Light-emitting element.

p0001The present invention relates to a light emitting element for use in a matrix display board.

p0002so far two kinds of elements were proposed for equipping billboards: these are the incandescent lamps and cathode ray tubes.

p0003The major disadvantages of the incandescent lamp is its high consumption (20 to 40 watts per element) and its relatively low efficiency (about 10 lumens per watt). Note also that the lamp has a modest duration of life (1000 hours on average), a color temperature, which is variable depending on the supply voltage at its terminals and a gradual decrease in its light intensity due to blackening inside the bulb according to the time of use. It has already been proposed such a light source to equip color display tables. In this case, use per unit element three lamps followed by color filters or simply three colored bulb lamps. One will however understand that, for the color, the disadvantages mentioned above for the black-white display is held. We owe the truth to say though that incandescent lights are inexpensive elements, easily interchangeable and easy to find on the market.

p0004Thus, in one embodiment, the black-white screen 4.3 m high and 8.6 m wide has 160 lines and 80 columns, which involves the use of 12,800 incandescent lamps. If the power of each lamp is 25 W, the power needed to light them all at full brightness will be 320 kW. So we understand that such a screen will prompt a considerable installed power, and no less important expenditure of energy.

p0005For its part, the cathode ray tube is used in screen installations in color, as can be seen in GB-A-2 053 547 and document US-A-4 326 150. Although it s' whether a tube whose bill is simplified compared to that we know of TV tubes, it remains very expensive and especially requires the use of very high voltages acceleration, greatly complicating embodiment of the assembly. This tube has for him, however the advantage of low power consumption compared to that of an incandescent lamp.

p0006To remedy the drawbacks mentioned above, the present invention provides a light emitting element characterized in that it comprises at least a discharge tube containing the low vapor pressure of mercury, the inner wall of said tube being coated with substance fluorescent.

p0007Such an element is known per se, but it was never offered in use in a matrix display panel. He currently is used in household lighting or in illuminated signs.

p0008In the first case, it is of variable length straight tubes or bent, having at each end of the electrodes consisting of a filament coated with an emissive oxide deposition. The gaseous atmosphere in the tube consists of argon for priming, pressure of a few millimeters of mercury, and a drop of mercury. The discharge in the mercury vapor causes essentially ultraviolet rays at the wavelength of 253.7 nm. The tube wall appears white appearance that gives the phosphor (phosphor) applied on the inside, and for converting ultraviolet radiation into visible light.

p0009Some neon signs also use so-called luminescent tubes where the gas discharge directly creates the light effect. In this case, the tube wall is either transparent or colored without harness the phenomenon of fluorescence. The storage ar described in GB-A-354 908 mentions filling neon tubes - which will give a red-orange color - or mercury - which will give a blue color -. In addition, this arrangement does not constitute a matrix table since it is composed of many straight segments of various lengths and nested so that we form such as letter or digit by the lighting of a number determined from these segments. The light emitting element described in DE-A-2031610 also uses neon tubes to equip a write scrolling display system. The cited material comprises three tubes emitting different colors. However, there is no reported means for mixing the colors which would produce the output of the element a light the resultant wavelength can vary along the entire visible spectrum. Typically, fluorescent tubes are unsuitable for the use we want to do in an array matrix display as for the three basic colors you have to combine the filling gas with the color of the tube, leading to elements that do not exhibit the same light intensity for each of three tubes.

p0010Compared with the incandescent lamp, the fluorescent tube has several advantages. It has a high luminous efficiency of about 40 lumens per watt, which leads to a comparable light output to a much lesser consumption. Its average life exceeds 7,500 hours, which helps to increase the reliability of the entire display panel. It also shows a very low heat, which has the effect of reducing the convective movements and therefore blackish streaks of dust brought by the convection. Finally, the tube has an invariable color temperature according to the brightness it produces as well as a very low blackening of the bulb - localized at the location of the electrodes - depending on its lifetime.

p0011Relative to the cathode ray tube, the fluorescent tube has a consumption approximately equal, by against its price is considerably lower and it does not require a power supply voltage very hau you. In addition, the number of electrodes is reduced to two.

p0012Thus, the use of the fluorescent tube in the display boards at large as expected in the present invention to provide a new benefit produced by low consumption, the quality of the transmitted images and its reasonable price.

p0013The invention will now be understood with the following description and gives, as an example and using the accompanying drawings, several embodiments of which:<ul><li>Figure 1 is a schematic representation of a display panel according to the prior art.</li><li>Figure 2 shows a white light emitting element with a single fluorescent tube according to a first variant of the invention.</li><li>3 shows a light emitting element equipped with three colored fluorescent lamps according to a second variant of the invention.</li><li>4 shows the element of Figure 3 front view according to a first arrangement of tubes.</li><li>Figures 5 and 6 show light emitting elements other arrangement of the tubes as shown in Figure 4.</li><li>Figure 7 is a circuit diagram showing a power principle of a light emitting element using three colored fluorescent lamps.</li><li>Figure 8 is a timing diagram showing the voltage supply and the respective currents flowing in each of the light emitting elements.</li><li>Figure 1 shows a matrix table as known from the prior art. The array proper 1 is equipped with two incandescent bulbs aligned in rows and columns next to each other. This arrangement commonly used in stadiums can at- dye very large as we have seen above. Linked to table 3 by the driver, there is a control room 4. This cabin is equipped with all equipment necessary for the transmission of static or animated images. It is thus possible to display texts such as sports scores, of advertising, of events animated ments or occasions such events by cameras, disks, tapes, etc. Each light emitting element is an incandescent bulb if the display is held in black-white. A device then makes it possible to vary the light intensity produced by the lamp to achieve the multiple shades of light that make up an image. In the case of color pictures, each of these elements comprise three incandescent lamps (red, green, blue) or three cathode ray tubes. Separately by varying the light intensity produced by the three tubes, one arrives at a resultant light that can cover the entire visible spectrum.</li></ul>

p0014As already explained in the preamble, the present invention aims to replace the incandescent lamp or cathode ray tube with at least one discharge tube generally called fluorescent tube to form the light emitting element. Figure 2 shows such an element 24. The fluorescent tube 5 is mounted in a compartment 6. To meet the physical laws that govern and recalling that the emitted light power is a function of the length of the tube, the tube 5 must have a certain length. To achieve this, we preferred to give here a U-shaped Thus the face 7 of the element remains within dimensions which are compatible with the proposed matrix display about 80 cm², Which is a square of 9 cm.

p0015however it will be understood that to use all the light radiation of the tube, therefore also that of its rectilinear portions, it will be necessary to provide a reflector system redirecting to b efore of the element the light issuing said straight portions. This can be achieved for example by means of a reflector located at the rear of the compartment instead of the surface 8, this reflector being completed according to the geometry of said compartment by a diffuse mirror forming the walls 9 of the compartment.

p0016The compartment shown in Figure 2 is that parallélipipédi-. One could imagine other geometries without departing from this invention. Thus, the fund could be triangular, the apex of the triangle lying at the point of connection of the tube and with the aim of improving the reflection effect presented by the walls. Similarly, the front panel may be provided with an anti-reflection system.

p0017The light emitting element that has just been described is used in the black-white paintings. We equip the a base element for electrical connections and simple attachment system to make it easily removable. Well designed, it will be easily interchangeable and very accessible to maintenance personnel.

p00183 shows a transmitter element of colored light 24 equipped with three fluorescent tubes. It does distinque the one shown in Figure 2 by the juxtaposition of three fluorescent lamps of different colors referenced 10, 11 and 12. As already stated above, this is the phosphor applied on the wall of the tube which converts the ultraviolet radiation of the discharge into visible light. Thus, in the element of Figure 3, the tube 10 radiates in the red (while using calcium borate as a fluorescent substance), the tube 11 in the green (willemite) and the tube 12 in the blue (tungstate calcium). With a mixture in suitable proportions of the various substances, white light is produced, and this mixture is used for the tube drawn in Figure 2.

p0019May be mentioned here that the three basic colors may also be obtained from three white tubes, each supplemented by a separate color filter located at the front of the tube. If this arrangement has the disadvantage of adding additional components and reduce the luminous efficiency, it has nevertheless him not implemented as tubes of a single white color that does not require special preparation for their fluorescent substance.

p0020If we vary independently the intensity of the light emitted by each of the three color tubes, is obtained at the output of the element a light the resultant wavelength may vary from violet to red, that is to -dire from 330 to 700 nm, and this as far as we take a step back from the front of the item.

p0021The same observations were made about the black-white element can be made to the color element (reflectors, form the compartment, anti-glare system, removable construction). For some special arrangements, care should be taken in addition to separate color tubes by partitions 13.

p0022Figure 4 is a front view of the element 24 of Figure 3. From this view, one can envisage other arrangements of tubes in the transmitter element where, for example,

p0023Figure 5 shows an arrangement where the tubes are disposed end to end to circumscribe a closed surface, here a triangle, and

p00246 shows a spiral arrangement where the observed tubes of the front face have portions of circles. The ends of these portions are bent at 90⁰ to form rectilinear portions extending behind the plane of FIG.

p0025Other arrangements than those shown in Figures 4, 5 and 6 may be envisaged without departing from the object of the present invention. Thus, the color element is not limited to the use of three tubes. A fourth tube, for example, could be added that, in some circumstances can improve the continuity of the light spectrum.

p0026For the application in question here, use will preferably fluorescent tubes with hot cathodes wherein at each end of the tube is placed an electrode constituted by a filament. The supply voltage is applied to each of the electrodes to cause discharge and the ignition of the tube. When using such a tube for domestic lighting on industrial frequency, the team usually a starter and a ballast inductance to limit the current. We know that this device causes a delay in the ignition, which can not naturally suited to this application where you want to display not only static text but still moving images from scenes taken from life (camera shooting or telecine). We therefore give preference to a high frequency power said that not only allows instant ignition of the tube, but a consumption reduction of about 20% as it is true that the light output of the tube increases with frequency. This arrangement also reduces the ballast as to its volume, which also causes a decrease in weight and price. Such power is briefly described in "Hexfet Databook, International Rectifier, 1981" under "fluorescent lighting".

p0027FIG 7 shows a possible scheme for supplying a light emitting element 24 according to the invention. Here, the element contains three fluorescent tubes 20 (red), 21 (blue) and 22 (green). A power generator 23 sized to power a plurality of elements produces a voltage Ug which frequency is chosen between 5 and 30 kHz from the voltage Us sector. The filaments 25-30 are fed through the common transformer 31 to the filaments 26, 28 and 30 and transformers 32, 33 and 34 respectively to power the filaments 25, 27 and 29. the primary of each transformer is connected to the power source Ug.

p0028Be mentioned that the transformer 31 may also be dimensioned to supply a plurality of tubes and not only the three tubes forming the light emitting element. To isolate galvanically the filaments 25, 27 and 29 corresponding filaments 26, 28 and 30, it is necessary to implement three separate transformers 32, 33 and 34 or a single transformer with several secondary windings. Note that these transformers are quite small since operating at high frequency.

p0029Lighting the tubes 20, 21 and 22 respectively acting on items 35, 36 and 37 placed in series in the circuit of the tube and are presented in the figure as switches. The each of the tubes respectively circuit is completed by an element 38, 39 and 40 which is intended to stabilize the current flowing in the tube. This element may be a resistor, an inductor or a capacitor. The first case is of little interest since the resistance causes additional losses. In the last two cases, it will be little clutter given the high operating frequency.

p0030According to the invention, the intensity of the light provided by each of the tubes will depend on the system time during which the switch will remain closed over a reference period that is fixed. Thus, if one chose wisely the constituent colors of each of the tubes and that rule separately the light flux emitted by each of them through the closing time from their respective switch, we finally obtain a color that will the result of the corresponding mixture of each of the luminous flux and which can extend over the entire visible spectrum.

p0031The switches 35, 36 and 37 can be in various forms, for example, under the triacs controlled by the video signals generated by an image pickup camera via an A / D converter and appropriate control logic. Here we find the means already known in the prior art and are applied in the matrix tables color found on the market.

p0032Figure 8 is a timing diagram showing the voltage supply Ug ali applied across the tubes and the current 1₂₀, I₂₁ and 1₂₂ flowing in each of them depending on the respective closing of the control elements 35, 36 and 37. In this diagram, the first line represents the supply voltage Ug supplied by the generator 23 (see Figure 7). This voltage is formed by the juxtaposition of reference periods T_r each comprising at least 64 alternating T_at. We light tube 20 (red) to the desired light intensity by closing the element 35 during a period T₁ T_r whereby a current 1₂₀ in the tube. The procedure is the same for 21 tubes (blue) and 22 (green) during periods T₂ and T₃ respectively whereby the resulting currents I₂₁ and 1₂₂. As explained above, the resulting color at the output of the element depend on the relative switch-on time of each of the tubes during the reference period. In other words, we can say that the light intensity emitted by a single tube will be controlled by inhibition of a variable number of cycles T_at during the reference period T_r. This is also true for a tube radiating white, so that this type of diet can also be applied to a matrix table black-white.

p0033In black-white matrix tables known today commonly used 16-tone gradations between black and white, allowing a suitable video image reproduction. In this case, a 4-bit digitized signal is sufficient. However, it will be noted that for a color image, first, it is desired a gradient of light intensity on a specific color - as for black and white - and, secondly, it must be measuring separately vary the light intensity of each of three tubes to create said predetermined color. It follows from this that a control based on a 4-bit signal is totally inadequate. Practical experiments have shown that it is necessary to provide at least 64 tones degraded, so that the reference period T_r which discussed about the figure 8 must be at least 64 vibrations, leading to feature a 6-bit digitized signal. Even better results are achieved with alternations 128 (7 bits) and 256 alternations (8 bits), which allows the system has Dapter visual perception according to a logarithmic function, for example.

p0034The power of the light emitting element is not limited to the description given above. In a not shown in the drawing variant, it varies either the number of cycles during the reference period, but the width of these alternations. It is thus leads to a pulse width modulation (PWM).