Patent Application
20090046223
The present invention relates to a discharge light source capable of reducing environmental load without using mercury, and to a lighting apparatus for driving such a light source.
Recently, as the digital television is becoming wider in screen and smaller in thickness, there is an increasing demand for larger size of liquid crystal display backlight. As the light source for liquid crystal display backlight, the conventional cold cathode fluorescent lamp is being replaced by solid light-emitting device such as light-emitting diode or organic EL element, and commercial products are partly developed. However, for the time being, the cold cathode fluorescent lamp may not be completely replaced in view of the viewpoints of efficiency of light emission, service life, and cost.
The fluorescent lamp uses a low-pressure glow discharge including mercury which is an environmental load, as an ultraviolet source for exciting phosphor as light-emitting material. In view of environmental protection, it is being desired to develop a light source having light emission efficiency equal to that of the existing fluorescent lamp without using mercury.
To achieve the purpose, it is required to develop a radiation source capable of emitting efficiently ultraviolet emission with wavelength (about 100 to 300 nm) enough to excite phosphors to radiate light effectively. Noticeable ultraviolet radiation medium other than mercury, which radiates ultraviolet by discharge, is a discharge plasma at low to medium pressure (about less than atmospheric pressure), which is mainly composed of rare gases. One photon of ultraviolet emission is finally converted to one photon of visible light by a phosphor, and the energy corresponding to the difference between ultraviolet emission energy and visible light energy makes loss. Hence, the wavelength of ultraviolet emission caused by discharge is preferred to be closer to that of visible light. Accordingly, among rare gas discharges, especially the discharge plasma which is mainly composed of xenon is considered useful since the wavelength of the radiated ultraviolet emission is relatively longer.
In xenon discharge, in particular, it is known that broad radiation efficiency is high around 172 nm, radiated upon dissociation of excimer (excited dimer) which is unstable bonding of xenon atoms in excited state and in ground state. Generally, generation, radiation, and dissociation of excimer are particularly high in efficiency in pulse after-glow. Accordingly, as compared with ordinary glow discharge, a higher efficiency is expected in the so-called dielectric barrier discharge having a dielectric layer serving as a charge barrier for cutting off current flow between the electrode and discharge space.
Therefore, regarding rare gas fluorescent lamps which using rare gas discharge caused by mainly xenon, particularly, one having a structure which uses glass tube wall of the discharge tube as dielectric layer of charge barrier has been intensively studied. As an example of such structure, the lamp disclosed in patent document 1 is shown in
During the lamp operation, a dielectric barrier discharge is generated between the internal electrode 1 and the external electrode 3 with the tube wall of the discharge tube 2 serving as charge barrier, and the ultraviolet emission is efficiently radiated from the filled rare gas such as xenon. This excites the phosphor layer to emit light.
Patent Document 1: JP-A-2002-042737
In the configuration shown in
In this method, however, an excessive electric power is applied to especially a portion near the internal electrode 1 in which ultraviolet emission efficiency is lower, and therefore the overall efficiency is lowered. When the length of the discharge tube 2 is changed depending on the television screen size, or the electric power provided to the backlight is changed by dimming according to the screen, a luminance is required to be kept uniform. However, such design of winding pitch of the external electrode 3 is extremely difficult, and it lacks flexibility in practical use.
The invention is directed to solve the above problems, and has a purpose to provide a rare gas fluorescent lamp which uses no mercury and is capable of providing uniform luminance distribution in the longitudinal direction even at low driving voltage and a lighting apparatus of the rare gas fluorescent lamp.
A lamp lighting apparatus according to the invention includes a fluorescent lamp, and a power supply circuit for supplying a driving voltage to the fluorescent lamp. The fluorescent lamp includes a discharge tube made of transmissive material having a phosphor layer formed on the inner surface of the discharge tube and filled with a discharge gas, a first internal electrode provided inside the discharge tube and at one end of the discharge tube, for applying a rectangular alternating voltage of high frequency, a second internal electrode provided inside the discharge tube and at the opposite end the discharge tube to the first internal electrode, an external electrode provided along the longitudinal direction of the discharge tube, and a capacitive element for discharging internal charge which is electrically connected to the second internal electrode.
A fluorescent lamp according to the invention includes a discharge tube made of transmissive material having a phosphor layer formed on the inner surface of the discharge tube and filled with a discharge gas, a first internal electrode provided inside the discharge tube and at one end of the discharge tube, for applying a rectangular alternating voltage of high frequency, a second internal electrode provided inside the discharge tube and at the opposite end the discharge tube to the first internal electrode, an external electrode provided along the longitudinal direction of the discharge tube, and a capacitive element for discharging internal charge which is electrically connected to the second internal electrode.
A liquid crystal display device according to the invention includes a liquid crystal display panel and a backlight device for illuminating the liquid crystal display panel. The backlight device includes the lamp lighting apparatus as described above.
The invention controls amount of residual charge inside the discharge tube by discharging the residual charge in discharge process to outside of the discharge tube through the capacitive internal charge discharge element. This lowers conductivity of the plasma when inverting the polarity at the dielectric barrier discharge, so that the discharge efficiency may be uniform throughout the overall length of the discharge tube. AS a result, a rare gas fluorescent lamp for backlight capable of providing uniform luminance distribution in the longitudinal direction of the discharge tube and has good efficiency is achieved.
Preferred embodiments of the invention are described below with reference to the accompanying drawings.
The discharge tube 102 is held at a distance of 3.0 mm (the shortest distance between the outer surface of the discharge tube 102 and the external electrode 103) from a flat-shaped external electrode 103 made of aluminum material, by spacers 105 made of insulating member such as silicone resin. The surface of the external electrodes 103 is treated by high luminance reflection coating. The term “flat-shaped” does not always mean to be perfectly flat. For example, it allows a shape having a width larger than the diameter of the discharge tube 102 and a carved shape with a radius of curvature larger than the distance to the axis of the discharge tube 102.
The one 101a of the internal electrodes 101 is used as an internal electrode for driving (referred to as “driving internal electrode”). A rectangular alternating voltage of 20 kHz in frequency is applied between the driving internal electrode 101a and the external electrode 103 from a power supply circuit for lighting lamp (not shown in
At the opposite end of the discharge tube 102 to the driving internal electrode 101a, an internal electrode 101b for adjusting an internal charge (referred to as “adjusting internal electrode”) having cup-shaped cold cathode similar to the driving internal electrode 101a is provided in the discharge tube 102. The adjusting internal electrode 101b is connected electrically and physically to the conductive member 104 at outside of the discharge tube 102, serving as an internal charge discharge element. The internal charge discharge element has a function of discharging outside the electric charge accumulated at the end of the discharge tube 102. The conductive member 104 is a flat member having conductivity and disposed in a plane parallel to the external electrode 103. Herein, both the adjusting internal electrode 101b and the conductive member 104 are set at floating potential. Preferably, the conductive member 104 is made from an aluminum plate of about 1 cm2 in area with the distance to the external electrode 103 of about 4.5 mm.
For understanding of such effect, progress of dielectric barrier discharge between the driving internal electrode 101a and the external electrode 103 is explained briefly with reference to
As the applied voltage of the driving internal electrode 101a becomes higher and the discharge gas is broken in insulation, firstly, the discharge is started near the internal driving electrode 101a at which electric field intensity is highest. As the discharge is started, a plasma is generated inside the discharge tube 102. Positive and negative charges in the plasma (mainly ions and electrons) drift to the driving internal electrode 101a and the external electrode 103 respectively in the space in the discharge tube 102 by the electric field between the driving internal electrode 101a and external electrode 103, and hence a lamp current flows. The electric charge (electron) drifting to the side of the external electrode 103 is accumulated on the tube wall of the discharge tube 102 because the tube wall of the discharge tube 102 which is an insulator serves as a charge barrier. The accumulated charges neutralize the electric field between the electrodes by the electric field generated from the accumulated charges. Therefore, near the driving internal electrode 101a where discharge was first started, discharge in discharge gas cannot be maintained, and the discharge stops.
As a result, the charges (referred to as “residual charges”) remaining in the space without drifting in the plasma generated by the initial discharge are present in a state similar to the so-called pulse after-glow plasma. The plasma behaves like a conductor having a finite electric resistance. Thus the leading end portion A of the residual charges become a pseudo internal electrode having a potential lower than the potential of the driving internal electrode 101a by the voltage drop across the residual charges.
On the other hand, since in the region ahead of the leading end portion A of the residual charges, charges are not accumulated in the tube wall of the discharge tube 102, the discharge can be started by the electric field caused by potential difference between the leading end portion A of the residual charges and the external electrode 103. Therefore, until the potential in the leading end portion A of the residual charges becomes lower than a discharge start voltage by the voltage drop in the plasma, or until the leading end portion A of the residual charges reach the end portion of the discharge tube 102, the discharge develops while repeating the above process in every small distance in the longitudinal direction, extending the plasma of the residual charges. Since the extending speed is usually very fast (more than 1×106 m/sec), at a frequency of about 20 kHz as in the present embodiment, a pulse-shaped lamp current flows immediately after the polarity of applied voltage is inverted. Almost no current flows in a half period until the polarity is inverted again (about 25 microseconds), and this period is a discharge stop period.
After completion of discharging, until the polarity of the applied voltage is inverted again, since the accumulated charges are maintained by the applied voltage, the discharge tube 102 is free from effective electric field. However, it takes more than scores of microseconds until the residual charges are lost completely (due to volume recombination and ambipolar diffusion), and some residual charges are left over at the time of next inversion of polarity.
The ultraviolet emission for exciting the phosphors in a rare gas fluorescent lamp using xenon mentioned above includes bright resonance line emission of 147 nm radiated from xenon excited atom, and continuum emission having a peak at 172 nm radiated when xenon excimer is dissociated. In particular, continuous radiation from excimer is high in efficiency, and therefore it is important for enhancing the lamp efficiency to generate the continuous radiation from excimer effectively. The excimer is generated by collision reaction of one xenon atom in excited state and two xenon atoms in ground state (three-body collision process). The xenon excited atom requires a higher energy for excitation than the mercury used in the general fluorescent lamp. For efficient production of xenon excited atom, a higher energy of electron (higher electron temperature) in the plasma is needed. Therefore, in the resonance line emission of 147 μm, electrons are accelerated in high electric field, and mainly pulsed radiation occurs when the polarity of the applied voltage is inverted.
On the other hand, since electrons are not present in the three-body collision process, formation of excimer and 172 nm continuous radiation continue after completion of discharge. If an electron current is present, the xenon excited atom once excited is easily ionized by collision against an electron of relatively low energy (cumulative ionization), and the forming efficiency of excimer is lowered in a plasma of high current density. Therefore, the lower current density is desired from the viewpoint of efficiency of continuous radiation from the excimer.
Applying the above discussion to the rare gas fluorescent lamp of the invention having the configuration shown in
When the applied voltage is low, at the opposite side of the driving internal electrode 101a, the luminance is lowered by the drop of the plasma potential contributing to the development of discharge. To the contrary, when the applied voltage is raised, the efficiency is lowered by the excessive current near the driving internal electrode 101a. That is, if it is attempted to progress the discharge further merely by raising the applied voltage, the current density elevates near the driving internal electrode 101a, and the ultraviolet emission efficiency drops, lowering the luminance. If much plasma of residual charges exist upon inversion of polarity of the applied voltage, the inside space of the discharge tube 102 has small electric resistance, and the current density becomes high at the time of discharge. In an extreme case, the discharge shrinks to be filamentary, and a streamer state (constricted state) may be observed. As known from the above discussion, in such state, the electron temperature is low, and the xenon excitation efficiency drops and the cumulative ionization is dominant, and hence the radiation efficiency of ultraviolet emission drops. In particular, at the opposite side end of the driving internal electrode 101a, the electric field intensity drops due to excessive presence of residual charges. Hence, even if the applied voltage is raised, the luminance is not increased sufficiently, and the applied voltage must be further raised.
The life of the residual charge is determined almost by the composition and pressure of the discharge gas. However, since the composition and pressure of the discharge gas are also related to the light emission efficiency and the lamp life, they cannot be determined independently. Accordingly, to decrease the residual charges sufficiently when inverting the polarity of the applied voltage generated by pulse discharge, it seems effective to wait until the residual charges recombine to disappear, that is, to prolong the discharge stop period by lowering the driving frequency. However, when the driving frequency is lowered, the number of times of discharge per unit time is decreased and the light emission quantity drops so that the light output from the lamp becomes smaller. Hence, to keep a required quantity of light, the number of lamps must be increased, and hence it is not practical to lower the frequency extremely.
The inventors of the present invention, on the basis of the discussion of physical process explained above, considered means of controlling positively the residual charges in the discharge tube 102 without waiting for spontaneous extinction. As a result, the inventors have concept provision of the internal charge discharge element according to the present invention.
The adjusting internal electrode 101b is connected to the conductive member 104 at outside, and is exposed to inside of the discharge tube 102, and is maintained at a potential nearly equal to the residual charge plasma in the discharge tube 102 both in the stop period and in discharge progress. Hence, the discharge is mainly limited to dielectric barrier discharge between the driving internal electrode 101a and the external electrode 103, and the adjusting internal electrode 101b may be considered not to contribute to the discharge.
Further effects obtained by the configuration of the present embodiment are explained below with reference to
In
Further according to the configuration of the present embodiment, it is enough to make use of an ordinary process of manufacturing a cold cathode fluorescent lamp to fabricate the discharge tube 102 provided with electrodes at both ends, and to connect the conductive member 104 to one electrode to form the adjusting internal electrode 101b with the other electrode used as the driving internal electrode 101a. Compared to the case of using circuit elements as internal charge discharge element (for example, a capacitor with high rating voltage and small capacity), the structure is very simple and great effects are obtained as mentioned above without largely modifying the process in mass production. Therefore, the cost increase can be suppressed to minimum.
As a result of multiple experiments, preferable internal charge discharge element in the present embodiment includes a conductive member 104 which is flat-shaped, has an area of 1 cm2 and a distance of about 4.5 mm between the conductive member 104 and the external electrode 103. In this case, the electric capacity is about 0.2 pF based on the result of measurement. The measuring method of the electric capacity is explained below.
As shown in
Of course, the required capacity Cx of the internal charge discharge element can be varied depending on various conditions, and hence the dimension of the conductive member 104 can be also changed. Increasing the area of the conductive member 104 and shortening the distance from the external electrode 103 allows the capacity Cx of the internal charge discharge element to be increased, increasing the charge amount to be discharged. As a result, it is possible to obtain the effect that makes the luminance distribution uniform even at a lower applied voltage.
However, at the same time, the discharge current increases, and the overall emission efficiency is lowered. For example, according to the experiment by the inventors of the present invention in which the area is varied with the distance fixed at 4.5 mm in the preferred embodiment, the efficiency is lowered by about 10% in the area of about more than 4 cm2. On the other hand, when the area of the conductive member 104 is too small, the effect for making uniform the luminance distribution is not obtained sufficiently.
The inventors have performed further experiments by varying the length of the discharge tube 102 and the distance between the discharge tube 102 and the external electrode 103, and studied the balance among effects of lowering of efficiency, uniform luminance in the longitudinal direction, and suppression of driving voltage. Then the inventors have discovered that a favorable range of electric capacity Cx of the internal charge discharge element is from 0.1 pF to 10 pF. If smaller than this range, the luminance distribution can not be uniform, or if larger than this range, the efficiency may drop and the light emission can not be stable, and flickering or worsening of characteristic is observed. Such favorable electric capacity may be realized arbitrarily by combination of area of conductive member 104 and distance from the external electrode 103, because the internal charge discharge element is a parallel flat capacitor.
The material of the conductive member 104 is not limited to aluminum, which can be metal. Instead of the conductive member 104, although not common, a capacitor element with high rating voltage and a capacity in a favorable range can be used.
The internal electrode 101 is a cup-shaped cold cathode, but the shape is not limited to this. The simpler shape can be applied, which can reduce cost, and the loss due to cathode decline may be reduced by coating with an emitter material. Similarly, regarding the external electrode 103, it is an aluminum flat plate with high luminance reflection coating in the present embodiment. However it may be formed of other material having a sufficient electric conductivity. For example, the discharge tube 102 may be formed in nearly a parabolic shaped and placed near the focus, which can enhance luminance on front. The surface of the external electrode 103 may be also formed as a diffusion plane.
A liquid crystal display backlight unit 300 shown in
In the present embodiment shown in
The material of the support member 106 may be selected properly in consideration of electric characteristics, aging effects and other conditions. In such a case, the area of the conductive member 104 and the distance to the external electrode 103 should be also designed appropriately.
Although in the second embodiment the external electrode 103 is a flat aluminum plate, the external electrode may be a conductor which is nearly flat-shaped and is provided for each discharge tube 102. In this case, preferably, all independent external electrodes 103 are set at the same reference potential.
The present invention realizes a fluorescent lamp excellent in uniformity of luminance at high efficiency without using mercury, and its utilization. Hence, it is very useful to liquid crystal display backlight, especially to a liquid crystal display backlight for television of wide screen.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-261986 | Sep 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2007/067908 | 9/14/2007 | WO | 00 | 5/29/2008 |
