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This invention relates to a cold cathode fluorescent tube assembly and a method of controlling a cold cathode fluorescent tube.
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When driving cold cathode fluorescent (CCF) lamps as used for the illumination of liquid crystal displays in applications such as lap-top computers and other information devices etc, the application often requires the ability to smoothly adjust the fluorescent lamp's brightness level between the fully on, and the fully off conditions.
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However, conventional fluorescent tubes normally exhibit a pop-on effect in terms of illumination levels when the tube first strikes following the application of sufficient voltage, and similarly exhibit a tendency to pop-off in terms of illumination level, when the supplied voltage is sufficiently reduced.
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This sudden turning on and turning off in terms of illumination levels, is apparent when a fluorescent lamp is driven by applied voltage or current control circuitry.
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These effects prevent absolute control of the tube's output brightness, and this applies both for single and in particular multi-tube drive applications, where one tube may illuminate and deprive power from other tubes in the system; until sufficient power is applied that the remaining tubes also illuminate, resulting in erratic and poor illumination control between the lamps.
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Conventionally, the solution to this has been to provide multiple circuits which operate fluorescent tubes or ordinary incandescent bulbs according to the required light level. This adds to the cost in both components and the manufacturing process.
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GB-A-1250376 describes a linear gas-discharge indicator for indicating the variation in a parameter. This makes use of a gas discharge tube having a pair of electrodes and a third electrode which extends alongside the tube and which is maintained negative with respect to the first electrode by a steady voltage of sufficient magnitude to start a glow discharge providing preionisation within the tube. A much lower second voltage difference is then applied between the third and second electrodes to cause a glow discharge along a portion of the second electrode. This is a relatively complex system requiring the use of three electrodes and the application of two different voltages and in practice would not generate sufficient light for illuminating other objects.
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In accordance with a first aspect of the present invention, a cold cathode fluorescent tube assembly comprises a cold cathode fluorescent tube having first and second terminals; and a voltage source connected to the terminals to generate a voltage therebetween, wherein the first terminal extends along the tube and is arranged such that progressively increasing or decreasing the voltage applied to the terminals by the voltage source causes fluorescence to progressively increase or decrease respectively along the tube.
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In accordance with a second aspect of the present invention, a method of controlling a cold cathode fluorescent tube having first and second terminals, the first terminal extending along the tube comprises progressively varying a voltage applied across the terminals to cause a corresponding progressive variation in fluorescence to occur along the tube.
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By providing a tube along which the first terminal extends, preferably for substantially its whole length, fluorescence progresses along the tube due to the effect of localised electrical discharge at points along the first terminal resulting from the voltage applied having to ionise gas in the tube over a reduced effective distance between the terminals. Therefore, the pop-on effect present in prior art fluorescent tubes is no longer present. By "substantially whole" we mean that the first terminal should terminate just short of the end of the tube. It could extend over a shorter distance (e.g. half the tube length) or over the entire tube length.
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This modification to the tube considerably improves the ability to smoothly control the tube's light intensity, by use of voltage or current control methods. In use it may be necessary to view a screen in almost complete darkness, e.g. when used in aircraft, making gradual illumination from very low light levels an important feature.
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The tube's normal pop-on effect in terms of illumination levels when sufficient power is applied is replaced with a more gradual, smoother increase in illumination level, which initially illuminates the tube dimly toward one end, and spreads gradually and controllably along the tube's length with increasing brightness, as applied power is slowly increased.
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Also the gradual reduction in applied power to the tube, results in the tube's brightness level more smoothly and controllably reducing in intensity and gradually receding, back along the tubes length until fully extinguished.
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The first terminal may take the form of a single wire running parallel to a centre axis of the tube which is electrically isolated from the second terminal, but preferably the first terminal comprises one of a twin spiral winding a running lace winding, a semi-spiral winding and a helical winding.
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Typically, in the case of a helical winding, this comprises a plurality of substantially equally spaced turns, although the spacing may be increased towards a remote end of the winding. This maintains control linearity towards the end of the tube.
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Preferably, the helical winding is wound at between 3 and 9 turns per inch, most preferably at 6 turns per inch.
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Examples of fluorescent tube assemblies in accordance with the present invention will now be described with reference to the accompanying drawings, in which:-
- Figure 1 shows a tube, winding and voltage source of a first example of an assembly according to the invention;
- Figures 2A-2D illustrate the illumination process of the tube of Figure 1;
- Figure 3 illustrates graphically the change in Lux level with control voltage;
- Figure 4 shows graphically the same data as Figure 3 with the Lux level magnified for clarity;
- Figures 5A-5C illustrate variants of windings for use in tube assemblies according to the invention;
- Figure 6 illustrates the tubes of a second example of an assembly according to the invention;
- Figure 7 illustrates two of the tubes of Figure 6 connected to a voltage source; and,
- Figure 8 shows a liquid crystal display illuminated by the second example.
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A cold cathode, glass envelope fluorescent tube 1 is shown in Figure 1 in which a pair of terminals 2,3 are mounted at opposite ends. A first one of the terminals 2 is extended by a winding 4 soldered to the first terminal 2 at one end and electrically insulated from the second terminal 3 at the other end. The terminals 2,3 are connected to an AC voltage generator 7. The voltage delivered across the terminals can be varied progressively from 0V to 12V via a control voltage input 8 of the generator 7.
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The winding 4 is generally constructed from a thin gauge wire e.g. tinned copper wire wrapped tightly around the tube 1 forming a uniform spiral winding at approximately 6 turns per inch. The winding 4 is cut off approximately ¼ inch from the second terminal 3 and secured against the tube's glass surface with transparent adhesive tape or other insulated method of bonding (not shown). The simple spiral has the practical advantages that it is simple and quick to apply by hand.
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The appearance of the tube during operation will be described with reference to Figures 2A-2D. As electrical power (or voltage) applied to the tube 1 is slowly increased from a fully off condition, the area "B" gradually becomes dimly illuminated, whilst area "A" remains unlit. (Figure 2A).
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With continuing and increasing application of power the unlit area "A" recedes down the tube, and is replaced by the dimly lit area "B" and a fully illuminated area "C". With approximately half power applied to the tube, the tube remains half illuminated with area "A" still completely un-illuminated, area "B" dimly lit, and area "C" fully illuminated as depicted in the Figure 2B.
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With continued increase in power the unlit area "A" disappears completely as the fully illuminated area "C" spreads down the tube, preceded by the partially illuminated area "B". (Figure 2C). This process continues gradually with smooth increase in applied power until the whole tube 1 is fully illuminated as depicted in Figure 2D.
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With reduction in applied power to the tube the reverse process occurs, and the light recedes back down the tube 1. First, the dimly lit area "B" returns as depicted in Figure 2C, followed by the fully off area "A" depicted in Figure 2B. The fully illuminated area "C" recedes back down the tube and gradually disappears until as in Figure 2A only the dimly lit area "B" remains. Finally, as power falls to zero area "B" slowly extinguishes and the entire tube becomes once again fully off.
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It should be noted that the dimly illuminated area B" gradually blends the un-illuminated area "A" with the fully illuminated area "C", and its boundaries are not as precisely defined as Figures 2A-2D portray. However, the area of merge between areas "A" and "C" as shown in Figures 2A-2D is approximately to scale.
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If, at any point in the process described with respect to Figures 2A-2D, the applied power were to be completely removed and then restored to the same level, the same areas and intensities of the tube illumination would also be restored. The winding 4 removes the relatively large hysteresis in lighting intensities that would otherwise normally be observed following a power interrupt, particularly at the lower levels of brightness.
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It is not essential for the turns to be equally spaced along the tube's length for the effect to work, but equally spaced turns enhance the smoothness of control in terms of the light output along the tube's length.
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However, it should be noted that towards the extreme non-connected winding end 5 of the tube, i.e. the last 10 to 15mm of a 90mm tube used in this example of the invention, the linear relationship between the number of turns per inch and the smoothness in spread of light along the tube 1 tended to deteriorate slightly. This slight deterioration in control linearity is indicated as the spread of light towards the tube's end spreads more rapidly for similar increase in control power, but this effect is only apparent in and confined to the last 15mm of the tube's length.
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This non-linear effect was overcome and corrected by a small reduction in the number of turns per inch from 6 to about 4 turns per inch during this last 15mm of the tube length.
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In Figure 3 the graph shows the relationship between the applied power generators control voltage and the measured Lux output from the 90mm tube. (Figure 4 shows the Lux output magnified for clarity, using the same data). The graph compares the same tube 1 measured with the winding 4 applied (wired), and without the winding 4 applied (unwired).
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The graph measurements were recorded starting with the control voltage set to 2.85 volts i.e. with the applied power in the fully off condition, and then plotted as the applied power to the tube was increased slowly to full, which occurred with a controlling input voltage applied at the control input 8 to the generator 7 at about 4.0 volts d.c. The controlling input voltage can be varied between 0-5Vac. The applied tube power was then slowly reduced back to zero, with the tube brightness again recorded at intervals, hence showing the resultant hysteresis curves, labelled "increase" and "decrease" (for both the wired and unwired tube conditions).
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It should be noted that the restricted input control range to the high voltage lamp generator of 2.8 volts to 4.0 volts, is purely a function of the control circuit itself and of no importance to the lamp's winding effect etc.
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As may be seen from the graph of Figure 4, with the winding 4 applied to the lamp the light level may be controlled smoothly from full brightness at about 1,600 Lux down to 0 Lux.
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As the graphs clearly show the "wired" tubes brightness is fully controllable over the entire range, the hysteresis between increase and decrease in applied power is particularly small, a desirable feature particularly at the lower light levels where the eyes are more sensitive to change.
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This is in contrast to the conventional arrangement without the winding 4 in which the natural tendency is for the lamp to "pop" on. Increasing power smoothly from the fully off condition results in a sudden and uncontrollable step in brightness increase from 0 to about 70 Lux, (Figure 3), when the applied signal is increased from 2.85 to 2.88 volts. This results in a total inability to set or retain a low level order of brightness. In addition, this situation is worse when the power is reduced from full brightness, and as the graph shows, even with a smooth reduction in applied power the tube brightness suddenly and uncontrollably drops from about 130 Lux to only 3 Lux when controlling voltage falls below 2.96 volts.
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As is indicated by the graph, the use of a winding 4 considerably enhances the brightness control of a cold cathode fluorescent tube particularly at lower light levels, and eliminates the rapid increase and decrease in light amplitudes normally observed as the tube reaches it's striking (pop-on) and turn-off voltages.
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An example of a tube assembly according to the present invention will now be described in more detail. A straight rod shaped cold cathode fluorescent tube of 4mm diameter and 90mm length was used. The maximum tube current was 4.0mA and the tube required 260Vac running voltage and 600Vac strike voltage.
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The wire used for the winding 4 was 0.22mm² (24 to 25 swg) tinned copper wire extracted from standard equipment type 7/0.2mm PVC coated wire with the insulation removed.
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However, larger diameter wires including 0.41mm² (22 swg) tinned copper wire gave equally good results. Generally, the practical minimum wire size for the windings would be 0.036mm² (35 swg), whilst a practical, maximum size would be approximately 0.410mm² (22 swg). Greater care must be taken when winding tubes with the thicker wire sizes to avoid mechanical strain particularly to the "wired electrode", and the glass tube itself. The larger wire sizes are less favourable to use for practical reasons as they are mechanically more difficult to bend tightly to the fragile glass tube's surface, and also more difficult to secure at the "open-circuit" end of the tube winding. It should also be noted that the greater the surface area of the tube that is covered with wire, then the less light output for the actual application is available. For this reason and for the practical reasons of attaching the wires, the relatively thin gauge wire 0.22mm² (24 to 25 swg) non-insulated tinned copper wire has been used, however PVC or PTFE insulated type wires would also work equally well.
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Good results were obtained with approximately 6 turns per inch. The accuracy of the winding 4 is not critical but the wire was wound tightly on the tube's glass surface to "optimise" the effect, and maintain a stable, secure and neat mechanical construction.
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Using 6 turns per inch or more, when a suitable voltage is applied, the illuminated tube area smoothly expands from the non-connected winding end 5 of the tube 1, towards the connected end 6 of the tube 1 as power is gradually increased until at full power the entire tube's length is illuminated.
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As applied power to the tube is gradually reduced, the light emitted from the tube 1 recedes from the full length, starting at the connected end 6 and retreats back towards the non-connected winding end 5, until the length of illumination finally diminishes to zero as power turns fully off.
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If the number of turns is reduced to say 3 turns per inch then the illuminated length of the tube has a tendency to step in increments equal to the resultant gap between each winding turn, as power is slowly increased or decreased to the tube. This results in stepped brightness level increments as the illuminated part of the tube length expands or recedes along the tube's length. This effect of the light stepping or incrementing along the tube 1 becomes less noticeable and disappears if the number of winding turns is increased to 6 per inch.
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In addition, when a circuit drives two or more tubes in parallel the application of the winding 4 to each tube 1, will again remove the undesirable and rapid increase and decrease in tube brightness, which is even more pronounced in multi-tube systems.
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If in the circuits described two conventional tubes were connected in parallel (i.e. without spacial first terminals), the first tube to strike (the one with the lowest strike voltage tolerance value), would tend to "hog" the current. As power is slowly increased in this system then the first tube to strike-on would become even brighter whilst the remaining tube would still be unlit.
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If the power applied was now gradually increased the second tube would also suddenly strike, and the current through both tubes would then suddenly become approximately equalised.
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This would have the effect that the first tube would now dim slightly from its former brightness and the second tube will suddenly become equally but fairly bright, giving the appearance to the observer of two separately stepped and uncontrolled brightness increases. Again if the power is gradually reduced both tubes will slowly dim, then as one tube suddenly turns off as it falls below its drop-out voltage, the remaining tube will suddenly become brighter as it receives the current that had been retained in the first tube. As power continues to be reduced by the operator so the brightness will again continue to fall until suddenly the remaining tube also turns off.
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These sudden stepped changes in light levels described are completely removed when each tube in the system has the winding applied, and the light becomes fully controllable as in the one tube system with winding.
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In addition to the substantially uniform spiral winding as described above, other winding types have been applied to both 90mm and 50mm long straight Cold Cathode Fluorescent tubes and tests have indicated equally good results.
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Figure 5A depicts a CCF tube 1 with a twin spiral winding 10. This was wound with two separate wires 11,12 both electrically connected to the same tube end electrode 2 but insulated from each other. These two wires were wound spirally at approximately 6 turns per inch. One in a clockwise, the other in an anti-clockwise direction, both wires also being isolated electrically from the remaining CCF tube electrode 3. The wire used was 0.22mm² (24 to 25 swg) non-insulated tinned copper wire, hence both wires were in electrical contact with each other at each of the spiral cross-over points 13 occurring down the tubes length.
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Figure 5B depicts a CCF tube with a "running lace" type winding 14, formed with 0.22mm² (24 to 25 swg) non-insulated tinned copper wire electrically connected to one CCF tube electrode 2 and electrically isolated from the other remaining electrode 3.
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Repeated test comparisons made between the three winding types in Figures 1, 5A and 5B, as applied to CCF tubes of the same size, have shown little to no apparent difference between either the smoothness of spread of light along the tube with increasing applied power, or receding of light back down the tube with reduction in applied power. In addition the stability of pre-set light levels returning to their former set levels following power interruptions was equally good for all three windings of Figures 1, 5A and 5B.
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Another possible winding involves adding additional wires to form a mesh. Alternatively, a semi-spiral winding 15 as shown in Figure 5C could be applied although this is less favoured due to the difficulties of attaching the wire to the tube glass surface. This winding does not completely wrap around the tube in a continuous spiralling manner, but completes a 360 degree wrap and then returns to form a 360 degree wrap in the opposite direction, this process repeating at alternate turns along the length of the tube.
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In another example, a conductive strip of tissue paper-thin conductive foil approximately 1/8th inch wide was attached to the tube's surface such that the strip lay flat along the tube's length. The foil was electrically connected to one of the tube's electrodes but isolated electrically from the remaining electrode. This worked reasonably well but the results were not as good in terms of lighting control as those obtained with the spiral type of tube winding. The spiral winding allows greater control by permitting adjustment of the turns ratio along the tube's body, to give increased linear control of the lights progression along the tube, with respect to applied power. In addition the conductive strip applied was nontransparent, and hence obscured some of the 4mm diameter tube's surface and reduced the total amount of visible light from the tube as a result, which is an undesirable feature.
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A single straight wire attached to one electrode and laid flat against the tube's glass surface and electrically isolated from the remaining electrode was tested, but control of the light was lost when nearing illumination of approximately 2/3rds of the tube's length, resulting in a sudden increase in light or "pop-on" type effect, the single wire suffering similar loss of control as the foil strip. Neither the single straight wire nor the foil strip provided the degree of control over the tube's output given by the various spiral type windings.
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In place of a wire winding, transparent or non-transparent conductive paint in the same form i.e. various spherical windings, could be used, or a CCF tube could be dipped vertically into a transparent conductive paint to form a complete coat around the tube extending from and hence electrically connected to one tube electrode 2 but leaving the remaining tube electrode 3 isolated.
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An example of the invention used for backlighting of Liquid Crystal displays will now be described with reference to Figures 6, 7 and 8.
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An arrangement of back-lighting for a practical Liquid Crystal Information Display has been derived using wire-wound CCF tubes as described above to give enhanced lighting control. The tube arrangement is shown in Figure 6. This comprises three tubes 16,17,18. Tubes 16 and 17 are driven from the same high voltage A.C. generator 19, as shown in Figure 7, whilst tube 18 is driven from a second entirely separate generator of identical design and build (not shown). The tubes 16, 17 are connected to the generator 19 by a common low tension line 20 and separate high tension lines 21,22. The two generators are employed to provide a degree of redundancy in the event of circuit failures which is desirable in some applications.
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The LCD display requires back-illumination to enhance its clarity both in total darkness, and whilst operating in partially lit conditions. This use requires good even lighting across the entire display, with the ability to adjust the light level emitted from the display as required by the operator. Figure 8 shows a liquid crystal display 23 in which the three tubes 16,17,18 as depicted in Figure 6 are placed between the rear of the LCD display 23 and a silvered reflector 24 such that light output 25 from the tubes is focused with minimum loss onto the display. Between the CCF tubes 16,17,18 and the LCD display 23 is a thin transparent light diffusor panel 26 to help defocus unwanted bright spots, and provide an even light intensity level falling over the entire display area.
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With reference to Figure 6 it will be noted that the three tubes are placed in a triangular formation, also the spiral windings are positioned such that one winding of a tube is electrically connected to a tube electrode at each corner. Note that with increase of applied power the light spreads along the tube, from the isolated spiral end of the tube towards the tube end with the electrode 2 and the spiral winding 4 connected, as indicated by arrows 27 for each tube.
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The power applied to both tubes 16,17 in the twin generator circuit is matched electronically within the twin generator circuit, such that tubes 16,17 are equally bright at all power settings. In addition the power applied to the third tube, 18 is matched by adjusting an output balance control in the single generator circuit. This balancing between the circuits simply applies an offset to one of the circuits 0 to 5 volt Tube power controls inputs 28, such that when both drive circuit's power controls are connected together and a common control voltage applied, the emitted light from each of the three tubes 16,17,18 is approximately equal over the entire control range from fully off (minimum brightness), to fully on (maximum brightness).
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As the power is slowly applied to the three tubes 16,17,18 via both the circuit's power control inputs, so light is emitted from the each of the tubes at one end, ie adjacent to a corner of the triangle, hence giving illumination from three localised areas, as though from three conventional light bulbs, (one at each corner of the triangle).
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With the reflector 24 behind the tubes and diffusor 26 between the tubes and the LCD display 23, even light at a dim level is achieved over the entire display area. As the control voltage is slowly increased, light smoothly spreads along each of the three tubes 16,17,18 in direction 27 shown in Figure 6, until as the control voltage reaches its maximum fully on (+5 volt) setting, so each of the three tubes is fully illuminated along the entire tube length and hence full brightness from the LCD display 23 is achieved.
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Upon resumption of power after an interrupt, the brightness level will always return to same pre-interrupt brightness level, irrespective of whether it was at fully brightness or very dim.
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Many versions of CCF tube are available in numerous combinations of length, diameters, and electrical parameters to which the invention would be applied. The details given are for the specific examples described, however the invention is equally applicable to any other cold cathode fluorescent tubes of similar mechanical and electrical characteristics, including specially shaped "specials" such as a "U" or "W" shape etc.
