KR20040013020A - Back-Light Unit utilizing Flat Fluorescent Lamp - Google Patents

Abstract

PURPOSE: A back light unit using a flat panel fluorescent lamp is provided to reduce a discharge starting voltage and consumption power of the lamp, improve light efficiency of the lamp, and obtain uniform discharge of the lamp. CONSTITUTION: A back light unit includes a flat panel fluorescent lamp(20), a diffusion member(31), and an inverter. The fluorescent lamp is composed of a back substrate(21), a front substrate(22), a fluorescent layer(25), and discharge electrodes(26,27) The front substrate is attached to the back substrate having a sealant(23) interposed between the two substrates. A discharge channel(30) is formed between the back substrate and front substrate. The fluorescent layer is coated on the discharge channel. A discharge gas is injected into the discharge channel. The discharge electrodes are respectively formed on the back substrate and front substrate, and creates dielectric barrier discharge. The diffusion member is located above the fluorescent lamp. The inverter supplies power to the discharge electrodes. The discharge channel is composed of a plurality of independent discharge spaces. The discharge electrodes are formed in a stripe shape.

Description

Back-Light Unit utilizing Flat Fluorescent Lamp

The present invention relates to a backlight unit using a flat fluorescent lamp, and more particularly to a backlight unit using the characteristics of a flat fluorescent lamp having an electrode structure for generating a dielectric barrier discharge.

In general, a display device is classified into a light emitting type and a light receiving type, and a light emitting type includes a CRT, a FED, a PDP, an organic EL, and a light receiving type, such as a liquid crystal display. The liquid crystal display itself does not have a structure that emits light and thus cannot implement an image unless external light is irradiated. Accordingly, an image must be realized by installing a backlight unit (Back Light) which is a separate light source.

The backlight device uses a light guide plate to produce light emitted from a cold cathode fluorescent lamp (CCFL) into a surface light source, and a plurality of cold cathode fluorescent lamps on a rear surface of a liquid crystal panel to form a surface light source. In this case, a discharge gas and a phosphor are placed between two sheets of flat glass, and a discharge is made into a surface light source.

As shown in FIG. 1, a flat fluorescent lamp 10 using two sheets of flat glass includes a rear substrate 11 and a front substrate 12 bonded to the rear substrate 11 via a sealing material 13. ) To form a discharge space. In addition, a phosphor layer 16 is formed on a lower surface of the front substrate 12, and a dielectric layer in which a plurality of discharge electrodes 14 having a predetermined pattern is embedded on the upper surface of the back substrate corresponding to the phosphor layer 16. 15) is formed, and the discharge space is filled with a discharge gas consisting of xenon (Xe), neon (Ne) and the like.

In this conventional flat panel fluorescent lamp 10, as the power is applied to the discharge electrode 14, the phosphor layer 16 is excited by the ultraviolet rays generated by the discharge between the discharge electrodes 14 and the surface of the liquid crystal display is imaged. This is implemented.

However, since the conventional flat fluorescent lamp uses xenon (Xe) or a mixed gas of xenon (Xe) and neon (Ne), not only the AC voltage applied to the discharge electrode is high but also the light efficiency is low as about 30 lm / W. Therefore, in order to irradiate as much light as necessary, the driving power must be increased, thereby increasing power consumption and generating a lot of heat. In addition, since xenon (Xe) gas used as the discharge gas emits ultraviolet rays of 147 nm to 173 nm, there is also a problem that an expensive phosphor should be used.

Some of these problems have been improved. A slightly advanced flat plate fluorescent lamp has been provided. The flat plate fluorescent lamp having a long discharge space formed of one discharge channel has both ends of the discharge channel. It has a structure in which an electrode is arranged.

That is, since the structure of the flat fluorescent lamp of the "-" shape has one long discharge channel, a large amount of discharge current flows in the discharge channel, and thus the light efficiency is increased. However, as the discharge channel is lengthened, the discharge start voltage is increased and the driving voltage is increased, and the leakage current is increased. In addition, as the liquid crystal display and the backlight device are enlarged in recent years, the discharge channel has a single discharge channel. The r-shaped flat fluorescent lamp has a problem in that its manufacture is also practically impossible because the length of the discharge channel is drastically increased.

In order to solve this problem, a method for manufacturing a flat fluorescent lamp (Korean Patent Publication No. 2001-0079377) forms a single discharge space consisting of a long discharge space having a "d" shape on the flat glass. In addition, because the glass of the weak material, which is difficult to be formed, must be heated to a moldable temperature, and the heated flat glass must be molded into a discharge space having a “d” shape, this also makes the manufacturing method difficult and does not completely solve the manufacturing method problems. I couldn't.

In addition, when power is applied to an external electrode to generate a discharge, cross talk is generated between discharge channels in which a strong discharge occurs in a specific discharge space or a severe vibration of the discharge plasma occurs in a long discharge space having an integrally formed “d” shape. There is a problem that is likely to occur. This is because the discharge current is easily confined to the adjacent discharge channel without being restricted when the discharge charges are moved through the passage of the internal discharge space where the electrode is located.

The above-mentioned problems occur in Japanese Patent Laid-Open No. 9-092208 and Korean Patent Laid-Open Nos. 5,903,096 / 5,509,841.

The present invention is to solve the problems of the prior art, the object of the present invention is to form a discharge channel in which each discharge space is formed in a stripe shape, the discharge start voltage and power consumption is low, the light efficiency is improved, uniform The present invention provides a backlight unit using a flat fluorescent lamp that can obtain a discharge.

Liquid crystal display backlight unit using a flat fluorescent lamp of the present invention for achieving the above object is a front substrate is fixed to the back substrate, the back substrate via a sealing material, between the back substrate and the front substrate is fixedly installed A discharge layer is formed, a phosphor layer is coated on a surface of the discharge channel, discharge gas is injected into the discharge channel, and discharge electrodes are formed on both sides of the rear substrate or the front substrate to cause dielectric barrier discharge. In the liquid crystal display backlight unit comprising a flat plate fluorescent lamp, a diffusion member is installed on the top of the flat plate fluorescent lamp at predetermined intervals, the frame is embedded with the flat plate fluorescent lamp and the diffusion member and an inverter for supplying power to the electrode. The discharge channel has discharge charges between adjacent discharge channels. To prevent the movement made of a plurality of independent discharge spaces of a stripe shape, of the strip-form The discharge electrode further includes an auxiliary electrode to lower the discharge voltage.

A mercury-containing metal and fixing means for fixing the mercury-containing metal at predetermined intervals of a plurality of independent stripe-shaped discharge spaces forming the discharge channel are further provided.

A white dielectric layer is coated on the lower surface of the discharge channel on which the phosphor layer is applied.

A white dielectric layer is coated on the bottom of the back substrate.

The upper portion of the partition wall that the back substrate and the front substrate is in contact with Fine gaps are formed to allow exhaust and mercury diffusion.

The structure of the discharge electrode uses either a mesh form having a predetermined empty space or a band shape in which an area of the empty space is gradually increased toward the direction of the corresponding discharge electrode.

The auxiliary electrode is connected to the discharge electrode, has a predetermined empty space, and is installed to extend in the longitudinal direction of the discharge channel to be close to the corresponding discharge electrode.

A plurality of auxiliary electrodes are independently installed between the discharge electrodes and the discharge electrodes, and are installed to float.

A plurality of auxiliary electrodes are independently installed between the discharge electrodes and the discharge electrodes, and are connected to the discharge electrodes by separate wires so that power is applied.

A white insulator layer is attached between the rear substrate and the frame.

According to the backlight unit using the flat fluorescent lamp according to the present invention, the uniform discharge by the formation of the independent discharge channel, the reduction of the driving voltage by the addition of the auxiliary electrode to the discharge electrode, the brightness of the flat fluorescent lamp by the white insulator There is a characteristic to improve the durability.

1 is a cross-sectional view of a conventional flat fluorescent lamp,

2 is a perspective view of a flat fluorescent lamp of the present invention;

3A and 3B are cross-sectional views of the backlight unit of the present invention to which the flat fluorescent lamp of FIG. 2 is attached, according to embodiments;

4A and 4B illustrate discharge channels, which are components of FIG. 2, according to embodiments;

5A and 5B are diagrams illustrating shapes of discharge channels, which are components of FIG. 2, according to embodiments;

6A, 6B, 6C and 6D are plan views showing various structural examples of electrodes.

<Code Description of Main Parts of Drawing>

20: flat fluorescent lamp 21: back substrate

22: front substrate 23: sealing material

24: partition 25: phosphor layer

26, 26 ', 27, 27': discharge electrode 28: reflective member

29: mercury-containing metal 30: discharge channel

31 diffusion member 32 white insulating layer

33: frame 34a, 34b: white dielectric layer

35a, 35b, 35c, 35d: auxiliary electrode 36: fixing means

Hereinafter, a configuration of an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Figure 2 is a perspective view of a flat fluorescent lamp of the present invention, Figures 3a and 3b is a cross-sectional view showing the backlight unit of the present invention with a flat fluorescent lamp of Figure 2 according to each embodiment, Figures 4a and 4b 2 is a view illustrating a discharge channel as a component of FIG. 2 according to embodiments, and FIGS. 5A and 5B are diagrams illustrating shapes of a discharge channel as a component of FIG. 2 according to embodiments. FIGS. 6A, 6B, 6C, and FIG. 6d is a plan view showing various structural examples of the discharge electrode and the auxiliary electrode.

As shown, the structure of the backlight unit using a flat fluorescent lamp according to the present invention is the rear substrate 21, the front substrate 22, the rear substrate fixedly installed by the sealing material 23 on the rear substrate 21 ( 21 and the discharge channel 30 formed between the front substrate 22, the phosphor layer 25 coated on the surface of the discharge channel 30, the discharge gas injected into the discharge channel 30, the back substrate (21) or a flat plate fluorescent lamp (20) provided with electrodes for generating dielectric barrier discharges on both sides of the front substrate (22), and the diffusion member (31) spaced at predetermined intervals above the flat plate fluorescent lamp (20). The flat fluorescent lamp 20, a frame 33 in which the diffusion member 31 is built and an inverter for supplying power to the electrode, the discharge channel 30 is easy to discharge discharge charge between adjacent discharge channels It is composed of discharge spaces of a number of independent stripes An electrode that causes the dielectric barrier discharge is a structure which is installed strip form the discharge electrode is installed or with the discharge voltage of the discharge electrode and the auxiliary electrode having a strip-form has to lower.

Here, the light irradiated from the phosphor layer 25 by the white dielectric layer 34a applied to the lower surface of the discharge channel 30 can be effectively emitted. In order to maximize the emission of light, the white dielectric layer 34b may be applied to the bottom of the back substrate 21.

The discharge channel 30 is formed by a plurality of discharge spaces formed between the rear substrate 21 and the front substrate 22. The discharge channels 30 are separated from each other so that the discharge spaces do not occur between the plurality of discharge spaces. It is formed in the form of a stripe.

In addition, the discharge channel 30 preferably has a width of 5 to 15 mm and a height of 2 to 5 mm in the discharge space in order to generate a uniform and stable discharge. For this reason, the width and the width of the discharge channel are defined. This is because if the cross-sectional area is too narrow, the driving voltage rises and the discharge becomes unstable. In addition, if the cross-sectional area of the discharge channel is too large, the driving voltage is lowered, but the discharge plasma is not formed through the entire area of the channel cross-section, but is partially formed. As a result, the phosphors do not emit light in the entire discharge channel. Because it occurs. In addition, it is preferable to reduce the non-light emitting area by narrowing the upper width of the partition wall forming the discharge space to about 2 mm.

In addition, the thickness of the phosphor layer 25 coated on the surface of the discharge channel 30 is the phosphor layer 25 coated on the lower surface of the front substrate 22 by the phosphor layer 25 coated on the rear substrate 21. It is possible to increase the reflectance by coating thicker than the thickness.

In addition, the white dielectric layer 34a is applied to the lower portion of the phosphor layer 25 of the rear substrate 21 to increase the reflectance, or the white dielectric layer 34b or the white insulator layer 32 is provided below the rear substrate 21. The reflectance can also be improved. Here, in order to effectively emit light to the outside of the front substrate, a thin layer of the phosphor layer 25 applied to the front substrate 22 is applied or the phosphor layer is not applied to the front substrate 22 but only to the rear substrate 21. It can also be produced in a coated transmission type.

The discharge channel 30 may have a different shape depending on the manufacturing method of the back substrate 21 and the front substrate 22. That is, the discharge channel may be manufactured in an inverted trapezoidal shape by sand blasting, laser processing, or grinder processing on the back substrate 21, or may be manufactured in a hemispherical shape by a molding processing method.

In addition, there is a method of forming a discharge channel by applying the sealing frit with a dispenser. Here, in the formation of the discharge channel 30 by the forming process method, the flatness of the substrate decreases as the area of the flat fluorescent lamp 20 increases, and thus the adhesion between the front substrate 22 and the rear substrate 21 worsens. Discharge may become unstable and requires manufacturing attention.

As the discharge gas injected into the discharge channel, an inert gas of argon (Ar), neon (Ne), helium (He), krypton (Kr), xenon (Xe), or a mixed gas thereof is used together with mercury (Hg). . As described above, mercury is injected into the discharge channel 30 to emit ultraviolet rays. The mercury is injected by injecting liquid mercury or mercury-containing metal 29 through an exhaust pipe (not shown). There is a method of injecting mercury by heating. The mercury (Hg) is vaporized with the temperature rise of the discharge channel 30 while the flat fluorescent lamp 20 is driven to diffuse into each discharge space. At this time, the diffusion of mercury greatly affects the uniformity of luminance of the flat fluorescent lamp 20. Immediately after the initial driving of the flat fluorescent lamp 20, diffusion is insufficient and a partial luminance difference occurs, but the temperature of each discharge channel is uniform. When is increased and mercury diffuses, the entire flat fluorescent lamp emits light uniformly. In order to facilitate the diffusion of mercury, Aging may be sufficient after fabrication of a flat fluorescent lamp, or a small passage may be provided in a part of the center portion of the partition wall 24 constituting a discharge channel between the discharge electrode and the discharge electrode. It is also possible to facilitate mercury diffusion between the discharge channels. However, when a small passage is provided in a part of the partition 24, the cross-sectional area of the discharge channel 30 decreases, or when the formed passage is located near the discharge electrode, the discharge is concentrated in a constant discharge channel or Since light emission may be weakened, in order to secure discharge stability, a fine gap may be formed in an upper portion of a partition wall between the rear substrate and the front substrate, and mercury may be diffused through the formed minute gap. In particular, when each independent discharge channel is formed as in the present invention, a very stable discharge can be generated even in a relatively unstable xenon (Xe) gas. In addition, if the mercury-containing metal 29 is fixed to each of the discharge space by fixing means 36 for fixing the mercury-containing metal 29 can be effectively diffused.

The discharge electrodes 26, 26 ', 27 and 27' of the planar fluorescent lamp of the present invention are formed in a band shape having a predetermined width on both sides of the front substrate 22 and the rear substrate 21. The structure of the electrode is very important for uniform light emission of the flat fluorescent lamp 20. That is, in order to uniformly emit light of the flat fluorescent lamp 20, it is necessary to first generate a stable and uniform discharge. To this end, discharge electrodes 26, 26 ', 27, and 27' are used to lower the discharge start voltage. ) Must be formed.

The discharge electrodes 26, 26 ′, 27, 27 ′ formed in the band shape may be formed in a mesh structure having a predetermined empty space in order to lower the discharge start voltage. Alternatively, it may be formed in a band-shaped structure in which the area of the empty space is gradually increased toward the corresponding discharge electrode. This is because when the strip-shaped discharge electrodes are formed to have such empty spaces, the distance between the discharge electrodes can be narrowed at the same power, so that the discharge start voltage is lowered.

In addition, the method of effectively lowering the discharge start voltage in a flat fluorescent lamp having a limited size is provided by discharging electrodes 26, 26 ', 27, and 27' with an auxiliary electrode. A secondary discharge is caused between 27 and 27 'to induce a discharge between the discharge electrodes 26, 26', 27 and 27 '. This secondary discharge is performed at a voltage much lower than discharge occurring between discharge electrodes located on both sides of the flat fluorescent lamp 20, and the discharge electrodes 26, 26 '(by forming a charge in advance in the discharge channel 30) ( 27) (27 ') to induce the discharge, and serves to lower the discharge voltage.

Referring to Figures 6a to 6d, it will be described in more detail as follows.

Discharge electrodes 26, 26 ', 27 and 27' located on both sides of the planar fluorescent lamp 20 have a band shape having a predetermined width, and the width of the discharge electrode is flat fluorescent lamp 20. Has a close relationship with the discharge start voltage. In general, when the width of the discharge electrode is wide, the distance between the discharge electrodes is relatively short, and the discharge start voltage is lowered. However, when the widths of the discharge electrodes 26, 26 ', 27, 27' are wide, the edge width of the backlight is increased and the non-light emitting area is widened. Therefore, the discharge electrodes 26, 26 '( There is a limit to widening the width of 27). Therefore, it is necessary to form the widths of the discharge electrodes 26, 26 ', 27, 27' within the range of the edge width of the limited backlight, and then lower the discharge start voltage using the auxiliary electrode. That is, under the condition that the discharge electrodes 26, 26 ', 27, 27' have the same width, it is preferable to provide an auxiliary electrode in order to lower the discharge start voltage of the flat fluorescent lamp.

The auxiliary electrode is formed between the discharge electrodes located on both outer sides of the rear substrate 21, and the distance between the auxiliary electrodes or between the discharge electrodes 26, 26 ′, 27, 27 ′ corresponding to the auxiliary electrodes. The distance of is shorter than the distance between the discharge electrodes, and the discharge starts before the discharge between the discharge electrodes 26, 26 ', 27, 27'. The auxiliary electrode may be connected to each of the discharge electrodes 26, 26 ′, 27, 27 ′ located on both sides of the rear substrate, and extended to the center portion of the rear substrate 21. 35a, 35b, 35c and a plurality of electrodes separated from each discharge electrode between the discharge electrodes 26, 26 ', 27, 27' located on both sides of the back substrate 21. There is a method of providing the auxiliary electrode 35d.

The areas of the auxiliary electrodes 35a, 35b, 35c connected to the respective discharge electrodes 26, 26 ', 27, 27', and installed to extend to the center of the rear substrate 21 are discharge electrodes. It is preferable to make it smaller than the area of (26) (26 ') (27) (27'). In other words, when the area of the auxiliary electrode 35a is large, the intensity of the discharge occurring at the auxiliary electrode 35a is increased, which causes the discharge between the discharge electrodes 26, 26 ′, 27, 27 ′ to be relatively weak. There arises a problem that the luminance uniformity of the fluorescent lamp 20 is lowered. Therefore, the area of the auxiliary electrode 35a is sufficient to cause an auxiliary discharge so that the discharge between the discharge electrodes 26, 26 ', 27, 27' is easily induced. To this end, a method of shortening the distance between electrodes while reducing the area of the auxiliary electrode has a method in which the electrode width is narrow and is formed to have a predetermined empty space such as polygon or circle, or the electrode is narrow and long in the discharge channel direction. In addition, the auxiliary electrodes having a small area in this manner may have any shape as long as they extend in the corresponding electrode direction.

Also, referring to FIG. 6D, the auxiliary electrodes 35d are formed independently from the discharge electrodes, and the distances between the auxiliary electrodes 35d and the discharge electrodes 26, 26 ′, 27, 27 ′ and There is a method of installing a floating auxiliary electrode to reduce the distance between the auxiliary electrodes 35d to cause an auxiliary discharge caused by a voltage induction phenomenon, and also reduce the number of auxiliary electrodes, and between the auxiliary electrodes or between the auxiliary electrodes and the discharge electrodes. Power may be supplied through a separate conductor through a power supply, that is, an inverter, without directly connecting the discharge electrode so that an auxiliary discharge occurs. As such, when power is separately supplied to the auxiliary electrodes through an inverter, the electrode area is reduced so as not to weaken the intensity of the discharge between the discharge electrodes, or the power is supplied temporarily or intermittently and the auxiliary electrodes are floated during operation. It is preferable.

In addition, the white insulating layer 32 is partially or entirely adhered between the electrode 26 and the frame 33 by the adhesive layers 32a and 32b, and the white insulating layer 32 is formed by the light emitted from the phosphor layer. It serves to increase the light efficiency by reflecting the, and at the same time it is also attached to the back substrate 21 serves to prevent the impact or damage of the flat fluorescent lamp 20 by vibration or shock at the same time.

In addition, the upper portion of the front substrate 22 is provided with a diffusion member 31 for diffusing light emitted from the phosphor, the diffusion member 31 and the front substrate 22 so that uniform light can be irradiated to the liquid crystal display panel Is installed at a predetermined distance. In addition, the diffusion member 31 may be configured by combining a transparent sheet such as acrylic and polycarbonate and a diffusion sheet, or may use a translucent acrylic or polycarbonate sheet having a diffusion function.

As described above, the present invention exhibits the effect of low discharge start voltage and power consumption, improved light efficiency, and uniform discharge.

Although the invention has been described in detail only with respect to the described embodiments, it will be apparent to those skilled in the art that modifications and variations can be made within the spirit and scope of the invention, and such variations or modifications will belong to the appended claims. .

Claims (10)

The front substrate 22 is fixed to the rear substrate 21 and the rear substrate 21 via the sealing material 23, and discharges between the rear substrate 21 and the front substrate 22 to which the rear substrate 21 is fixedly installed. The channel 30 is formed, a phosphor layer 25 is coated on the surface of the discharge channel 30, discharge gas is injected into the discharge channel 30, and the back substrate 21 or the front surface of the discharge channel 30 is formed. A planar fluorescent lamp 20 composed of discharge electrodes 26, 26 ', 27, 27' that cause dielectric barrier discharges on both sides of the substrate 22, and a predetermined upper portion of the planar fluorescent lamp 20. In the liquid crystal display backlight unit, the diffusion member 31 is spaced apart from each other, the frame 33 having the flat fluorescent lamp 20 and the diffusion member 31 embedded therein, and an inverter for supplying power to the electrode. , The discharge channel 30 is composed of a plurality of independent stripe-shaped discharge spaces so that discharge charges do not easily move between adjacent discharge channels 30, The discharge electrode (26) (26 ') (27) (27') is a liquid crystal display backlight unit, characterized in that provided in the form of a band. The method according to claim 1, wherein the mercury-containing metal 29 and the fixing means 36 for fixing the mercury-containing metal 29 at predetermined intervals of a plurality of independent stripe-shaped discharge spaces forming the discharge channel 30. Liquid crystal display backlight unit characterized in that it is further provided. The liquid crystal display backlight unit as claimed in claim 1 or 2, wherein a white dielectric layer (34a) is coated on the lower surface of the discharge channel (30) on which the phosphor layer (25) is applied. The liquid crystal display backlight unit as claimed in claim 1, wherein a white dielectric layer (34b) is coated on the bottom of the back substrate (21). According to claim 1, wherein the back substrate 21 and the front substrate 22 is sealed via a sealing material 23, the one side of the partition space of the discharge space that the back substrate 21 and the front substrate 22 contact. Liquid crystal display backlight unit, characterized in that the fine gap is formed on the top The structure of any one of claims 1 to 5, wherein the structure of the discharge electrodes 26, 26 ', 27, 27' is in the form of a mesh having a predetermined empty space or in the direction of the corresponding discharge electrode. The liquid crystal display backlight unit, characterized in that any one of the strip form is arranged to gradually increase the area of the empty space. The method of claim 1, wherein the discharge electrodes 26, 26 ', 27, 27' are further provided with an auxiliary electrode to lower the discharge voltage, the auxiliary electrodes (35a, 35b, 35c) are The discharge is connected to the discharge electrodes 26, 26 ', 27, 27', has a predetermined empty space, and closes the corresponding discharge electrodes 26, 26 ', 27, 27'. Liquid crystal display backlight unit, characterized in that installed to extend in the longitudinal direction of the channel (30). The discharge electrodes 26, 26 ′, 27, 27 ′ are further provided with auxiliary electrodes to lower the discharge voltage, and the auxiliary electrodes 35d are disposed on the discharge electrodes 26 ( 26 ') (27) (27') and a plurality of discharge electrodes (26) (26 ') 27, 27' (27 ') are independently installed, the liquid crystal display backlight unit, characterized in that the installation is floating. The discharge electrodes 26, 26 ′, 27, 27 ′ are further provided with auxiliary electrodes to lower the discharge voltage, and the auxiliary electrodes 35d are disposed on the discharge electrodes 26 ( 26 ', 27, 27' and a plurality of discharge electrodes 26, 26 ', 27, 27' are independently installed, but discharge electrodes 26, 26 ', 27, 27' are provided. LCD display backlight unit, characterized in that is installed so that the power is applied to be connected to a separate conductor. 2. The liquid crystal display backlight unit as claimed in claim 1, wherein a white insulator layer (32) is attached between the rear substrate (21) and the frame (33).