CA2235215A1 - Methods of controlling the brightness of a glow discharge - Google Patents

Abstract

Methods of controlling the brightness of a glow discharge which switches from a low brightness state to a high brightness state a given time after the start of an excitation pulse are described. In the first method, conventional pulse duration modulation produces a dimming ratio much greater than the ratio of duty factor variation. In the second method, a plurality of sets of pulses having different fixed durations but variable repetition rates are employed. In the third method, a plurality of sets of pulses having different relative durations have their pulse durations modulated in synchrony.

Description

CA 0223~21~ 1998-04-17 METHODS OF CONTROLLING THE BRIGHTNESS OF A GLOW DISCHARGE
This invention relates to methods of controlling the hrigh~n~ss of a glow discharge.
The methods rel,ate particularly, though not exclusively, to light sources for b~r~ hting 5 liquid crystal displays.
Glow discharge light sources are increasingly being used as b~rklight~ for liquid crystal displays. Such b~r~ ht~ must be capable of high brightn~s~ for use in direct sunlight, and have applications in vehicle in~ u~.ent displays, aircraft cockpits etc. When such displays are used in low light conditions, or when the observer is wearing image 10 intensifying goggles to improve night vision, such high source brightn~ss becomes a disadvantage. For this reason a nu~l~bel of methods of ~limming LCD b~r~ ht~ have been developed.
One method of controlling the brightn~s of a glow discharge light source is to use a train of excitation pulses and to modify the duration of the pulses. This is known as pulse 15 duration modulation, and the brightn~ss of the light source can be reduced in ~ po,Lion with the average power supplied to the lamp. There are, however, a number of drawbacks with such techniques. In US 5,349,273 for example it is disclosed that only a 20:1 ~limming range is possible because of significant ilh~ ion non-u~ir~l"~ily at low lamp currents, and because of a reduction in output voltage of the controller resnlting in non-excitation of 20 the discharge. Most commercially available fluorescent lamp ~limm~ have a ~iimming range of less than 150 to 1.
In US 5,420,481 a supplementary set of electrodes are used to operate a glow discharge in a dirrt;lc"- manner in a low brightn~ss regime. By switching from one set of electrodes to the other set it is possible to achieve a .li"""i..g range approaching 10,000:1 2s (or 80dB) from 3000 cd m~2 to 0.3 cd m-2. However the m~ximllm brightn~ of this lamp is not high enough for good contrast displays in bright sunlipht, and the provision of extra electrodes and ~wil~;llillg cilc-uiLIy increases cost and decreases reliability and convenience of use. There can also be a discontinuous change in brightn~ss when ~.wi~hillg from one set of electrodes to the other set.
According to a first aspect of the invention there is provided a method of controlling the bri~htn~ss of a discharge capable of op~,ldlillg in a first condition having a first brightn~ss and in a further condition having a dirrelc,lt brightn~ss, the said conditions occurTing in adjacent time periods, the method comrri~ing a) supplying r.f. energy to the discharge as a train of pulses, and b) controlling the duration of the pulses, thereby controlling the ratio of the time spent by the discharge in the first condition to the time spent by the discharge in the further condition in any given time period, such that any change in the duty factor CA 0223~21s 1998-04-17 WO 97/15172 pcT/Gss6/o2499 of the train of pulses is proportionally less than the resulting change in bri~htnPs.c of the discharge.
This method ca~Ll provide brightnPs~ control which is continuously variable over a b~ightnPcs range in excess of other known methods, the brightness range being ~ fi~ gly S greater than the range of duty factor variation.
Preferably the method is such that in the first condition r.f. energy is mainly electric field coupled to the discharge and in the further condition r.f. energy is mainly ~,n~gnPtic field coupled to the discharge. The r.f. energy is advantageously mainly electric field coupled to the discharge at the start of a given pulse.
According to a second aspect of the invention, there is provided a method of controlling the brightnçs,c of a glow discharge capable of operating in a first condition having a first b~ightne~ and in a furtner condition having a dirr~lcnl bri~htnPs~, the said conditions occu~Tin~ in adjacent time periods, the method comprising a) supplying r.f. energy to the discharge as a plurality of sets of pulses, each set having a dirrel~nl pulse duration, at least one set having a pulse duration snfficjently short that the discharge is in the said first condition for the whole duration of each pulse in the said at least one set, and at least are further set having a further pulse duration s~lffi~ ntly long that the discharge passes into both conditions during each pulse in the said at least one further set, and b) controlling the repetition rate of the pulses comprising the at least one further set of pulses, thereby controlling the ratio of the time spent by the discharge in the first condition to the time spent by the discharge in the second condition in any given time period.
This method can provide a plurality of b1ightnPc~ levels which are less susceptible to temperature variations and other variables which are difficult to control.
According to a third aspect of the invention, there is provided a method of controlling the blightnPs~ of a glow discharge capable of o~eldlillg in a first condition having a first blightn~c~ and in a further condition having a dirr~rellt hlightnPs,s, the said conditions occnrring in adjacent time periods, the method comprising a) supplying r.f. energy to the discharge as a plurality of sets of pulses, each set having a respective pulse duration, at least one set having a pulse duration sufficiently short that the discharge is in the said first condition for the whole duration of each pulse in the said at least one set, and b) controlling the duration of the pulses in each of the sets of pulses in synchrony with the other sets.
This method can also provide a plurality of brightnPss levels which are less susceptible to lell~ dlure variations and other variations which are difficult to control.
-CA 0223~21~ 1998-04-17 EmbodilllenL~ of the invention will now be described, by way of example only, with reference to the accompanying diagld~ tic drawings in which:, Figure 1 shows trains of pulses according to the first aspect of the invention, Figure 2 shows the h.LGI~iL~r of light emitted by the discharge during the pulses 5 shown in Figure 1.
Figure 3 shows the brightnloss of a discharge as a function of pulse duration at a pulse repetition rate of 100 Hz.
Figure 4 shows the brightn~-ss of a discharge as a function of pulse duration at a pulse repetition rate of 10,000 Hz.
lo Figure 5 shows a block diagram of the pulse controller used to give the results of Figure 3 and Figure 4.
Figure 6 shows trains of pulses according to a second aspect of the invention.
Figure 7 shows a pulse train according to a third aspect of the invention.
Figure 8 shows a pulse train according to a fourth aspect of the invention.
Flat inductively coupled discharge lamps have been developed as high pGlr~ ulceb;~c~ htc for liquid crystal devices. Such b~ lightc have been described in detail in W09507545 which is incorporated herein by reference. A lamp of the type described in W09507545 is employed to generate the discharge in the following specific embodiments of a method of controlling the brightnto~ of a discharge. The lamp comprises a sealed 20 quartz envelope filled with a low pressure mixture of mercury and argon. One surface of the envelope carries a lumin~scent m~t.ori~l such as a layer of a phosphor. The envelope is placed adjacent a spiral external driving electrode to which r.f. energy at 13.56 MHz is supplied in a train of pulses.
Figure l(a) shows sch~m~tir~lly a ~irst train of pulses according to a first aspect of 25 the invention. Figure l(b) shows a second train of pulses according to a first aspect of the invention. The time period between pulses starting is constant in the two cases, but the duration of the pulses is dirrelGllt in the two cases, resulting in a dirrGlGnt duty factor.
Figure 1 (c) shows a third train of pulses having the same period but yet another duty factor.
In each of these Figures the x axis corresponds to time. The y axis in each case is schPm~tic 30 in that it is equal to zero between pulses of r.f. energy and non-zero during each pulse of r.f. energy. The top of each pulse of r.f. energy is shown to be oscillating merely to help the reader recognise at which times the r.f. energy is applied. In the case of Figure 1 (a) the pulse duration is 4 ms and the time between pulses is 6 ms. The duty cycle is therefore ~ 40% and the frequency of the pulses is 100 Hz. The lumin~nre of a discharge lamp excited 3s by 13.56 MHz r.f. power in this manner would typically be 4000 cd m -2.
The inventors have observed that during each pulse the bri~htness of the discharge of a lamp of the kind described in W09507545 is not constant. In particular there are two distinct conditions or regimes in which the lamp operates during each pulse. In the first CA 0223~215 1998-04-17 condition (marked 4 in Figure 1), which is generally the first condition when the pulse of r.f. energy is applied to the discharge, the brightn~ss of the discharge is fairly low. This condition persists for a time 6 shown in Figure la. The discharge then quickly flips into a second condition, labelled S in Figure la, which lasts for a time 7 until the r.f. energy is no longer supplied to the discharge. The brightness of the discharge in this second condition is typicaUy 30 to 100 times brighter than in the first condition. The hlLensi~y of light emitted by the discharge with time during the pulses shown in Figure la is shown sche~ 11y in Figure 2a. The same reference numerals are used to denote the same time periods and conditions in the two Figures.
It is believed that the two conditions having dirr~ lL brightness are due to the r.f.
energy being coupled into the glow discharge via different mech~nicm~. At high peak r.f.
powers, the energy is coupled into the glow discharge via the m~gn~tic field generated by the external spiral electrode. This method of coupling is very effl~ient but it takes a f~nite time for the glow discharge to be able to enter this condition.
For example, when starting a 40 watt m~gn~ticzllly coupled discharge this delay might be 1.5 milliseconds. In the time between the glow discharge 'striking' and the onset of the m~gnetic field coupled condition as described previously, energy is initially coupled into the glow discharge via the electric field generated between ~ ent coils in the spiral electrode.
For sufficiently low r.f. powers, only electric field coupling is observed. However, for higher powers the electricaUy coupled initial discharge will flip into the more efficient magnetically coupled discharge after a short delay. This delay time depends upon a number of facts such as larnp telllpel~Lur~, electrode geometry, and input power. However for a given set of conditions the delay time is well defined. As a result, by choosing an apployliate modulation frequency (such as a few hundred Hertz) it is possible tocontrollably reduce the r.f. pulse duration (and hence duty factor) such that there is a smooth transition from electric field coupling foUowed by m~gn~tic field coupling to electric field coupling alone.
The effect of reducing pulse duration is shown in Figures l(b) and l(c). In Figure l(b) the frequency has been kept constant at 100 Hz, but the pulse duration (17) has been reduced from 4 ms (in Figure l(a) to 3 ms, and the time between pulses (18) increased to 7 ms. As the width of the pulse decreases, so the proportion of time spent by the discharge running in the first condition (via electric field coupling) increases, thereby reducing the bnghtn~ss of the lamp. Eventually, as the pulse duration is reduce~l, the pulse of r.f. energy is not long enough to enable the lamp to switch into the second condition. This state of affairs is shown in Figure l(c), where the pulse duration (19) has been reduced to less than 1.5 ms, and the time between pulses (23) increased to still give a pulse repetition rate of l 00 Hz.

CA 0223~21~ 1998-04-17 The ~ uL~ iLy of light emitted by the lamp when being operated as in figure l (b) and (c) is shown schematically in Figures 2(b) and (c) respectively. Once again, the same reference numerals are used to denote the same fea~ules in each respective Figure. The average lllmin~nce or brightn~ of the discharge in Figure 2a, b and c is proportional to the 5 area under the graph in each case.
It is apparent that the average lllmin~n~e or bri~htn~c~ of the discharge decreases with decreasing pulse width, but it is also ~")art;lll that this decrease is proportionally much greater than the decrease in duty factor of the pulse train, due to the large difference in brightnl-c~ or lnmin~nce of the first and second conditions of the discharge.
Figures 3 and 4 show how the h.. n;~ ce of a typical discharge according to the invention varies with duty factor. The y axes in the figures corresponds to the l--min~n~e expressed in cd m~2, whilst the x axes denote pulse duration. Figure 4 shows how the discharge behaves at a pulse repetition rate of 10 kHz, whilst Figure 3 shows the behaviour at 100 Hz. The x axes are linear whilst the y axes are lo~ l"nic In Figure 4, the data points m~rk~d with a triangle were measured when increasing pulse duration, whilst the data points marked with a square were ...ea~ulcid when decreasing the pulse duration. The fact that the two sets of data points do not lie on the same curve is an indication that at high repetition rates (and correspondingly short pulse durations) hysteresis becomes important.
This is most likely due to the possibility of bypassing the first (electric field coupling) condition if the time since the discharge was last in the second (m~gn~tic field coupling) condition is less than a characteristic relaxation time of the glow discharge. If the time between the end of a m~gn.otic field coupled r.f. energy pulse and the start of a subsequent pulse is sufficiently short that populations of electrons ions and radicals in the lamp have not had time to relax back to the values present during electric field coupling or before any excitation began, then the subsequent pulse may go straight into the second (m~gnçtic field coupling) condition without passing through the first condition. From the experim~-nt~l results shown in Figure 4 this can be calculated to be approximately 80,us for the particular lamp and input power shown in Figure 4.
In Figure 3 the data points were taken at a repetition rate of 100 Hz, so that the length of time between pulses was always greater than 100,us so that such hysteresis is not observed.
It will be observed from Figure 4 that there will be a ~i nific~nt step in brightnto~
between the regime in which electric field coupling is the only coupling m.o~h~ni~m and the regime in which magnetic field coupling is present. Such a "bri~htn~s~ gap" is un~lesir~hle for applications such as b~lrlighting of displays. The "bri~htn~s~ gap" is less pronounced in the case of Figure 4 when the pulse repetition rate is lower. The reasons for this are not well understood. One effect which can be used to overcome this gap in brightn~s~ is to CA 0223~21s 1998-04-17 wo 97/lS172 PcT/Gss6/02499 change the r.f. power being delivered in each pulse. It is observed that the time duration of the first (electric field coupling) condition depends upon the power being supplied to the discharge. If the power is high, the time before the discharge switches into its second condition is short. If the power is re~lnced, the time before the discharge switches into its 5 second condition is greater. There is a critical power level below which the second condition is never achieved. By co~ g variation of the duty cycle with variation in the r.f. power supplied during each pulse it is possible to miti~1~ the disadvantage of a brightn.os~ gap.
A block ~ ~m of the system which controls the pulse duration is shown in Figure 5. A 14 volt d.c. power supply is provided at input tertnin~l~ 39 and 40. This powers an NE566 Function Generator integrated circuit (32). This circuit provides a triangular output waveform at output 34. The repetition rate of this waveform is regulated by an RC
network (33) which is provided on a neighbouring part of a common PC~B. In normal use the frequency is not adjusted. The triangular output waveform is supplied as one input (35) 1S to an LM 311 comparator integrated circuit (37). The other input to the comparator is provided by a d.c. level set by an adjustable potentiometer (36). The d.c. level acts to trigger the c~m~lor twice per cycle as the tri~n~ r waveform passes through a predet~llni.led level whilst increasing and again whilst decreasing. Thus the output of the comparator (38) will be in the shape of a square wave, with the duration of each pulse 20 de~rrnin.--l by the d.c. level set by the potentiometer. (~h~nging the d.c. Ievel by adjusting the potentiometer will alter the square wave pulse duration at output termin~l~ 41 and 42 without ~lSt~ring the repetition rate of the pulses.
The second aspect of the invention provides a method of controlling the brightn~cs of a glow discharge which miti~t~s the disadvantage of the "brightn~s~ gap" as described 2s above.
Figure 6(a), (b) and (c) illustrate ~ree different pulse trains according to this second aspect of the invention. In this figure, as in Figure 1, the x axes corresponds to time and the y axes correspond to the presence or absence of r.f. energy. In each case the pulse train co~ lises a plurality of sets of pulses (in the present example two sets), the sets of pulses 30 having dirr~ repetition rates and having .lirrt;l~ellt pulse durations. The duration of the first set of pulses (30) is arranged to be such that the glow discharge will always be in the first condition. That is, it will be mainly electric field coupled for the whole duration of each pulse in the set. In the present case each pulse in the first set has a duration of 0.2 ms and a gap of 0.3 ms. In Figure 6(a) every l5th pulse in the pulse train is arranged to have a 3s duration of 1.6 rns, forrning a further set (31) of pulses having a lower repetition rate and a different duration. The period of the longer pulses will be (0.5 ms x 14 + 1.6 rns) or 8.6 ms, yielding a repetition rate of just over 116 Hz. In figure 6(b) every 12th pulse in the pulse train is arranged to have a duration of 1.6 rns. The period of the set of longer pulses CA 0223~21~ 1998-04-17 WO 97/1~i172 PCT/GB96/02499 in this case will be (0.5 ms x 11 + 1.6 ms) or 7.1 ms. In Figure 6(c) every 9th pulse in the pulse train has a duration of 1.6 ms, giving a period of (0.5 ms x 8 + 1.6 ms) or 5.6 ms.
In this way, the repetition rate of the further set of pulses (i.e. longer pulses in the present example) is increased, whilst the repetition rate of the set of shorter pulses remains S the same. When the average brightnPss of the glow discharge produced in each case is compared, it is found that a 'grey scale' of dirr~,.c;l.l average brightn~sc levels has been produced. The 'brightn~ss gap' beLweell each grey level is not as large as that produced by the pulse trains in Figure 1 because all the pulses in the train do not have their durations increased at the same time.
To compare the hrightn~sc levels of the examples shown in Figure 6 we must calculate the average brightn~cs in each case. For example, if we assume that the lumin~nce in the first condition is equal to 1, and that in the second condition is equal to 50 (in albiLl~uy units), and that the switch from one condition to the other occurs after 1.5 ms pulse duration, then the average brightnesc in the example of Figure 6(a) will be (1 x 0.2 ms x 14 + 50 (1.6 - 1.5)) x 116 Hz or 905 a~ y units. Figure 6(b) will be (1 x 0.2 ms x 11 +50(1.6-1.5))xl41HzorlO14~bill~yunits,andFigure6(c)willbe(1xO.2msx8+
50 (1.6 - 1.5 ms)) x 179 Hz or 1179 ~biLI,~ur units. If only short pulses were used the average brightn~sc would be 1 x 0.2 x 2 kHz = 400 all.iLIdly units. Brightnt~sc below 905 ~biL,d.~y units may be produced by increases the number of short pulses between long pulses above the fifteen shown in Figure 6(a).
However, because it is undesirable to have the light appear to flicker it is important to keep the repetition rate above the critical fusion frequency for an observer (which may typically be 70 - 90 Hz. If hrightn~cces below 400 ~bill~uy units were required,conventional pulse time modulation techniques may be used on the shorter pulses alone.
In the example of Figure 6, the brightest possible condition is where a long pulse occurs each time, with in this example a 0.3 ms gap between pulses.
It is important to keep the gap between successive pulses sufficiently long so that the next pulse does not start to glow in the further (m~gn~tic field coupled) condition, thereby bypassing the first condition completely.
The trains of pulses shown in Figure 6(a) may be generated by a pulse generator triggered under colll~uLel control according to the following algc~iLhlll:-1. Reset pulse counting shift register to read zero.
2. Generate a pulse of duration 0.2 ms.

3. Add 1 to number in pulse counting shift register.

4. Wait for 0.3 ms.

5. If pulse counting shift register does not read " 14", go to 2.

6. If pulse counting shift register reads 14, continue.

7. Generate a pulse of duration 1.6 ms.

CA 0223~21~ 1998-04-17 8. Wait for 0.3 ms.

9. Go to 1.
To control discharge brightn~.c, the integer '14' in steps 5 and 6 would be altered.
For example, it rnay be altered to "11" to give the pulse train of Figure 6(b), or "8" to give 5 the pulse train of Figure 6(c). The generation of the pulses in steps 2 and 7 may be p~;lrc,lll,ed by different pulse generators. The pulse time control means employed can take many forms whilst l~ i..g with the scope of the present invention. Persons skilled in the pulse control art will be able to design many circuits which would be able to produce the pulse trains of Figure 6.
The third aspect of the invention provides a further method of controlling or regulating the hrightn~s of a glow discharge which also mitig~tes the disadvantage of the brightness gap and temperature variation effects as described above.
Figure 7 illustrates a pulse train according to this third aspect of the invention. The pulse train comprises a sequence of 6 pulses, each pulse having a dirrt;l~ duration. The 15 train of 6 pulses is repeated to form a continuous pulse train. The train of pulses therefore comprises, in effect, 6 sets of pulses each set having the same repetition rate but a dirr.,~
duration. In the example shown in Figure 7, the pulse durations are as follows:- 2 ms (50), 1.2 ms (51), 1.8 ms (52), 1.4 ms (53) and 1.6 ms (54).
There is a gap of 0.5 ms (55) between each pulse. The brightnç~s control according 20 to this aspect of the invention is achieved by ch~nging the duration of all the pulses, but keeping the ratio of the pulse durations from set to set constant.
The duration of the pulses therefore becomes 2xd, 1.2xd, 1.8xd, 1.4xd and 1.6xd,with d being varied to adjust glow discharge brightn~s~.
As d is varied, a dirrt;lt;~lt number of the pulses in a given time period will have a 25 duration long enough to excite the glow discharge into the second (m~gn~tic field coupled) condition having a higher brightness. Thus there are, in effect, a plurality of 'grey-levels' depending on how many of the sets of pulses have a duration greater than some critical duration (in the present example 1.5 ms). The embodiment as described would yield 6 grey levels, but greater or few levels would be provided by having a different number of sets of 30 pulses.
Figure 8(a) and 8(b) each show a pulse train according to an advantageous embodiment of the invention. The method is employed to control a two dimensional array con~i~ting of two discharges as previously described. The discharges are spatially adjacent one another. One is supplied with the train of pulses as shown in Figure 8(a), and the other 3s with the train of pulses as shown in 8(b). Thus, ~dj~e~t discharges are supplied with r.f.
power in different time intervals. As a result, there is a reduced interference caused by a plurality electrom~gn~tic fields being coupled to a given discharge simlllt~n~ously. For two nearest neighbour discharges this is possible if the duty factor of each pulse train is less than WO 9711!;17Z PCT/GB96/02499 50%. For a square array having 4 nearest neighbours, a duty factor of less than 25% for each of the plurality of pulse trains would enable all spatially ~r1j~cent discharges to be excited during diL~~ time periods. In general, a duty factor of less than 100/u % is required for an array having u nearest neighbours.
s Finally, the contents of the accompanying abstract is incol~ol~led herein by reference.

Claims (10)

1. A method of controlling the brightness of a glow discharge capable of operating in a first condition (4) having a first brightness and in a further condition (5) having a different brightness, the said conditions occurring in adjacent time periods, the method comprising a) supplying r.f. energy to the discharge as a train of pulses (1, 2, 3), and b) controlling the duration of the pulses, thereby controlling the ratio of the time spent by the discharge in the first condition to the time spent by the discharge in the further condition in any given time period, such that any change in the duty factor of the train of pulses is proportionally less than the resulting change in brightness of the discharge.

2. A method of controlling the brightness of a glow discharge capable of operating in a first condition (4) having a first brightness and in a further condition (5) having a different brightness, the said conditions occurring in adjacent time periods, the method comprising a) supplying r.f. energy to the discharge as a plurality of sets of pulses (30, 31), each set having a different pulse duration, at least one set (30) having a pulse durationsufficiently short that the discharge is in the said first condition for the whole duration of each pulse in the said at least one set, and at least one further set (31) having a further pulse duration sufficiently long that the discharge passes into both conditions during each pulse in the said at least one further set, and b) controlling the repetition rate of the pulses comprising the at least one further set of pulses, thereby controlling the ratio of the time spent by the discharge in the first condition to the time spent by the discharge in the second condition in any given time period.

3. A method of controlling the brightness of a glow discharge capable of operating in a first condition (4) having a first brightness and in a further condition (5) having a different brightness, the said conditions occurring in adjacent time periods, the method comprising a) supplying r.f. energy to the discharge as a plurality of sets of pulses (50, 51, 52, 53, 54), each set having a respective pulse duration, at least one set (51) having a pulse duration sufficiently short that the discharge is in the said first condition for the whole duration of each pulse in the said at least one set, and b) controlling the duration of the pulses in each of the sets of pulses in synchrony with the other sets.

4. A method as claimed in claim 3 in which successive pulses have different durations.

5. A method as claimed in any preceding claim in which in the first condition r.f. energy is mainly electric field coupled to the discharge at the start of a given pulse.

6. A method as claimed in any preceding claim in which in the further condition r.f. energy is mainly magnetic field coupled to the discharge.

7. A method as claimed in any preceding claim in which the duty factor of the train of pulses is less than 50%.

8. A method as claimed in any preceding claim in which the pulse repetition rate is greater than the critical fusion frequency for an observer.

9. A method as claimed in any preceding claim in which the pulse repetition rate is less than the frequency of the r.f. energy being supplied.

10. A method as claimed in any preceding claim in which r.f. energy is supplied to an array of glow discharges in a train of pulses, such that spatially adjacent glow discharges are supplied with a pulse in a different time period.