Back-lighting apparatus for screen.

0001This invention relates to back-lighting apparatus for screens for various kinds of display devices, especially that for the liquid crystal display panel of word processors, personal computers, television sets and instrument panels.

0002The conventional back-lighting apparatus for the liquid crystal display panel described above having a light source such as a spherical lamp or LED has a system as shown in Fig. 14, and the back-lighting apparatus having a cylindri­cal light source such as a fluorescent lamp adopts a system as shown in Fig. 15 or Fig. 16 respectively.

0003In other words, the system indicated in Fig. 14 is de­signed to illuminate the front display panel 23 with the light beam emitted by the spherical light source 22, with those illustrated in Figs. 15, 16 being designed to illuminate the front display panel 23 with the light beam emitted by the cylindrical light sources 22, but any of the three systems has the following drawbacks.

0004In the system indicated in Fig. 14, the light source 22 lies right under the front display panel 23, thus bringing about such drawbacks as uneven brightness on the front dislay panel 23 and thickening of the back-lighting apparatus in design.

0005The system indicated in Fig. 15 has a dot pattern screen 24 between the cylindrical light source 22 and the front dis­play panel 23 as a means to correct uneven brightness on the front display panel 23 which occurs when the light emitted by the cylindrical light source 22 installed in the lamp house 21 is thrown directly to the front display panel 23. The system indicated in Fig. 16 has a semi-cylindrical lamp house 21, the back plate at the opposite end of the front display panel 23 having aluminum film deposited on the concave surface of its inside to use it as the concave reflective surface 25, whereby a lightbeam is reflected to the front display panel 23.

0006The system indicated in Fig. 15, however, poses problems such as high manufacturing cost of the dot pattern screen 24 and demand for high accuracy in installing said dot pattern screen 24, and also it has a drawback that the light beam emitted from the light source 22 cannot be efficiently used because the light cast to the front display panel 23 is sup­pressed by the dot pattern screen 24. The system indicated in Fig. 16 has a drawback that it is difficult to reduce the thickness of the back-lighting apparatus to less than 20 mm because of the limitation that it has a combination of a semi-­cylindrical structure and a light source positioned just under the front display panel 23.

0007Thus, it is a first object of the invention to provide a back-lighting apparatus for a screen that has a sufficient brightness and uniform distribution of brightness.

0008It is a second object of the invention to provide a back-­lighting apparatus for a screen capable of reducing thickness in design.

0009It is a third object of the invention to provide a back-­lighting apparatus for a screen inexpensive and simple in structure. <ul id="ul0001" list-style="none"><li>Fig. 1 is a plan view illustrating the first example of a back-lighting apparatus for a screen according to the invention,</li><li>Fig. 2 is a transverse sectional view along the arrows II-II of Fig. 1,</li><li>Fig. 3 (A), (B) are transverse sectional views of special cylindrical light sources respectively suitable for a back-­ lighting apparatus for a screen of the invention,</li><li>Fig. 4 is a plan view illustrating the second example,</li><li>Fig. 5 is a transverse sectional view along the arrows V-V of Fig. 4,</li><li>Fig. 6 is a plan view illustrating the third example,</li><li>Fig. 7 is a transverse sectional view along the arrows VII-VII of Fig. 6,</li><li>Fig. 8 is a plan view illustrating the fourth example,</li><li>Fig. 9 is a transverse sectional view along the arrows XI-IX of Fig. 8,</li><li>Fig. 10 is a plan view illustrating the fifth example,</li><li>Fig. 11 is a transverse sectional view along the arrows XI-XI of Fig. 10,</li><li>Fig. 12 is a plan view illustrating the sixth example,</li><li>Fig. 13 is a transverse sectional view along the arrows XIII-XIII of Fig. 12, and,</li><li>Figs. 14 through 16 show structures of conventional back-­lighting apparatus for screen respectively.</li></ul>

0010The first example shown in Figs. 1 and 2 has, as a basic structure, light sources 32 at the insides of more than one of the four side walls for forming the lamp house 31, a light reflective surface 34 being located at the inside of the back plate which is opposite to the front display panel 33 of the lamp house 31, and a light diffuser 35 being installed in the space between the front display panel 33 and the light re­flective surface 34 in such a way that it separates said space to specifically locate the light sources 32 at the side of the light reflective surface 34.

0011By so doing, even if the light sources 32 themselves are placed eccentrically near the inside of the side wall but not in the middle of the lamp house 31, most of the direct light beam emitted from the light sources 32 is blocked by the light diffuser 35 separating the front display panel 33, and reflected uniformly on the light reflective surface 34 at the inside of the back plate, after which the light beam permeates the light diffuser, thus illuminating the front display panel 33 with sufficient brightness and uniform distribution of brightness.

0012The shape of the lamp house does not particularly matter. But in making the back-lighting apparatus for screens to be used for the liquid crystal display panel for the word processor, personal computer, television set and instrument panel, it is appropriate to have a lamp house made into a thin, substantially parallelepipedic shape.

0013There is also no particular restriction on the material of the lamp house 31, but it is appropriate to mold it with synthetic resin such as ABS resin.

0014The alternate long and short dash lines in Figs. 1, 4, 8 and 10 represent a virtual line which shows a bisector of the light diffuser 35 looking from both light sources 32, 32.

0015Although the shape of the light source 32 may be either spherical or cylindrical, it is appropriate to have usually a cylindrical light source such as a hot cathode-ray tube (fluorescent tube) or cold cathode-ray tube.

0016It is generally appropriate that the light sources 32 are located at the inside of the side walls forming the lamp house 32, but when the light sources are of the spherical type, it is especially advisable to have them at the inside of each corner made up of two adjacent side walls of the four side walls forming the lamp house.

0017As a special light source, it is effective to use a light source which is equipped with a light reflective film (light reflective layer) at the side facing the side wall of the lamp house in efficiently casting the light beam from the light source.

0018Fig. 3 (A), (B) show the structure of the light sources 32 in the concrete. In Fig. 3 (A), 32a is a glass tube form­ing the cold cathode-ray tube, 32b is a light reflective film layer provided at the inside of the cold cathode-ray tube, and 32c is a fluorescent film layer provided at the inner side of the glass tube, with part of the light reflective film layer 32b and the fluorescent film layer 32c missing at an angle of about 70 degrees. This missing angle may be about 180 degrees.

0019Needless to say, a special cold cathode-ray tube embodied in the transverse sectional view in Fig. 3 (B) can be used instead of the special cathode-ray tube as described above.

0020The special cold cathode-ray tube shown in Fig. 3 (B) is provided with the light reflective film layer 32b at the outside of the glass tube 32a and the fluorescent film layer 32c at the inside of the glass tube 32.

0021The light reflective surface 34 has an inverted V-type ridgy convex toward the front display panel within an alti­tudinal difference of plus and minus of 4.5 mm in the middle between both light sources.

0022With the view to improving the mirror properties and adhesive strength, the light reflective surface was treated as follows: the surface was first given an undercoat; then alumi­num, which provides high reflectance,was deposited by evapo­ration method; and on top of it, the surface was top coated with transparent resin as a surface protective layer.

0023The light reflective surface 34 is intended to throw back the light beam from the light sources 32 or the light beam emitted through the light refractor by the light source 32, to the front display panel 33 of the lamp house 31 uni­formly.

0024Therefore, the material or shape of the light reflective surface does not matter.

0025It is necessary to locate the light reflective surface 34 at the inside of the back plate which lies at the opposite end of the front display panel 33 of the lamp house 31, but needless to say, it does not matter whether the inside surface itself of the back plate forming the lamp house 31 is planish­ed for this purpose or whether a separate light reflective surface is installed at the inside of the back plate forming the lamp house 31.

0026In order to achieve uniform distribution of brightness on the front display panel 33, one of the objects of this in­vention, more effectively, it is desirable to have the light reflective surface 34 made convex toward the front display panel 33 so that the light beam reflected by the light re­flective surface 34 securely reaches a proper site of the front display panel 33 (usually the center of the front dis­play panel) at a distance from the light source.

0027It is therefore preferable that the light reflective surface 34 is convex along the center line drawn between both light sources 32, showing a gradual descent toward both light sources 32 if two light sources 32 are used, that is, if a pair of light sources are provided at the insides of two op­posed side walls of the four side walls forming the lamp house 31.

0028When four light sources 32 are used, that is, when four light sources are provided at the insides of the four side walls forming the lamp house 31, respectively, it is ap­propriate that the light reflective surface 34 is convex in the middle area at regular interval from the four light sources 32, showing a gradual descent toward each light source 32.

0029By taking such an arrangement, the light reflected by the light reflective surface 34 illuminates the whole front display panel 33 of the lamp house 31 with sufficient bright­ness and uniform distribution of brightness more effectively.

0030Moreover, it is also effective to make the light re­flective surface 34 like Fresnel lens in both reducing the volume and weight of the entire back-lighting apparatus for screen and in ensuring the reach of the reflected light to the proper site of the front display panel 33 away from the light source 32.

0031In other words, a Fresnel reflective surface -- a plate-­like relfector with serrations on the reflective surface -- may be used as the light reflective surface of this invention. In this case, the above object can be effectively attained by providing many serrations on the light reflective surface in such a way that the tilting angles of the serrations are devised to allow more of the reflected light to reach the area of the front display panel 33 which is away from the light sources.

0032It is effective to have a light diffuser 35 between the front display panel 33 and the light reflective surface 34 at the inside of the back plate of the lamp house 31 in attaining more even distribution of brightness on the front display panel 33 of the lamp house 31.

0033It particularly does not matter what kind of light diffuser 35 is to be used because the light diffuser is intended to ensure even distribution of brightness on the front display panel 33 by diffusing the light beam in the lamp house 31 before it illuminates the front display panel 33.

0034In order to effectively embody the object of the present invention, for instance, it is preferable to provide a light diffuser 35 consisting of a light-permeable plate with micro-­irregularities on its one or both sides.

0035The light diffuser surface is given either ordered micro-irregularities with a specific directional property or disordered micro-irregularities with no specific di­rectional property. Generally speaking, the light diffuser with ordered micro-irregularities on its surface is suitable for attaining uniform distribution of brightness on the whole front display panel by improving the brightness in a specific direction, while the light diffuser with disordered micro-irregularities on its surface is suited to improve the whole distribution of brightness in no specific direction.

0036In some cases, it may be effective to have a light dif­fuser 35 with ordered micro-irregularities on one side and disordered micro-irregularities on the other side of the light diffuser 35.

0037The light diffuser 35 shown in Fig. 2 consists of a 1.2­mm thick, transparent acrylic resin plate (the surface fac­ing the light reflective surface 34) with inumerable dis­ordered irregularities on it, and has a V-type valley-like convex toward the light reflective surface 34 within an altitudinal difference of plus and minus of 5.5 mm in height at the area facing the ridgy convex of the light reflective surface 34.

0038The present invention is characterized in that the light diffuser 35 described above is provided in the space between the front display panel 33 and the light reflective surface 34 at the inside of the back plate in such a way that it separates said space with the light sources 32 near the light reflective surface 34.

0039By so doing, as explained before, even if the light sources 32 themselves are placed eccentrically near the in­side of the side wall but not in the middle of the lamp house 31, most of the direct light beam emitted from the light sources 32 is blocked by the light diffuser 35 separat­ing the front display panel and reflected uniformly once on the light reflective surface 34 at the inside of the back plate, after which the light beam permeates the light diffuser to diffuse uniformly, thus illuminating the front display panel 33 with sufficient brightness and uniform distribution of brightness.

0040As the light diffuser 35 of the present invention, it is especially preferable to have a light diffuser with such a tilting surface that gets nearer the light reflective surface 34 at the inside of the back plate as it gets away from the light sources 32.

0041This is because when the light beam emitted from the light source 32 is converted to the reflected light on the light reflective surface 34 on the inside of the back plate, the reflected light at the area of the light reflective surface 34 near the light source 32 is strong and that at the center area at a distance from the light source is weak.

0042That is, in order to illuminate the front display panel in an even distribution of brightness equalizing the strong reflected light on the area of the light reflective surface 34 near the light source 32 and the weak reflected light on the area of the light reflective surface at a distance from the light source 32, the interval between the light reflective surface 34 at the inside of the back plate and the light diffuser 35 should be made greater at the area near the light source 32 and smaller at the area far away from the light source 32. Therefore, it is fit for the purpose to have a light diffuser 35 with a tilting surface that gets nearer the light reflective surface 34 at the in­side of the back plate as it gets away from the light source 32.

0043More specifically speaking of the above, when a pair of light sources 34 are arranged at the insides of two opposed side walls of the four side walls forming the lamp house 31 (or when two light sources are used), it is most preferable to provide a light diffuser 35 presenting a tilting surface (plain or curved surface) which is the closest to the light reflective surface 34 near the center line drawn between the two opposed light sources 32.

0044The back-lighting apparatus for screen of the Example 1 composed as such was lighted with a power of 7.2 W for each cold cathode-ray tube. As a result, the brightness on the front display panel 33 measuring 140 mm in width and 230 mm in length was a maximum of 940 cd/m² and a minimum of 800 cd/m², indicating that the distribution difference of brightness was within about plus and minus of 8%.

0045For comparison's sake, the brightness distribution of the conventional back-lighting apparatus for screen shown in Fig. 15 was measured. The maximum brightness was 650 cd/m² and the minimum brightness 470 cd/m², with the dis­tribution difference of brightness standing at about plus and minus of 16%.

0046The brightness distribution of the conventional back-­lighting apparatus for screen shown in Fig. 16 was also measured. The maximum brightness was 540 cd/m² and the minimum brightness 400 cd/m², with the distribution differ­ence of brightness standing at about plus and minus of 16%.

0047The above measurements were all performed with Minolta model CS-100 color-difference meter. The measurements of brightness distribution on the following examples were also performed with the same instrument.

0048The Example 2 shown in Figs. 4 and 5 adopts the con­struction in which two cylindrical light sources 32 are arranged at the insides of two opposed side walls of the four side walls forming the lamp house 31.

0049This Example is the same as Example 1 except that as a pair of cylindrical light sources 32, 32, two Elbum model FLE6-270AE cold cathode-ray tubes were used; concave re­flectors 36, 36 presenting a concave toward the cylindrical light sources 32, 32 are additionally provided between the two light sources 32, 32 and the side walls forming the lamp house 31.

0050The concave light reflector 36 is located between the side wall making up the lamp house 31 and the light source 32, presenting a concave surface toward the light source 32.

0051The concave light reflector 36 is not an essential com­ponent in all embodiments of the present invention, but this component, if installed, throws back the light emitted from the light source 32 toward the middle of the lamp house, so that the light from the light source 32 is effectively di­rected to the light reflective surface 34 on the inside of the back plate, thus further improving the brightness of the front display panel 35.

0052The back-light apparatus for screen of the Example 2 composed as such was lighted with a power of 7.2 W for each cold cathode-ray tube. As a result, the brightness on the front display panel measuring 140 mm in width and 230 mm in length was a maximum of 910 cd/m² and a minimum of 780 cd/m², indicating that the distribution difference of brightness was within about plus and minus of 8%.

0053The Example 3 shown in Figs. 6 and 7 has four cylindri­cal light sources 32 at the insides of the four side walls forming the lamp house 31. The components of the back-light­ing apparatus for screen in the Example 3 are basically the same as in the case of Example 2.

0054However, Example 3 is different from Example 2 in that: as light sources 32, Elbum model FLE6-270AE cold cathode-ray tubes are provided at the insides of the four walls forming the lamp house 31; the number of concave reflectors 36 is increased to 4; and the shapes of the light reflective surfaces 34 and light diffusers 35 are changed.

0055That is, the light reflective surface 34 in this Example has a quadrangular pyramid-like convex toward the front dis­play panel 33 within an altitudinal difference of 4.5 mm at around the middle (intersection of the two diagonal lines of the light reflective surfaces 34) of the four light sources 32. The light diffuser 35 has an inverse quadrangular pyramid-­like convex toward the light reflective surface 34 within an altitudinal difference of 5.5 mm at the area facing the quadrangular pyramid-like convex of the light reflective surface 34.

0056When two pairs of light sources are provided at the in­sides of the four side walls forming the lamp house 31 (or when four light sources are used), it is preferable to have a light diffuser presenting a tilting surface (plain or curv­ed surface) which is the closest to the light reflective surface at around the intersection of the two center lines connecting the two light sources arranged at opposite ends (or near the intersection of two diagonal lines on the light reflective surface).

0057The back-lighting apparatus for screen composed as such was lighted with an electric power of 7.2 W for each cold cathode-ray tube. As a result, the brightness on the front display panel 33 measuring the same as in the case of Example 1 was a maximum of 1540 cd/m² and a minimum of 1320 cd/m², with the distribution difference of brightness standing at plus and minus of 7%.

0058Each of the Examples 4 through 6 shown in Figs. 8 through 13 illustrates the construction comprising the light reflector 41 combined with the above-described Examples 1 through 3. The Example 4 in Figs. 8 and 9 corresponds to the Example 1, the Example 5 of Figs. 10 and 11 to the Example 2, the Example 6 in Figs. 12 and 13 to the Example 3, respectively, all being the same except for installing the light reflector 41.

0059In each of the Examples 4 through 6, a light reflector 41 is provided in the area between the light source 32 and the light reflective surface 34, in addition to the light diffuser 35 shown in the Examples 1 through 3 mentioned above, in order to further improve uniform distribution of bright­ness on the front display panel 33 of the lamp house 31.

0060This light reflector 41 is designed to use effectively the light beam emitting downwardly (in this case, at the side of the light reflective surface) from the light sources 32 by specifically reflecting it toward the central direction of the front display panel 35. Therefore, its kind, material or shape do not particularly matter.

0061By so doing, even if the light sources 32 themselves are placed eccentrically near the insides of the side walls but not in the middle of the lamp house 31 light beam emitted from the light sources 32 is reflected uniformly once by the light reflective surface 34 at the inside of the back plate, thus illuminating the front display panel 33, so that the light beam emitted downwardly from the light sources 32 is reflected toward the center by the reflector 41.

0062Also, by so doing, the light beam near the light sources 32 is induced near the center of the front display panel 35 so as for it to be used fully with the result that the front display panel 35 is illuminated with a sufficient brightness and uniform distribution of brightness.

0063In order to attain the aim of the present invention more effectively together with the light diffuser 35, for example, like the light reflective surface 34, the surface itself under the light source 32 forming the lamp house 31 is planish­ed to be used as the light reflector 41. Needless to say, a separate light reflector 41 is installed under the light source 32 forming the lamp house 31.

0064The Examples 4 through 6 are characterized by that the light reflector 41 described above is installed in the area under the light source 32. It is needless to say that the light reflector 41 may be concave or Fresnel or the like in shape to ensure reach of the light beam to a proper site of the front display panel 35 (usually the center of the front display panel) at a distance from the light source 32.

0065In the Example 4 shown in Figs. 8 and 9, the light re­flective plate measuring 12 mm in width and 230 mm in length was used as the light reflector 41, and the back-lighting apparatus for screen was lighted with a power of 7.2 W for each cold cathode-ray tube. As a result, the brightness on the front display panel 33 measuring 140 mm in width and 230 mm in length was a maximum of 940 cd/m² and a minimum of 805 cd/m², indicating that the distribution difference of brightness was within about plus and minus of 7.7%.

0066The Example 5 shown in Figs. 10 and 11 is the same as Example 4 except that Toshiba Litech cold cathode-ray tubes ⌀ 6.5 x 270 EX not painted were used and that concave re­flectors 36, 36 presenting a concave toward the cylindrical light sources 32, 32 are additionally provided between the two light sources 32, 32 and the side walls forming the lamp house 31.

0067The back-lighting reflective apparatus for screen in Example 5 composed as such was lighted with a power of 7.2 W for each cold cathode-ray tube. As a result, the brightness was a maximum of 915 cd/m² and a minimum of 780 cd/m², indi­cating that the distribution difference of brightness was within about plus and minus of 7.9%.

0068The Example 6 shown in Figs. 12 and 13 is different from the Example 5 in that: Toshiba Litech cold cathode-ray tubes ⌀ 6.5 x 270 EX not painted were used; they were installed at each inside of the front surfaces of the four side walls form­ing the lamp house 31; the number of the concave reflector 36 was increased to 4; shapes of the light reflective surface 34 and light diffuser 35 were changed.

0069The back-lighting apparatus for screen in the Example 6 composed as such was lighted under the same conditions as that of the Example 5. As a result, the brightness was a maximum of 1560 cd/m² and a minimum of 1330 cd/m², indicating that the distribution difference of brightness was within about plus and minus of 8%.