafter referred to as a “mean value”). Assuming values of two resistances to be RA and RB (RA>RB), the above-described condition can be expressed by 200×(RA−RB)/(RA+RB)<1% or 10%. By rewriting that formula in terms of RA/RB, RA and RB can be regarded as being equal to each other when the resistance value RA is less than 101% of the resistance value RB. Also, a variation of RA and RB can be regarded as being small when the resistance value RA is less than 111% of the resistance value RB.
Further, in the exemplary embodiment of the present invention, the temperature difference that can be regarded as indicating the “same temperature” is, strictly speaking, a temperature difference resulting when the “equal resistance” is obtained based on the above-mentioned formula (1), and such a temperature difference differs depending on not only the temperatures (T0 and T) serving as references to define the temperature difference, but also the activation energy Ea.
As seen from quantitative calculations based on the above-mentioned formula (1), when the resistance variation is relatively small at a low value (0.05 eV) of the activation energy Ea and at a high temperature (about 330K) of the resistor, the resistance value varies less than 1% if a temperature change is less than 2K.
Further, even when the resistance variation is relatively large at a high value (1 eV) of the activation energy Ea and at a low temperature (about 270K) of the resistor, the resistance variation remains less than 1% if a temperature change is less than 0.06K. Thus, when the temperature change is less than 0.06K, the resistance change can be considered as causing substantially no problems. Accordingly, it is most desirable that two temperatures are regarded to be at the “same temperature” when the temperature change is less than 0.06K.
Moreover, as seen from calculations based on the above-mentioned formula (1), even when the resistance variation is relatively small at a low value (0.05 eV) of the activation energy Ea and at a high temperature (about 330K) of the resistor, the resistance value varies 10% or more if the temperature change is 20K or more. Accordingly, the exemplary embodiment of the present invention can be most desirably applied to the case where there occurs a temperature difference of 20K or more during the driving. Stated another way in a reversed view, the resistance variation of 10% or more does not generate unless there occurs a temperature distribution varying 20K or more. On the contrary, at the activation energy of 0.1 eV or more, the resistance variation of 10% or more generates if there occurs a temperature change of 10K or more at 270 to 330K. In practice, therefore, the exemplary embodiment of the present invention is desirably applied to the case where a material having the activation energy of 0.1 eV or more is used.
On the other hand, when the resistance variation is relatively large at a high value (1.0 eV) of the activation energy Ea and at a low temperature (about 270K) of the resistor, the resistance value varies 10% or more if the temperature change is 0.6K or more. It is not desirable to use, as the resistor, a material having a resistance value that is apt to vary depending on such a slight temperature change. Meanwhile, when the activation energy is 0.6 eV or less, the resistance variation of 10% or more does not occur at about 270K for the temperature change of less than 1K. Further, when the activation energy is 0.1 eV or more, the resistance variation of 1% or more occurs for the temperature change of 1K or more.
From the above-described point of view, it is typically considered that, when a material having the activation energy of not less than 0.1 eV and not more than 0.6 eV is used as the resistor 2, the temperature difference of less than 1K can be regarded as indicating the “same temperature”. Hence, the above-described temperature range as a unit for division of the reference temperature distribution is desirably set to 1K.
A method of reducing nonuniformity in a displayed image according to the exemplary embodiment of the present invention will be described below. A first exemplary embodiment is to reduce a variation in the resistance values of the resistors 2 during the driving. A second exemplary embodiment is to reduce a variation in brightness in consideration of respective temperatures of the light emitting devices 1 connected to the resistors 2.
A method of setting a resistance distribution among the resistors 2 in the first exemplary embodiment of the present invention is described with reference to
In the light emitting apparatus 10, resistance values of the resistors 2 positioned at PLOW and PHIGH at the temperature (same temperature) T0, i.e., at the time when the temperatures of the resistors are the same, are assumed to be respectively R1LOW0 and R1HIGH0. Resistance values R1LOW and R1HIGH of the resistors 2 positioned at PLOW and PHIGH in a state providing the reference temperature distribution are expressed, based on the above-mentioned formula (1), by the following formulae (2) and (3), respectively:
R
1LOW
=R
1LOW0exp{(Ea/kb)×(1/TLOW−1/T0)} (2)
R
1HIGH
=R
1HIGH0exp{(Ea/kb)×(1/THIGH−1/T0)} (3)
In this first exemplary embodiment, the resistance values in the state providing the reference temperature distribution are made equal to each other. In other words, R1LOW=R1HIGH=R1EQ are to be held. R1EQ is a resistance value necessary for properly driving the light emitting device 1 and is set as appropriate. From the formulae (2) and (3), it is understood that R1LOW0 and R1HIGH0 are required to be given by the following formulae (2′) and (3′), respectively:
R
1LOW0
=R
1EQexp{(Ea/kb)×(1/T0−1/TLOW)} (2′)
R
1HIGH0
=R
1EQexp{(Ea/kb)×(1/T0−1/THIGH)} (3′)
Further, from the formulae (2) and (3), the relationship between R1LOW0 and R1HIGH0 is expressed by the following formula (4):
R
1HIGH0
=R
LOW0exp{(Ea/kb)×(1/TLOW−1/THIGH)} (4)
Accordingly, if R1HIGH0 and P1LOW0 are in the relationship expressed by the formula (4), the resistance values of the resistors in the state providing the reference temperature distribution can be made equal to each other. When the formula (4) is rewritten into;
R
1HIGH0
/R
1LOW0=exp{(Ea/kb)×(1/TLOW−1/THIGH)} (4′)
the right side of the formula (4′) takes a value larger than 1, thus resulting in R1HIGH0/R1LOW0>1. In other words, the resistance variation can be reduced by setting the resistance value R1HIGH0 of the resistor at T0, which is positioned at PHIGH, to be larger than the resistance value R1LOW0 of the resistor at the same temperature T0, which is positioned at PLOW. For all the resistors, more desirably, the resistor having a higher temperature in the reference temperature distribution is set to have a larger resistance value at the same temperature.
A range of the resistance value R0 at an arbitrary point PXY within the light emitting area and at the temperature T0 can be determined by measuring at least the point PMAX where the temperature is maximized, and the point PMIN where the temperature is minimized.
Assuming that resistance values of the resistors 2 positioned at the points PMAX and PMIN, which provide a maximum temperature TMAX and a minimum temperature TMIN in the reference temperature distribution, are respectively R1MAX0 and R1MIN0 at the temperature T0, resistance values R1MAX and R1MIN of the resistors 2 positioned at the points PMAX and PMIN in the state providing the reference temperature distribution are expressed, based on the above-mentioned formula (1), by the following formulae (5) and (6), respectively:
R
1MAX
=R
1MAX0exp{(Ea/kb)×(1/TMAX−1/T0)} (5)
R
1MIN
=R
1MIN0exp{(Ea/kb)×(1/TMIN−1/T0)} (6)
Because R1MAX=R1MIN=R1EQ are required to be satisfied, it is understood from the formulae (5) and (6) that R1MAX0 and R1MIN0 are required to be given by the following formulae (5′) and (6′), respectively:
R
1MAX0
=R
1EQexp{(Ea/kb)×(1/T0−1/TMAX)} (5′)
R
1MIN0
=R
1EQexp{(Ea/kb)×(1/T0−1/TMIN)} (6′)
Further, the relationship between R1MAX0 and R1MIN0 is expressed by the following formulae (4):
R1MAX0=R1MIN0exp{(Ea/kb)×(1/TMIN−1/TMAX)} (7)
Accordingly, a resistance value R1XY0 at an arbitrary point PXY, which takes a temperature TXY, at the same temperature T0 is required to be set within the range expressed by the following formula (8):
R
1MIN0
≦R
1XY0≦R1MIN0exp{(Ea/kb)×(1/TMIN−1/TMAX) } (8)
On that occasion, if R1XY0 at the point taking the temperature TXY near TMIN is set to a value near R1MAX0, such setting may often increase the variation contrary to the intention. To avoid the unintended setting, R1XY0 is desirably required to satisfy not only the above formula (8), but also the following formula (9):
R
1XY0
≦R
1EQexp{(Ea/kb)×(1/T0−1/TXY)} (9)
Such requirement corresponds to the fact that, when TXY is TLOW, R1XY0 is set to fall within a range larger than R1MIN0and not larger than R1LOW0 expressed by the formula (2′). Also, such requirement corresponds to the fact that, when TXY is THIGH, R1XY0 is set to fall within a range between R1MIN0 and R1HIGH0 expressed by the formula (3′). By setting R1XY0 to fall within the above-mentioned range, the resistance value of the resistor in the state providing the reference temperature distribution can be made closer to R1EQ with no necessity of always satisfying R1HIGH0>R1LOW0. The case where the sign of inequality holds in the formula (9) corresponds to the fact that, in
While the above description has been made about the method of giving a distribution to the resistance values at the same temperature depending on the reference temperature distribution, the following description is made about a method that can be more universally carried out in practice. When the resistors 2 are arrayed in the two-dimensional matrix pattern as described above, a temperature rise is relatively small in the peripheral portion of the light emitting apparatus 10 where heat is more apt to radiate, and it is relatively large in a central portion where heat is less apt to radiate. Accordingly, from the viewpoint of the concept of the “greatest common divisor”, i.e., from the viewpoint of dividing the reference temperature distribution into main basic regions, the reference temperature distribution can be imaginarily simply divided such that the temperature is high in a central portion and gradually lowers toward a peripheral portion. The expression “central portion” implies a center in a part of the light emitting apparatus 10, which has a maximum width, including the surroundings of the center (i.e., a central area). The expression “peripheral portion” implies an area positioned nearer to an edge (end) within 10% of the distance from the center to the edge. For example, when the light emitting apparatus 10 has a rectangular shape, the center is a middle point (crossed point) of diagonals of the rectangle. In that case, it is desirable that the reference temperature in the central portion provides the maximum (highest) temperature in the actually measured temperature distribution, and that the reference temperature in the peripheral portion farthest away from the central portion provides the minimum (lowest) temperature therein.
Further, the resistance values of the resistors at the same temperature are set so as to gradually reduce in the direction toward the peripheral portion from the central portion, namely, with an increase of the distance from the central portion. The direction toward the peripheral portion from the central portion (i.e., the direction away from the central portion) implies all directions radially extending from the central portion.
By setting the resistance values of the resistors as described above, nonuniformity in the displayed image attributable to the temperature distribution can be satisfactorily reduced even when the actually generated temperature distribution slightly deviates from the imaginarily set one.
As described above, the first exemplary embodiment can suppress a variation in the resistance values of the resistor having a relatively high temperature and the resistor having a relatively low temperature, which occurs during the driving of the light emitting apparatus 10. Therefore, nonuniformity in the brightness can be reduced without making the light emitting apparatus more complicated. In particular, a more satisfactory result is obtained when the light emitting apparatus is driven under the predetermined conditions used for setting the reference temperature distribution. In other words, a more satisfactory result is typically obtained when the light emitting apparatus is driven at the environmental temperature of 300K for 60 minutes with brightness of 50% for all the light emitting devices.
The first exemplary embodiment has been described as reducing a variation in the resistance values of the resistors 2 in the state providing the reference temperature distribution. A second exemplary embodiment will be described below in connection with the case of reducing nonuniformity in brightness among the plurality of light emitting devices 1 in the state providing the reference temperature distribution.
As described above, nonuniformity in brightness among the plurality of light emitting devices 1 can be reduced in the first exemplary embodiment. In some light emitting device, however, the light emission efficiency of the luminous body varies upon a temperature change even with the light emitting device driven at the same current or voltage. In an organic EL device, for example, the light emission efficiency of the luminous body increases with a temperature rise. Conversely, the light emission efficiency of a light emitting diode, for example, decreases with a temperature rise.
Taking into account such a tendency, in this second exemplary embodiment, a variation in the light emission efficiency generated among the light emitting devices 1 due to the temperature distribution is reduced by setting the resistance values of the resistors 2 to be different from each other. More specifically, resistance values R2MIN, R2LOW, R2HIGH, and R2MAX of the resistors are set to different values depending on respective brightness-temperature characteristics of the light emitting devices 1 so that the brightnesses of the light emitting devices 1 are held constant.
The principle for giving a distribution to the resistance values of the resistors from the above point of view is described in a quantitative way. The distribution of the resistance values (i.e., the resistance distribution) at the same temperature is determined depending on a temperature-brightness characteristic: L=g1(T′), a brightness-current characteristic: L=f1(I), and a current-voltage characteristic: I=h1(V1) of the luminous body (or the light emitting device 1). Here, L is the brightness of the luminous body, T′ is the temperature of the luminous body, and g1(T′) represents a function of T′. Also, I is the current flowing through the light emitting device 1, and f1(I) represents a function of the current I. V1 is the voltage applied to the light emitting device 1, and h1(V1) represents a function of V1. In addition to the temperature characteristic of the luminous body, the temperatures of other components of the light emitting device 1 can also be taken into consideration as required.
Because the resistor 2 is connected in series to the light emitting device 1, the current I flows through the resistor 2 when the voltage V is applied to both the light emitting device 1 and the resistor 2 through the wirings 3. When the resistor 2 has a resistance value R′, a voltage V2 applied to the resistor 2 is expressed by V2=R′I.
Accordingly, a voltage V1 applied to the light emitting device 1 is expressed by V1=V−R′I. This formula can be rewritten into I=h1(V1)=h1(V−R′I). Thus, I=h2(V, R′) is obtained for the current I as a function of V and R′. By substituting the function I=h2(V, R′) in L=f1(I), L=f2(V, R′) is obtained for the brightness L as a function of V and R′.
Therefore, the relationship between T′ and R′, i.e., R′=g2(T′) can be determined from g(T′)=f2(R′)=Lc so that the brightness L takes a constant value Lc when the driving voltage V is set to a constant value Vc.
That relationship can be determined, as described above, depending on the temperature-brightness characteristic, the brightness-current characteristic, and the current-voltage characteristic of the light emitting device 1. In other words, if the characteristics of the light emitting device 1 are experimentally confirmed even when those characteristics are not theoretically determined, a proper distribution can be given to the resistance values of the resistors during the driving based on those characteristics.
A method of setting the distribution in the resistance values of the resistors 2 according to the second exemplary embodiment of the present invention will be described below with reference to
This second exemplary embodiment can be advantageously applied to the case where the temperature T′ of the light emitting device 1 is substantially equal to the temperature T of the resistor 2. The temperature of the light emitting device 1 and the temperature of the resistor 2 are substantially the same, for example, when the light emitting device 1 and the resistor 2 are arranged at close positions, and/or when the light emitting device 1 and the resistor 2 are interconnected through a material having good thermal conductivity. This second exemplary embodiment can also be advantageously applied to the case where the temperature distribution among the light emitting devices 1 and the temperature distribution among the resistors 2 show a similar tendency.
While the following description is made about the case where the temperature distribution among the light emitting devices 1 and the temperature distribution among the resistors 2 show a similar tendency, it is equally applied to the case where the temperature T′ of the light emitting device 1 and the temperature T of the resistor 2 are substantially the same. Also, in the case described below, the light emitting device 1 has such a brightness-temperature characteristic that the brightness increases as the temperature rises.
It is assumed that, when a temperature distribution of TMIN<TLOW<THIGH<TMAX occurs among the resistors 2 as in the first exemplary embodiment, a similar temperature distribution of T′MIN<T′LOW<T′HIGH<T′MAX also occurs among the light emitting devices 1. Here, respective temperatures of the light emitting devices 1 at the four points PMIN, PLOW, PHIGH, and PMAX are T′MIN, T′LOW, T′HIGH, and T′MAX.
Respective resistance values R′ of the resistors 2 connected to the light emitting devices 1 during the driving, when the light emitting devices 1 take respectively the temperatures T′=T′MIN, T′LOW, T′HIGH and T′MAX during the driving, are set based on the above-described formula R′=g2(T′), i.e., the relationship obtained from the brightness-temperature characteristic.
A line B illustrated in
In the brightness-current characteristic of a self-light emitting device, the brightness L usually increases as the current I increases. Therefore, the brightness can be reduced by reducing the current, and hence by increasing the resistance value of the resistor 2 connected to the light emitting device 1.
When the brightness-temperature characteristic of the light emitting device 1 is such that the brightness increases as the temperature rises, an increase of the brightness can be suppressed by setting the resistance value of the resistor 2 during the driving, which is connected to the light emitting device 1 having a higher temperature, to a larger resistance value.
Thus, because of the relationship that the resistance value of the resistor is set to be larger as the temperature of the light emitting device 1 increases, the resistance values of the resistors during the driving are set so as to satisfy the relationship of R2MIN<R2LOW<R2HIGH<R2MAX.
On the other hand, as described above, R′ is also related to the temperature T of the resistor 2 as expressed by the formula (1). Accordingly, respective resistance values R0′=R2MIN0, R2LOW0, R2HIGH0 and R2MAX0 of the resistors at the same temperature T0 can be each determined from the following formula (10):
g
2(T′)=R0′exp{(Ea/kb)×(1/T−1/T0)} (10)
Because the resistor 2 has the negative resistance-temperature characteristic, the resistance values R0′ of the resistors 2 at the same temperature T0 are set in the relationship of R2MIN0<R2LOW0<R2HIGH0<R2MAX0. In other words for the light emitting device 1 having a higher temperature during the driving and the resistor 2 connected to the relevant light emitting device 1, the resistance value is required to be set to a larger value at the same temperature. Hence, in the self-light emitting device, the line B in
Thus, this second exemplary embodiment can be advantageously applied to the case where the brightness-temperature characteristic of the light emitting device 1 is such that the brightness increases as the temperature rises.
On the other hand, when the brightness-temperature characteristic of the light emitting device 1 is such that the brightness decreases as the temperature rises, a reduction of the brightness can be suppressed for the light emitting device 1 having a higher temperature and the resistor 2 connected to the relevant light emitting device 1 by setting the resistance value during the driving to a smaller value. This second exemplary embodiment can be satisfactorily applied unless, in the range between TMIN and TMAX in
While the above description is made about the case where the temperature distribution among the light emitting devices 1 exhibits a similar tendency to that of the temperature distribution among the resistors 2, nonuniformity in the brightness can be likewise reduced by setting the resistance value during the driving so as to satisfy the formula of R′=g2(T′) even when the temperature distribution among the light emitting devices 1 does not exhibit a similar tendency to that of the temperature distribution among the resistors 2. In particular, this second exemplary embodiment can be advantageously applied to the case where an extent of the temperature distribution among the light emitting devices 1 is smaller than that of the temperature distribution among the resistors 2.
According to the second exemplary embodiment, as described above, nonuniformity in the brightness can be more satisfactorily reduced when temperature distribution occurs among both the light emitting devices 1 and the resistors 2. As in the first exemplary embodiment, the most satisfactory result is obtained when the light emitting apparatus is driven under similar conditions to those providing the reference temperature distribution.
A method of making the resistance values different from each other at the same temperature in the first and second exemplary embodiments will be described below, for example, in connection with the light emitting apparatus according to the exemplary embodiment of the present invention in which a cathode luminescence device is used as the light emitting device 1.
The following description is made in connection with the electron emitting device of the surface conduction type. A typical structure, a manufacturing method, and characteristics of the electron emitting device of the surface conduction type are disclosed in, e.g., Japanese Patent Laid-Open No. 2-56822. Also, typical structures, manufacturing methods, and characteristics of the electron emitting device of the layered type are disclosed in, e.g., Japanese Patent Laid-Open No. 2001-167693 and No. 2001-229809.
One end of the resistor 22 is connected in series to the electron emitting device 20. The scanning-signal device electrode 25 and the information-signal device electrode 26 are each a connecting member having a low resistance and are formed in such shapes as facilitating respectively the connection between the one end of the resistor 22 and the electron emitting device 20 and the connection between the electron emitting device 20 and the second wiring 24. The other end of the resistor 22 is connected in series to the first wiring 23 through an extended electrode 27. The extended electrode 27 is also a connecting member having a low resistance and is formed in such a shape as facilitating the connection between the resistor 22 and the first wiring 23.
An insulating film 28 secures insulation in an area where the first wiring 23 and the second wiring 24 cross each other. The insulating film 28 is also disposed partly between the extended electrode 27 and the first wiring 23. The extended electrode 27 connected to the electron emitting device 20 through the resistor 22 is connected to the first wiring 23 via a contact hole 29 formed in the insulating film 28.
While the electron emitting devices 20 on the rear plate 11 are substantially the same, the phosphors on the face plate 13 emit lights in different colors. Therefore, when the electron emitting devices 20 and the phosphors are arranged to face each other to form the light emitting devices, the light emitting devices to be compared with each other for compensation are ones corresponding to the phosphors emitting lights in the same color. Using the addresses assigned as illustrated in
In the light emitting apparatus having the above-described structure, one electron emitting device 20, one phosphor 14, and one resistor 22 constitute one light emitting unit 5. The light emitting apparatus is constituted by arraying a plurality of light emitting units 5 (having the addresses A-1, A-2, . . . , A-6, etc.).
The light emitting apparatus using the above-described electron emitting devices may often have a variation in electron emission characteristics. When the simple matrix wiring is employed as described above, there is a possibility that, if one electron emitting device is short-circuited for some reason, a large current flows in such an excessive amount as damaging the electron emitting device and the voltage can no longer be applied to the other electron emitting devices.
To overcome the above-mentioned problem, the resistor 22 connected to each electron emitting device 20 is required to have a high resistance. In other words, a variation in the emission current-applied voltage characteristic of the electron emitting device 20 can be reduced by connecting a resistor having a high resistance in series to the electron emitting device 20 and restricting the current flowing into the electron emitting device 20 with the provision of the resistor. Further, the resistor having the high resistance can suppress a large current from flowing to the wirings even if the electron emitting device 20 is short-circuited, and can prevent damage of the other electron emitting devices. For those reasons, the resistance value of the resistor is desirably in the range of not less than 1 kΩ and not more than 10 GΩ. For example, when an emission current of 100 μA flows, a resistance value of 10 kΩ generates a voltage drop of 1 V.
Moreover, since the electron emitting device is basically manufactured through a photolithographic process, the resistor is formed as a thin film. To obtain the above-mentioned resistance value in that case, a material of the resistor desirably has the volume resistivity of not less than 10−3 Ωm or 1 kΩ/(unit square) with a thickness of 1 μm.
Such a high-resistance material can be selected from among various materials including, e.g., Si, a-Si, Si—C, TaN, amorphous carbon, DLC, cermet, silicide, an oxide semiconductor, a nitride semiconductor, ATO (Antimony-containing Tin Oxide), SnO2, WGeON, PtAlN, AlN, and ZnO. Many of those materials have semiconductor characteristics and exhibit the negative resistance-temperature characteristic. For example, the activation energy Ea is about 0.05 eV for AuSiON, about 0.1 eV for PtAlN, about 0.14 eV for TaN, about 0.3 eV for WGeON, and about 0.8 eV for a-Si. The thin film resistor can be formed by suitable one of methods including vacuum film-forming processes, e.g., vacuum vapor deposition, sputtering, and plasma CVD, as well as spin coating, spraying, etc.
As described above, the light emitting apparatus using the electron emitting devices has the structure suitable for practicing the first exemplary embodiment of the present invention.
The second exemplary embodiment of the present invention can also be suitably applied to the light emitting apparatus using the electron emitting devices. More specifically, as described above, the rear plate 11 including the electron emitting devices 12 and the face plate 13 including the phosphors 14 are arranged to face each other, and a space between both the plates 11 and 13 is held in the vacuum state. This provides a structure in which heat generated from heat sources, such as the electron emitting devices 12 and the wirings, are hard to conduct to the phosphors 14. Therefore, an extent of temperature distribution generated among the phosphors 14 on the face plate 13 on the side closer to the display surface is smaller than that of temperature distribution on the rear plate 11 including the resistors 2. Hence, the second exemplary embodiment can also be suitably applied to the light emitting apparatus using the electron emitting devices.
When the second exemplary embodiment is applied to the light emitting apparatus using the electron emitting devices, the brightness-current characteristic of the light emitting device can be expressed by L=f1(I)=κ(η×I)γ. Herein, κ is light emission efficiency of the phosphor, η is efficiency of the electron emitting device, and γ is a gamma characteristic of the phosphor. The current-voltage characteristic of the electron emitting device is expressed by the Flower-Nordheim's formula I=h1(V1)=aV12exp(−b/V1). Herein, a and b are coefficients. Accordingly, when the resistor 2 having a resistance value R′ is employed, I=a(V−R′I)2exp{−b/(V−R′I)} is obtained. The brightness-temperature characteristic L=g(T′) can be provided as a temperature characteristic of the phosphor light-emission efficiency κ that is quantitatively determined depending on the type of the phosphor.
A method of practically giving a distribution to the resistance values of the resistors will be described below. As seen from the formula (1), the resistance values at the reference temperature T0 can be made different from each other by a method of changing the shapes of the resistors and a method of changing materials of the resistors such that the volume resistivity and/or the activation energy Ea are set to different values. From the viewpoint of compensating for the temperature distribution with higher accuracy in the exemplary embodiment of the present invention, it is desirable to prepare many variations for setting of the resistance values. In the case of forming thin-film resistors, however, the manufacturing process becomes very complicated if different materials are used for forming the plurality of resistors. For that reason, the resistors are formed by using the same material, and the resistance values of the resistors are made different from each other by changing any of the length, the width and the thickness of the resistor, or a combination of those parameters.
Alternatively, the effective length of the resistor 22 may be changed by changing the lengths of portions of the extended wiring 27 and the scanning-signal device electrode 25, which are overlapped with the resistor 22, while the length of the resistor 22 itself is held constant. Although the resistor 22 in
Although the resistor 22 has a rectangular shape in
While the direction in which the current flows is the direction of length of the resistor 22 (i.e., the direction parallel to a substrate) in the above description, the direction of the current may be the direction of thickness of the resistor 22 (i.e., the direction vertical to the substrate). As still another example, a distribution may be given to the resistance values of the resistors by changing a contact area between the resistor and the wiring.
The other end of the scanning-signal device electrode 25 is connected to the electron emitting device 20 (not shown in
Thus, by changing the contact area between the resistor 22a and the wiring 23, a distribution is given to the resistance values of the resistors. As an alternative, the resistance distribution may be given by changing the thickness of the resistor 22a.
While the present invention will be described below in more detail in connection with Examples, it is to be noted that the present invention is not limited to the following Examples.
A method of fabricating the components of the image display apparatus, illustrated in
First, a method of fabricating the rear plate 11 used in Example 1 is described. A typical array of the electron emitting devices 12 in the light emitting apparatus of
In Example 1, the electron emitting device 20 having the structure illustrated in
A glass commercialized under the trademark of PD-200 (made by Asahi Glass Company, Ltd.) and having a thickness of 2.8 mm is used as a substrate, and a SiO2 film having a thickness of 200 nm is formed on the glass substrate by coating.
A Ti film having a thickness of 5 nm and a Pt film having a thickness of 200 nm are formed on the glass substrate. Then, the Ti/Pt films are patterned by the photolithography to form the scanning-signal device electrode 25 and the information-signal device electrode 26. Each of those device electrode 25 and 26 has volume resistivity of 0.25×10−6 (Ωm). Further, the scanning-signal device electrode 25 is trimmed in a later-described step such that an electrode portion connected to the electron emitting film 21 has a width of 20 μm and an electrode portion connected to the resistor 22 has a width of 10 μm.
After forming a TaN film, the resistor 22 is patterned into a predetermined shape. The resistor 22 has a thickness of about 1 μm and a width of 20 μm. Lengths of individual resistors 22 are changed so as to give a distribution depending on respective positions within the display region. A manner of giving the length distribution will be described later.
The information signal wiring 24 and the extended wiring 27 are formed by a screen printing process using a silver paste. The information signal wiring 24 has a thickness of about 10 μm and a width of 20 μm.
Under the scanning line wiring 23 formed in a later step, the insulating layer 28 having a thickness of 30 μm and a width of 200 μm is formed by a screen printing process using an insulating paste. The opening 29 is formed in the insulating layer 28 in a portion of its region overlapped with the extended wiring 27.
On the insulating layer 28, the scanning signal wiring 23 having a thickness of 10 μm and a width of 150 μm is formed by a screen printing process using a silver paste. In the same step, a lead wiring and a lead terminal for connection to an external driving circuit are also formed in a similar manner (though not illustrated).
An organic palladium-containing solution is applied to between the device electrodes 25 and 26 by an ink jet applicator while the applied solution is adjusted to have a dot diameter of 50 μm. Then, a palladium oxide (PdO) film having a maximum thickness of 10 μm is obtained by carrying out a high-temperature baking process in air.
An energization process is carried out on the palladium oxide film under an atmosphere containing hydrogen gas. As a result, the palladium oxide is reduced to form the electron emitting film 21 made of palladium, and a crack is partly formed in the electron emitting film 21 at the same time.
Thereafter, an energization process (activation process) is carried out on the electron emitting film 21 in an atmosphere under 1.3×10−4 Pa, thus depositing a carbon film on the electron emitting film 21. As a result, the electron emitting device 20 including the electron emitting portion 21a is obtained.
A method of fabricating the face plate 13 will be described below with reference to
A glass (PD-200) is used as a substrate of the face plate 13, and the phosphors 14a, 14b and 14c are formed on the underside of the substrate. In this Example 1, to display a color image, P22 phosphors in three primary colors of red, green and blue, which are generally used in the field of CRT, are used as the phosphors 14a, 14b and 14c. The black matrix 30 is arranged so as to separate the phosphors 14a, 14b and 14c in the X-direction, and to separate individual pixels in the Y-direction. The black matrix 30 is effective in not only absorbing electrons, but also absorbing extraneous light to suppress reflection of the extraneous light at the display surface. A black pigment paste and a phosphor paste are used respectively as the black matrix 30 and the phosphors 14a, 14b and 14c. The black matrix 30 and the phosphors 14a, 14b and 14c are formed on the face plate 13 by screen-printing the respective pastes and baking them.
Thereafter, a metal back (not shown) serving as a reflective layer is formed by smoothing the surfaces of the phosphors 14a, 14b and 14c and vapor-depositing Al thereon in vacuum with a thickness of 100 nm. The face plate 13 is thus fabricated.
Finally, the frame 15 is arranged, as illustrated in
In the image display apparatus constructed as described above, voltages are applied to the electron emitting devices through the respective wirings. Further, an image is displayed by applying a voltage to the metal back of the face plate 13 through a high-voltage terminal. At that time, 0 or 10 V is applied to the information signal wiring 24, 0 or −20 V is applied to the scanning signal wiring 23, and 15 KV is applied to the metal back. An electric circuit as a driving unit is installed at a position slightly deviated rightward from the center as viewed from the backside of the rear plate 11.
A method of setting the resistance values of the resistors 22 will be described below. In this Example 1, the resistance value of the resistor 22 is set to be larger at the same temperature T0 in a region taking a higher temperature in the temperature distribution that occurs when an image is displayed by the image display apparatus.
Further, the resistor 22 in this Example 1 is made of TaN and has activation energy of 0.14 eV and volume resistivity of 0.01 Ωm. Additionally, the same temperature T0 is set to 300K, and the constant resistance value REQ during the operation is set to 10 kΩ.
Conditions for measuring the temperature distribution during the driving will be described below. The temperature distribution in the display region is measured when all the pixels of the image display apparatus are lit up at the environmental temperature of 300K with a gradation of 100% and a temperature change is saturated. Also, the temperature distribution is measured by attaching 25 thermocouples in a matrix pattern to the backside (i.e., the surface not including the electron emitting devices) of the rear plate 11. The temperature change has become small after the lapse of 60 minutes. Although the temperature is measured from the backside of the rear plate 11, the thermocouple shows substantially the same value as the temperature of the corresponding resistor 22. The result of the temperature measurement in the light emitting apparatus of this Example 1 provides a temperature distribution that is asymmetric and exhibits a higher temperature at a position slightly deviated rightwards from the center as viewed from the backside.
In the reference temperature distribution, an average temperature is 317K, a maximum temperature is T=320K at a point slightly deviated leftwards from the center in
On the basis of the reference temperature distribution illustrated in
The resistance values at room temperature (300 K) are set at the above-mentioned points such that a resistance at the point slightly deviated leftwards from the center has a maximum resistance value RMAX0=14.1 kΩ, and a resistance RE0 at the lower right point has a minimum resistance value RE0 =RMIN0=11.9 kΩ. The resistance values at opposite edges are set such that a resistance value RL0 substantially at the middle point in the left edge line is RL0=RHIGH0=13.8 kΩ, and a resistance value RR0 substantially at the middle point in the right edge line is RR0=RLOW0=13.4 kΩ. Thus, a distribution is given to the resistance values such that the difference between the maximum value and the minimum value is 17% with respect to the middle between the maximum value and the minimum value.
In this Example 1, the distribution is given to the resistance values by changing the pattern width of the resistor 22 as illustrated in
With that setting, the resistance values when the light emitting apparatus is driven under the same conditions as those used in measuring the reference temperature distribution become approximately 10.0 kΩ that is set as the constant resistance value REQ. Further, as a result of displaying images under the above-described driving conditions while the display pattern is changed in several ways, an image having small nonuniformity in its displayed view is obtained for any of the display patterns. More specifically, some nonuniformity in the displayed image is observed immediately after startup of the image display apparatus, but the nonuniformity reduces to an unappreciable level in several minutes.
Example 2 of the present invention will be described below. The basic structure and the manufacturing steps in Example 2 are the same as those in Example 1 and hence a description thereof is omitted. In this Example 2, the resistance values of the resistors 22 are set such that, in both longitudinal direction and the transverse direction of the image display apparatus, the resistance values of the resistors in a central portion are larger than those of the resistors in an edge portion. Therefore, this Example 2 can be suitably applied to the case of (temperature in the edge portion)<(temperature in the central portion).
As in Example 1, the resistor 22 in this Example 2 is made of TaN and has activation energy of 0.14 eV and volume resistivity of 0.01 Ωm. Similarly, the constant temperature T0 is set to 300K, and the constant resistance value REQ during the operation is set to 10 kΩ.
A temperature distribution is measured in a similar manner to that in Example 1.
In this Example 2, the reference temperature distribution is set based on the temperatures measured in Example 1 such that the resistance value at the same temperature reduces as the distance from the center increases. As illustrated in
On the basis of the reference temperature distribution illustrated in
In this Example 2, the distribution is given to the resistance values by changing the pattern length of the resistor 22 as illustrated in
Further, as a result of displaying images under similar driving conditions to those in Example 1 while the display pattern is changed in several ways, an image having small nonuniformity in its displayed view is obtained for any of the display patterns.
Example 3 of the present invention will be described below. This Example 3 employs a structure in which, as illustrated in
Part of the manufacturing steps, which is the same as that in Example 1, is not described here.
First, the scanning-signal device electrode 25 and the information-signal device electrode 26 are formed on a glass substrate. Then, an a-Si film is formed thereon by sputtering and is patterned to form the resistor 22a on the scanning-signal device electrode 25. The resistor 22a has a thickness of about 60 nm and a width of 20 μm. The resistor 22a in this Example 3 is made of a-Si and has activation energy of 0.8 eV and volume resistivity of 100 Ωm.
Then, the information signal wiring 24 is formed and the insulating layer 28 is further formed. The opening 29 is formed in the insulating layer 28 in a portion of its region overlapped with the resistor 22a on the scanning-signal device electrode 25. The opening 29 has a width of 15 μm and a length varied so as to provide a distribution among the opening lengths. A manner of providing the length distribution will be described later.
Then, the scanning signal wiring 23 is formed on the insulating layer 28. Finally, the electron emitting film 21 and the electron emitting portion 21a are formed.
A method of setting the resistance distribution will be described below. In this Example 3, as in Example 1, the resistance value of the resistor 22a is set to be larger at the same temperature T0 in a region taking a higher temperature in the temperature distribution that occurs when an image is displayed by the image display apparatus. The reference temperature distribution (
In this Example 3, the distribution is given to the resistance values by changing the area of the opening 29. More specifically, the area of the opening 29 is changed by changing the length of the opening 29 as illustrated in
At room temperature (300K), as in Example 1, the resistance values become approximately the constant resistance value 10.0 kΩ at all the points when the temperature distribution illustrated in
Further, as a result of displaying images under similar driving conditions to those in Example 1 while the display pattern is changed in several ways, an image having small nonuniformity in its displayed view is obtained for any of the display patterns.
Example 4 of the present invention will be described below. Example 4 mainly differs from Example 1 in the structure of the electron emitting device 12 on the rear plate 11 of the light emitting panel illustrated in
Manufacturing steps of the rear plate 11 will be described below.
First, a Cu wiring is formed on a substrate 33 by the photolithography to form the scanning signal wiring 34 (
In the multi-strip-shaped portion 38 and thereabout, the insulating layer 39 (SiN), the insulating layer 40 (SiO2), and the patterned gate electrode 36 (TaN) are stacked on the substrate 33 (
Methods of manufacturing the other components including the face plate 13 and a method of forming a panel are similar to those described above, and hence a description thereof is omitted.
A method of setting the resistance distribution will be described below. In this Example 4, as in Example 1, the resistance value of the resistor 42 is set to be larger at the same temperature T0 in a region taking a higher temperature in the temperature distribution that occurs when an image is displayed by the image display apparatus. The reference temperature distribution is set to the same as that (illustrated in
The constant resistance value REQ during the operation is set to 1.0 MΩ. This Example 4 differs from the other Examples in the arrangement of the resistor 42 and the position used to define the resistance value. In this Example 4, plural strips of the lower-potential side cathode 41 are arranged in parallel to constitute one electron emitting device. Therefore, the resistor 42 is also arranged in the form of strips corresponding to the lower-potential side cathode 41. The resistance value represents a resistance value between the multi-strip-shaped lower-potential side cathode 41 and the multi-strip-shaped cathode electrode 35.
On the basis of the reference temperature distribution illustrated in
In this Example 4, the resistance value is adjusted by changing a distance l between the lower-potential side cathode 41 and the cathode electrode 35 as illustrated in
Further, as a result of displaying images under similar driving conditions to those in Example 1 while the display pattern is changed in several ways, an image having small nonuniformity in its displayed view is obtained for any of the display patterns.
Comparative Example 1 represents the case where the rear plate has the same structure as that in Examples 1 and 2, but a distribution is not given to the resistance values, namely, all the resistors 22 are formed in the same shape. The resistor 22 is made of TaN that has activation energy of 0.14 eV. All the resistance values are set to become REQ=10 kΩ at room temperature (300K).
An average of the resistance values of the resistors when the temperature distribution illustrated in
Comparative Example 2 represents the case where the structure is the same as that in Example 4, but a distribution is not given to the resistance values. The resistor 42 is made of WGeON that has activation energy of 0.3 eV. All the resistance values are set to become REQ=1.0 MΩ at room temperature (300K). An average of the resistance values of the resistors 42 when the temperature distribution illustrated in
Comparative Example 3 represents the case where the structure is the same as that in Example 3 and a distribution is not given to the resistance values. The resistor 22 is made of a-Si that has activation energy of 0.8 eV. All the resistance values are set to become REQ=10 kΩ at room temperature (300K).
An average of the resistance values of the resistors when the temperature distribution illustrated in
To confirm the versatility of Examples, the effect has been checked while changing the ambient temperature under which the image display apparatus is operated (i.e., the environmental temperature).
In Example 1 (activation energy: 0.14 eV), a more satisfactory result, i.e., a smaller extent of nonuniformity in the displayed image, is obtained in comparison with Comparative Example 1 having the same activation energy. In Example 2, the nonuniformity in the displayed image appears in part of edge portions at relatively low temperature, but the nonuniformity in the displayed image over the entire screen is more satisfactorily reduced to a lower extent than in Example 1.
In Example 4 (activation energy: 0.3 eV), a more satisfactory result, i.e., a smaller extent of nonuniformity in the displayed image, is obtained in comparison with Comparative Example 2 having the same activation energy, and the nonuniformity in the displayed image is reduced to a half or below that in Comparative Example 1 having a lower level of the activation energy. Further, in Example 3 having a higher level of activation energy (0.8 eV), the nonuniformity in the displayed image appears at relatively low temperature (environmental temperature of 280K) and relatively high temperature (environmental temperature of 320K), but an image having a smaller extent of the nonuniformity in the displayed image is obtained in comparison with Comparative Example 3 having the same activation energy. In addition, comparing with Comparative Example 1 having a lower level of the activation energy, more satisfactory results are obtained at both relatively low and high temperatures in Example 3. In Examples 1 to 4, because the resistance values are set to become optimum at the environmental temperature of 300K, the nonuniformity in the displayed image may occur at relatively high and low temperatures. As a matter of course, however, the resistance values may also be set to become optimum at either relatively high or low temperature.
As described above, the nonuniformity in the displayed image during the operation of the image display apparatus can be reduced by previously giving a distribution to the resistance values at the same temperature.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2008-272126 filed Oct. 22, 2008, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-272126 | Oct 2008 | JP | national |
