JPH08273600A - Low-pressure mercury vapor discharge lamp and lighting system using same - Google Patents

Abstract

PURPOSE: To prevent impure gas released into a bulb from raising starting voltage or causing blackening of the bulb. CONSTITUTION: A phosphor coating 7 which differs in thickness along the direction of bulb axis is formed over the inner surface of a bulb 1 having an electrode 5. A mercury releasing metallic base 8 is secured to the thicker bulb end of the phosphor coating 7 inside the bulb. With the mercury releasing metallic base secured to the thicker bulb end of the phosphor coating, impure gas emitted from the mercury releasing metallic base or a mercury alloy is adsorbed by the phosphor coating to prevent diffusion of the impure gas.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-pressure mercury vapor discharge lamp such as a fluorescent lamp in which mercury is quantitatively sealed using a mercury-releasing metal substrate and an illuminating device using the same.

[0002]

2. Description of the Related Art Generally, a mercury vapor discharge lamp represented by a fluorescent lamp is constructed by sealing electrodes at both ends and enclosing a predetermined amount of mercury in a bulb having a fluorescent film on the inner surface. However, as a method of sealing mercury in the bulb, a method of dropping liquid mercury from an exhaust pipe connected to the end of the bulb is general. However, such an encapsulation method is difficult to quantitatively enclose mercury, and in order to solve this, mercury is made into an alloy form to make a mercury alloy pellet, and the mercury alloy pellet is put into a valve, A method of heating this mercury alloy from the outside to release mercury has been studied and put to practical use. This mercury alloy pellet is, for example, an alloy of zinc and mercury, in which zinc serves as a mercury-releasing metal substrate, and mercury is excessively melted in this, and excess mercury is released when heat is applied. After the excess mercury has been released, there remains a mercury-releasing metal substrate with an appropriate amount of mercury forming an alloy with the metal. With such a mercury alloy pellet, since mercury can be handled in the form of alloy granules, quantitative control is easy and the amount of the enclosed can be regulated with high accuracy.

[0003]

However, the mercury alloy pellets as described above often adsorb external impure gas during the manufacturing process and storage of the alloy pellets.
When such a mercury alloy pellet is put into a bulb and heated from the outside, an impure gas is also released into the bulb together with mercury.

Such an impure gas causes a rise in the starting voltage, and adheres to the bulb wall to cause blackening, which causes a problem that the luminous flux is reduced at an early stage. The present invention has been made based on such a situation, and its object is to focus on the fact that the phosphor coating has a gettering action, and to impure impurities emitted from a mercury alloy or a mercury-releasing metal substrate. It is an object of the present invention to provide a low-pressure mercury vapor discharge lamp and a lighting device using the same, which can prevent an increase in starting voltage and blackening of a bulb by allowing a coating film to be adsorbed.

[0005]

According to a first aspect of the present invention, a phosphor coating is formed on the inner surface of a bulb having an electrode means, and the bulb wall temperature during lamp operation is exceeded and the bulb softening temperature is formed in the bulb. The phosphor coating comprises a mercury-releasing metal substrate that melts below the temperature and does not substantially reabsorb mercury after mercury emission. The phosphor coating has a film thickness difference along the bulb axis direction. A low-pressure mercury vapor discharge lamp characterized in that the above-mentioned mercury-releasing metal substrate is fixed to the end of the bulb having the thicker film thickness.

According to a second aspect of the present invention, in the low-pressure mercury vapor discharge lamp according to the first aspect, the difference in film thickness of the phosphor film is 1.3 times or more the maximum film thickness with respect to the minimum film thickness. Is characterized by.

According to a third aspect of the present invention, in the low-pressure mercury vapor discharge lamp according to the first or second aspect, the bulb seals the glass tube and the end of the glass tube and supports the electrode. The mercury-releasing metal substrate is located at an end of the bulb having a thicker film thickness of the phosphor coating and is fixed to a boundary portion between the glass tube and the stem.

The invention of claim 4 provides the low-pressure mercury vapor discharge lamp according to any one of claims 1 to 3,
The valve has a ring shape, and the mercury-releasing metal substrate is fixed to an end portion of the ring valve opposite to the end portion to which the exhaust pipe is connected.

According to a fifth aspect of the present invention, in the low-pressure mercury vapor discharge lamp according to the fourth aspect, a node is formed at an end of the annular bulb, and the mercury-releasing metal substrate is fixed to the node. It is characterized by

The invention of claim 6 provides the low-pressure mercury vapor discharge lamp according to any one of claims 1 to 5,
The mercury-releasing metal substrate is mainly composed of zinc. The invention according to claim 7 is the low-pressure mercury vapor discharge lamp according to any one of claims 1 to 6, a lighting circuit device for lighting the lamp, and an illumination to which the fluorescent lamp and the lighting circuit device are attached. A lighting device comprising: a fixture.

[0011]

According to the invention of claim 1, since the mercury-releasing metal substrate is fixed to the bulb end portion of the phosphor coating having a larger film thickness, the mercury-releasing metal substrate and the mercury-releasing metal substrate in the alloy state before mercury emission are used. The impure gas released to the is well adsorbed on the phosphor coating. Since the phosphor has a getter action for adsorbing an impure gas by nature, and since the larger the volume, the better the adsorption ability, the mercury-releasing metal is attached to the end of the bulb with the larger thickness of the phosphor coating. If the substrate is fixed, the impure gas emitted from the mercury-releasing metal substrate will be adsorbed by the phosphor coating having a large adsorption capacity at a position close to the substrate, and thus it will be prevented from diffusing over the entire bulb. Therefore, the proportion of the impure gas released from the mercury-releasing metal substrate and the mercury alloy remaining in the valve is reduced, the rise of the starting voltage is suppressed, and the blackening of the valve can be prevented. When the phosphor coating is formed on the inner surface of the bulb, a difference in film thickness occurs along the axial direction of the bulb depending on the flow condition of the phosphor coating liquid. Therefore, the mercury-releasing metal substrate may be fixed to the end of the bulb having the larger film thickness.

According to the second aspect of the invention, when the difference in the film thickness of the phosphor film is 1.3 times or more the maximum film thickness, the mercury emission is emitted to the end of the valve having the larger film thickness. Since the metal substrate is fixed, a remarkable effect can be obtained as compared with the case where it is fixed on the opposite side.

According to the invention of claim 3, since the mercury-releasing metal substrate is fixed to the boundary portion between the glass tube and the stem, this portion is a portion where no phosphor coating exists,
When the mercury is evaporated and scattered from the mercury alloy, even if it is heated from the outside, the phosphor coating is not thermally deteriorated.

According to the fourth aspect of the invention, since the bulb is bent in a ring shape, a significant difference in film thickness occurs in the phosphor coating during bending. In particular, the end portion side to which the exhaust pipe is connected extends much more than the other end side. Therefore, if the mercury-releasing metal substrate is fixed to the end portion of the valve with the thicker film thickness, the getter function is further enhanced and the starting voltage is increased. Is suppressed and the valve is prevented from blackening.

According to the fifth aspect of the present invention, since the mercury-releasing metal base is fixed to the node formed at the end of the annular valve, the fixed contact area of the mercury-releasing metal base is large and it is difficult to drop it.

According to the sixth aspect of the present invention, since the mercury-releasing metal substrate is mainly composed of zinc, the alloy of zinc and mercury melts above the wall temperature of the bulb during lamp operation and below the softening temperature of the bulb. It can be easily melted and fixed to the valve in this temperature range by heating from the outside. According to the invention of claim 7, it is possible to provide an illumination device having the features of the fluorescent lamp.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the first embodiment shown in FIGS. In the figure, reference numeral 1 denotes a bulb of a ring-shaped fluorescent lamp, which is formed by bending a tube made of soda lime glass containing a lead component into a substantially ring shape. Both ends of the valve 1 are opposed to each other, and flare stems 2 and 2 are sealed to the both ends. Exhaust pipes 3 and 3 and wells 4 and 4 are airtightly penetrated through these stems 2 and 2, respectively, and a filament electrode 5 is installed on these wells 4 and 4. The electrode 5 is a tungsten wire 2
It is formed by forming a heavy coil or triple coil,
An emitter (not shown) is applied.

The exhaust pipe 3 on one side is closed before the stem 2 is welded to the valve 1, and the exhaust pipe 3 on the other side is closed.
Are sealed off after the sealing step. The ends of the valve 1 are formed with nodes 6a and 6b having a diameter reduced by molding so that the valve 1 can be easily gripped by a chuck on the bending machine side when the valve 1 is bent.

A phosphor coating 7 is formed on the inner surface of the bulb 1. The phosphor coating 7 is made of a three-wavelength light emitting phosphor or a white light emitting phosphor, and is applied to the entire inner surface of the bulb 1. When the phosphor coating 7 is formed on the inner surface of the bulb 1, when the bulb 1 is a straight tube which is not yet bent, the bulb axis is supported in the vertical direction, and the phosphor solution is flown from the upper end opening. Apply to the inner surface of the valve. In this case, since the phosphor solution drops by gravity, the phosphor coating 7 becomes thicker on the lower end side than on the upper end side of the bulb 1. Therefore, the phosphor coating 7 has a film thickness difference along the tube axis direction of the bulb 1. In the case of such a straight tube type bulb, the film thickness difference between both ends of the phosphor coating 7 is about 1.3 times.

Moreover, since the lamp of this embodiment has a ring shape, it can be obtained by bending the straight glass tube. In this bending process, the exhaust pipe 3 which is not yet sealed and protruded from one end of the valve 1 is connected to an exhaust machine (not shown), and the node portion 6a on the exhaust pipe 3 side is held by a chuck of the exhaust machine. Then, the node portion 6b on the other end side is held by the chuck on the bending molding drum side. In this state, the bulb is heated to soften it, and the bending drum is rotated to wind the glass tube around the outer periphery of the drum. Then, the valve is bent into an annular shape along the outer periphery of the bending drum. At this time, the valve 1 is wound around the drum while pulling the valve so as not to cause wrinkling, and therefore the valve 1 has one end side provided with the opened exhaust pipe 3 extended more than the other end side. Along with this extension, the phosphor coating 7 formed on the inner surface also tends to extend largely on the one end side provided with the open exhaust pipe 3 as compared with the other end side. Therefore, the phosphor coating 7
Is thicker on the other end side without the exhaust pipe than on the one end side with the opened exhaust pipe 3. Phosphor coating 7 in this case
The film thickness difference may be 1.5 times or more.

A mercury-releasing metal substrate 8 is fixed in the bulb 1. The mercury-releasing metal substrate 8 is made of, for example, an alloy of zinc and mercury, and is in the form of pellets in which mercury is excessively melted before being charged into the bulb 1. The pellets have a mercury concentration of 50 to 60% by weight. It is granular and has a diameter of about 1.5 mm.

Such alloy pellets are injected into the valve 1 through the open exhaust pipe 3 and are heated from the outside to release excess mercury, and are simultaneously deposited on the valve wall. In this case, the mercury alloy is fixed to the end of the bulb 1 on which the phosphor coating 7 is not formed, and is fixed to the end of the fluorescent coating 7 having a larger film thickness.

In the case of the ring-shaped fluorescent lamp of the present embodiment, the mercury alloy pellets are fixed to the already sealed bulb end of the exhaust pipe 3 side, and are located closer to the bulb end than the node 6b. It is welded to the boundary between the glass tube and the stem 2. Mercury is released by this heat welding, and thus the mercury alloy pellet after welding is an alloy with zinc in which an appropriate amount of mercury remains, which is referred to as a mercury releasing metal substrate 8. After releasing excess mercury, the mercury-releasing metal substrate 8 does not substantially reabsorb mercury, and does not reabsorb so much as to affect the mercury vapor pressure in the valve, unlike amalgam. An ionizable medium containing a rare gas such as mercury and argon evaporated from the alloy pellets is enclosed.

Such a lamp is manufactured as follows. A straight tube type tube made of soda lime glass is supported with the bulb axis in the vertical direction, and the phosphor solution is flown from the upper end opening and coated on the inner surface of the bulb. When this phosphor coating 7 is dried, it is heated from the outside and the phosphor coating 7 is heated.
To bake. As a result, the phosphor coating 7 is formed thicker on the lower end side of the bulb 1 than on the upper end side thereof.

At both ends of the glass tube, a stem 2 made of lead glass having an electrode 5 and an exhaust pipe 3 is sealed. At this time, one exhaust pipe 3 corresponding to the lower end side of the valve 1 is closed, and the other exhaust pipe 3 corresponding to the upper end side is open. When sealing the stem 2, the nodes 6a and 6b are molded at the boundary between the glass tube and the stem 2 while the glass is soft.

After this, the valve 1 is placed in a vertical posture with the valve end portion to which the opened exhaust pipe 3 is connected facing upward, and the exhaust pipe 3 is connected to an exhaust machine (not shown), and the node of the upper end portion is connected. The portion 6a is held by the chuck of the exhaust machine, and the node portion 6b at the lower end is held by the chuck on the bending drum side. In this state, the entire valve is heated to soften it. After this softening, the bending drum is rotated to wind the softened valve 1 around the outer circumference of the drum. Then, the valve 1 is bent into an annular shape along the outer circumference of the bending drum. At this time, the gas pressure in the valve 1 is increased to prevent the valve from being crushed, and at the same time, the valve is pulled and wound around the drum to prevent the valve from wrinkling.

By such bending, the valve end, that is, the node 6b, to which the exhaust pipe that is not opened is connected.
The film thickness of the phosphor coating 7 on the side gradually becomes larger than the film thickness on the opposite end.

After the bending is completed, the exhaust gas is exhausted from the exhaust pipe 3 which is opened while heating the entire lamp to replace the rare gas. Then, the mercury alloy pellets are injected into the bulb 1 through the exhaust pipe 3. After this, the exhaust pipe 3
Tip off.

Next, the posture of the bulb 1 is changed to move the charged mercury alloy pellets to the other end on the side of the node 6b,
The joint 6b is located at the boundary between the glass tube and the stem 2. In this state, the mercury alloy pellets are heated from the outside. The heating temperature at this time is higher than the temperature of the bulb tube wall at the time of operating the lamp, but lower than the softening temperature of the glass. By this heating, the mercury alloy pellets release excess mercury and are melted and fixed to the boundary portion between the glass tube of the node portion 6b and the stem 2. Therefore, the mercury-releasing metal substrate 8 after the excess mercury is released is melted and fixed to this boundary portion.

When the above mercury alloy pellets are heated,
It is desirable to satisfy the following conditions. 5 for mercury and zinc
0% by weight: 50% by weight of a mercury alloy pellet with a diameter of about 1.5 mm exceeds the temperature of the bulb tube wall during lamp operation,
In addition, the temperature is lower than the softening temperature of the glass, that is, the temperature of the bulb wall is about 300 ° C.
Hold for 0 seconds. This causes the mercury alloy pellets to release excess mercury and melt. After that, the mercury alloy pellets are solidified by natural cooling.
A minute vibration of about 9 to 0.35 G is applied for 20 seconds to shake the mercury alloy pellets in a molten state so that the molten mercury alloy pellets are made to fit on the glass surface to improve the wettability.

As a result, the mercury-releasing metal substrate 8 becomes the node portion 6b.
It will be fixed to the boundary between the glass tube and the stem 2. The fixed position of the mercury-releasing metal substrate 8 is the bulb end of the phosphor coating 7 having the larger film thickness.

The fluorescent lamp having the above-described structure is attached to the lighting fixture 10 as shown in FIG. 3 to form a lighting device. The luminaire 10 includes a reflector 11 and a ballast housing 12, and the ballast housing 12 houses a ballast 13 as a lighting circuit component. The fluorescent lamp attached to the lighting fixture 10 is kept on by the ballast 13.

The operation of the fluorescent lamp and the lighting device of the first embodiment having such a configuration will be described. When the lamp is turned on, mercury is ionized and excited to emit ultraviolet rays, which are converted into visible light by the phosphor coating 7 and emitted to the outside of the bulb 1. The light emitted from such a lamp is reflected, for example, via the reflector 11 of the lighting fixture 10 and functions as downward illumination.

In the above fluorescent lamp, the mercury alloy pellets and the mercury-releasing metal substrate 8 left after the excess mercury is emitted from the pellets are fixed to the end of the thicker bulb in the phosphor coating 7. Therefore, even when the impure gas is emitted from the mercury pellets or from the mercury-releasing metal substrate 8, the impure gas is adsorbed by the gettering action of the phosphor coating 7 at a position close in distance.
In particular, since the phosphor coating 7 near the mercury-releasing metal substrate 8 is formed to have a large film thickness, the getter action is large,
Therefore, the impure gas emitted from the mercury pellets or from the mercury-releasing metal substrate 8 is adsorbed by the phosphor coating 7 in the thick portion before being diffused into the space of the bulb 1. For this reason, the impure gas is not mixed with the discharge medium containing mercury and rare gas, so that the starting voltage is not increased and the phosphor and the bulb wall are not blackened.

The film thickness difference of the phosphor coating 7 is remarkable when the mercury-releasing metal substrate 8 is fixed to the thicker bulb end when the maximum film thickness is 1.3 times the minimum film thickness or more. The effect is obtained. If the film thickness difference is less than 1.3 times, a significant difference due to the distinction cannot be confirmed. In particular, in the case of a ring-shaped fluorescent lamp, when the bulb 1 is bent, the end portion side to which the open exhaust pipe 3 is connected expands much more than the other end side, and at the same time, the phosphor coating 7 also expands greatly and the film thickness increases. Become thin. Therefore, the thickness of the phosphor coating becomes thicker at the end opposite to the end to which the exhaust pipe of the bulb is connected, and for example, the thickness difference may reach 1.5 times. Therefore, if the mercury-releasing metal substrate 8 is fixed to the thicker valve end portion, the effect becomes remarkable.

Further, the lamp of the above-mentioned embodiment is a ring-shaped fluorescent lamp, and since the ring-shaped fluorescent lamp has the nodes 6a and 6b at the end of the bulb 1, the mercury-releasing metal substrate is provided at one of the nodes 6b. If 8 is fixed, the mercury-releasing metal substrate 8
The contact area with the valve becomes large and the adhesion strength becomes large, so it is difficult to fall off.

Further, if the mercury-releasing metal substrate 8 is further fixed to the boundary portion between the glass tube and the stem 2, this portion is a position where the phosphor coating 7 does not exist, so that mercury is evaporated and scattered from the mercury alloy pellets. When the phosphor coating 7 is heated from the outside, the phosphor coating 7 is not thermally deteriorated.

If the mercury-releasing metal substrate 7 is formed of a mercury alloy containing zinc as a main component, this alloy is 3
When heated at a temperature of about 00 ° C. for 30 seconds, excess mercury is released and melted and adhered to the bulb 2. Since this heating temperature exceeds the temperature of the bulb wall at the time of operating the lamp, but is lower than the softening temperature of the bulb, there is no fear of softening the bulb and thermal deformation can be prevented.

In the lighting device of FIG. 3 using such a fluorescent lamp as a light source, blackening of the lamp itself is prevented, so that a high luminous flux can be maintained for a long period of time.

In the above-mentioned embodiment, the case of the ring type fluorescent lamp has been described, but the present invention is not limited to this, and a straight tube type fluorescent lamp can be used as in the second embodiment shown in FIG. it can. In the case of a straight tube type fluorescent lamp, when the phosphor coating 7 is formed on the inner surface of the straight tube type bulb 2, the bulb axis is supported in a vertical posture, and the phosphor solution is flown from the upper end opening and applied to the inner surface of the bulb. To do. In this case, since the phosphor solution flows down, the phosphor coating 7 is formed thicker on the lower end side of the bulb 1 than on the upper end side thereof.

Therefore, if the mercury-releasing metal substrate 8 is fixed to the bulb end portion of the phosphor coating 7 having a larger film thickness, the gettering action of the phosphor coating 7 causes release from the alloy pellets or the mercury-releasing metal substrate 8. The impure gas generated will be adsorbed by the enemy.

Further, the present invention is not limited to the fluorescent lamps shown in the above-mentioned respective embodiments, but in addition, bent fluorescent lamps such as U-shaped, W-shaped, H-shaped and saddle-shaped, or in short, mercury alloy pellets are used. It can be applied to a low pressure metal vapor discharge lamp in which mercury is enclosed in the bulb.

[0043]

As described above, according to the first aspect of the present invention, the mercury-releasing metal substrate is fixed to the bulb end portion of the phosphor coating having the thicker film thickness. The impure gas released at the previous time becomes better adsorbed on the phosphor coating, preventing the impure gas from diffusing through the bulb. Therefore, the rise of the starting voltage is suppressed and the blackening of the valve is prevented.

According to the second aspect of the present invention, when the difference in film thickness of the phosphor coating is 1.3 times or more, the mercury-releasing metal substrate is fixed to the end of the bulb having the larger film thickness. A remarkable effect can be obtained rather than sticking to the.

According to the third aspect of the present invention, since the mercury-releasing metal substrate is fixed to the boundary portion between the glass tube and the stem, this portion is a portion where no phosphor coating exists, and mercury is evaporated and scattered from the mercury alloy. In this case, the phosphor coating is not thermally deteriorated even if it is heated from the outside.

According to the fourth aspect of the invention, since the bulb has a shape bent in an annular shape, a large film thickness difference occurs in the phosphor coating, and particularly the end portion side to which the exhaust pipe is connected is the other end side. Therefore, if the mercury-releasing metal substrate is fixed to the end of the valve having the larger film thickness, the getter function is further enhanced, the rise of the starting voltage is suppressed, and the blackening of the valve is also prevented.

According to the fifth aspect of the present invention, since the mercury-releasing metal base is fixed to the node formed at the end of the ring-shaped bulb, the fixed contact area of the mercury-releasing metal base is large and it is difficult to drop it. According to the sixth aspect of the present invention, since the mercury-releasing metal substrate is mainly composed of zinc, the alloy of zinc and mercury melts above the bulb wall temperature during lamp operation and below the softening temperature of the bulb. By heating, it can be easily melted and fixed to the valve in this temperature range. According to the invention of claim 7, it is possible to provide an illumination device having the features of the fluorescent lamp.

[Brief description of drawings]

FIG. 1 is a plan view of a ring-shaped fluorescent lamp showing a first embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view showing an end portion of the ring-shaped fluorescent lamp of the same embodiment.

FIG. 3 is a diagram showing a configuration of an illumination device using the ring-shaped fluorescent lamp of the embodiment as a light source.

FIG. 4 is a sectional view of a straight tube type fluorescent lamp showing a second embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Bulb 2 ... Stem 3 ... Exhaust pipe 4 ... Wells 5 ... Electrodes 6a, 6b ... Nodes 7 ... Phosphor coating 8 ... Mercury emitting metal substrate 10 ... Lighting fixture 13 ... Ballast

Claims (7)

[Claims] 1. A bulb in which a discharge medium containing mercury and a rare gas is sealed; electrode means for generating a discharge in the bulb; and a film thickness difference formed along an axial direction of the bulb formed on an inner surface of the bulb. A phosphor coating; fixed inside the bulb to the thicker bulb end of the phosphor coating,
A low-pressure mercury vapor discharge characterized by comprising: a mercury-releasing metal base that melts above the bulb wall temperature during lamp operation and below the softening temperature of the bulb and that does not substantially reabsorb mercury after the mercury is released. Electric light. 2. The low-pressure mercury vapor discharge lamp according to claim 1, wherein the phosphor coating has a maximum film thickness of 1.3 times or more of a minimum film thickness. 3. The bulb includes a glass tube and a stem that seals an end portion of the glass tube and supports the electrode, and the mercury-releasing metal substrate has a thick phosphor coating film. The low-pressure mercury vapor discharge lamp according to claim 1 or 2, wherein the low-pressure mercury vapor discharge lamp is located at an end of the other bulb and fixed to a boundary portion between the glass tube and the stem. 4. The valve has a ring shape, and the mercury-releasing metal substrate is fixed to an end of the ring valve opposite to the end to which the exhaust pipe is connected. A low-pressure mercury vapor discharge lamp according to any one of claims 1 to 3. 5. The low-pressure mercury vapor discharge lamp according to claim 4, wherein a node is formed at an end of the annular bulb, and a mercury-releasing metal substrate is fixed to the node. 6. The mercury-releasing metal substrate is mainly composed of zinc.
The low-pressure mercury vapor discharge lamp described in. 7. A low-pressure mercury vapor discharge lamp according to any one of claims 1 to 6, a lighting circuit device for lighting the lamp, and a lighting fixture to which the fluorescent lamp and the lighting circuit device are attached. A lighting device comprising: