Abstract:
The present invention provides a lighting system powered by a system power source. The lighting system includes a ballast in operative connection with the system power source where the ballast is designed to generate a lamp input signal. A lamp input line is operatively connected to receive the lamp input signal. Further, a gas discharge lamp is in operative connection to the lamp input line configured to receive the lamp input signal. An amplitude modulation circuit is then placed in operative connection to the lamp input line, where the amplitude modulation circuit is configured to periodically modulate amplitudes of the lamp input signal prior to the lamp input signal being received by the gas discharge lamp. Operation of the amplitude modulation circuit results in a periodic amplitude modulation of the lamp input signal and eliminating visual striations otherwise occurring in the lamp.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to improving the visual appearance of linear fluorescent lamps, and more particularly, to the elimination of visual striations which may occur in gas discharge lamps. Generally, a gas discharge lamp will have an elongated gas-filled tube having electrodes at each end. A voltage between the electrode accelerates electrons movement. This causes the electrons to collide with gas atoms producing positive ions and additional electrons forming a gas plasma of positive and negative charge carriers. Electrons continue to stream toward the lamp&#39;s anode and the positive ions toward its cathode sustaining an electric discharge in the tube and further heating the electrodes. The electric discharge causes an emission of radiation having a wavelength dependent on the particular fill gas and the electrical parameters of the discharge. 
     A fluorescent lamp is a gas discharge lamp in which the inner surface of the tube is coated with a fluorescent phosphor. The phosphor is excited by the ultraviolet radiation from the electric discharge and fluoresces, providing visible light. 
     During operation of a gas discharge lamp, such as a fluorescent lamp, a phenomenon known as striations can occur. Striations are zones of light intensity, appearing as dark bands. This phenomenon can give a lamp an undesirable strobing effect. An example of the striation phenomenon is shown in FIG. 1, which depicts a linear fluorescent lamp  10  employing Krypton added as a buffer gas to improve the efficacy of the lamp. In FIG. 1, lamp  10  has striation zones  12  which appear as the dark bands moving along the length of the lamp. Striations in gas discharge lamps are known to occur in cold applications and in other contexts such as Krypton content lamps. 
     A variety of theories as to why striations occur have been set forth. For example, in U.S. Pat. No. 5,001,386 to Sullivan, it is stated that striations are believed to occur as a result of high-frequency currents re-enforcing a standing wave of varying charge distribution between the lamp electrodes. 
     Sullivan attempts to solve the striation problem by injecting a dc component superimposed on top of a driving ac current. A disadvantage to this technique, is the requirement that existing typical high-frequency ballasts in the marketplace must be removed and replaced with a unique ballast capable of injecting the dc bias component. Also, by adding the dc bias it is possible to cause damage to the lamp, by moving mercury in the lamp to one end, creating an unbalanced light output. It has also been suggested that increasing the crest factor in a lamp lighting system will eliminate the usual striations. However, increasing the crest factor may also increase the stress on a lamp, which will lead to a shorter lamp life. 
     Therefore, it would be beneficial to provide a retrofit or upgrade of existing units which does not require the replacement of typical high-frequency ballasts now in place. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a lighting system powered by a system power source. The lighting system includes a ballast in operative connection with the system power source where the ballast is designed to generate a lamp input signal. A lamp input line is operatively connected to receive the lamp input signal. Further, a gas discharge lamp is in operative connection to the lamp input line configured to receive the lamp input signal. An amplitude modulation circuit is then placed in operative connection to the lamp input line, where the amplitude modulation circuit is configured to periodically modulate amplitudes of the lamp input signal prior to the lamp input signal being received by the gas discharge lamp. Operation of the amplitude modulation circuit results in a periodic amplitude modulation of the lamp input signal and eliminating visual striations otherwise occurring in the lamp. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a typical fluorescent lamp having striation zones creating a strobing effect to an end user; 
     FIG. 2 illustrates a standing pressure wave in a closed organ pipe; 
     FIG. 3 depicts a high-level view of a system implementing the concepts of the present invention; 
     FIG. 4 illustrates a standard arc current forcing function or lamp input current; 
     FIG. 5 depicts a lamp input current obtained by use of the concepts of the present invention; 
     FIG. 6 sets forth a more detailed view of the amplitude modulation circuit of the present invention; 
     FIG. 7 depicts a further embodiment of an amplitude modulation circuit; 
     FIG. 8 shows an amplitude modulation circuit integrated into a lamp; 
     FIG. 9 sets forth an amplitude modulation circuit as a module connected to a lamp; 
     FIG. 10 depicts an amplitude modulation circuit inserted within a ballast; and 
     FIG. 11 illustrates a system for operating a plurality of lamps with a single amplitude modulation circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As depicted in FIG. 1, the striation zones  12  generate an undesirable visual effect to an end user. In addressing this problem, the inventors applied a null hypothesis to describe the striation phenomenon, and propose the physics behind striations can be modeled as a standing pressure wave  14  in an enclosed organ pipe  16 , such as shown in FIG.  2 . The frequency of resonance for a closed pipe is given by:          f   n     =       n     4                 l                               C   p       C   v                       P   0         ρ   0                                  
     where  1  is length unit, n is harmonic, c p  is molar capacity as constant volume, c v  is molar capacity at constant pressure, P 0  is undisturbed gas pressure and ρ 0  is density of gas outside compression zone. 
     Using this hypothesis, the inventors developed a circuit which periodically modulates the input current to the lamp. By altering the modulation of the current in this periodic manner, the repetitive resonance frequencies which are believed to create the striations are interfered with thereby eliminating the visual appearance of striations. 
     FIG. 3 sets forth an exemplary lamp lighting system  20  which incorporates the concepts of the present invention. An input power source  22  supplies power through an input filter  24  to a ballast  26 . A lamp input line  30  supplies an input current signal from ballast  26  to lamp  32 . Also connected to input line  30  at junction  34 , is an amplitude modulation circuit  36  according to the present invention. Amplitude modulation circuit  36  alters the input current carried on input line  30  at periodic intervals by interjecting a periodic amplitude modulation signal. Operation of amplitude modulation circuit  36  results in an altering of at least portions of the input signal to modulate the input current. 
     To illustrate the results achieved by circuit  20 , attention is directed to FIGS. 4 and 5 which show lamp input current signal for an Argon/Krypton fluorescent lamp. As may be seen in FIG. 4, shown is a lamp input current signal  38  in a conventional lighting system, not implementing the amplitude modulation circuit of the present invention. 
     As illustrated by line  40 , the peaks of the input signal  38  are all substantially equal. Implementation of amplitude modulation circuit, and as shown in FIG. 5, permits the selective and periodic altering of the lamp input current signal  42 , whereby the value of the input signal or portions of the input signal are modulated in a controlled manner. For example, as shown in FIG. 5, whereas peak  44  and peak  46  are substantially at equal values, the value of peak  48  has been modulated to a lower value. More specifically, in this embodiment, the values of  44  and  46  are approximately 214 mA, whereas the modulated value for peak  48  is approximately 200 mA. Therefore, there is a differential of substantially 14 mA. This differential is sufficient to remove the visual striations from an operating lamp, caused by the repeating resonance signals. 
     It is also to be noted that modulation is made to the value of the input lamp current, and not to its frequency. Particularly, the time periods T 1 , T 2  and T 3  in FIG. 5 are not altered from FIG. 4 or from each other. 
     Turning to FIG. 6, shown is an embodiment of the amplitude modulation circuit  60  according to the present invention which may be implemented as a separate module attached to the lamp, or a circuit which may also be integrated into the lamp. Circuit  60  of FIG. 6 is placed in series with the lamp, by its connection to lamp input line  30 , via a current transformer  62  and a capacitor  64 . Current transformer  62 , which in this embodiment is an inductor, but may be implemented in other known designs, is used to acquire energy from the input line  30  by acquiring at least a portion of the lamp input current carried on lamp input line  30 . Although not shown in this figure but disclosed in previous figures, input line  30  receives a lamp input signal from ballast  26  (FIG.  3 ). The portion of current acquired by current transformer  62  is rectified by full bridge rectifier  68  including diodes  70 - 76 . Zener diode  78  permits for the build-up of a voltage  80  (+VDD) which in one embodiment may be approximately 5 volts, sufficient to power logic electronics used in circuit  60 . The design of circuit  60  meets the desired low power consumption requirements, and therefore the energy obtained via current transformer  62  is sufficient. 
     Signal line  82 , which includes resistive element  84 , carries a half-wave rectified signal  88 , which is converted into a voltage and appears at the input of Schmidt trigger  90 . The Schmidt trigger  90  generates a substantially digital output  92 , which is then supplied to flip-flop  94 . The flip-flop  94  is essentially a divide-by-two device whereby the output signal  96  becomes half the frequency of the input lamp current signal. Also shown in the circuit of FIG. 6 is common capacitor  97 . 
     Output signal  96  is used to control the operation of transistor  98 . Particularly, transistor  98 , which acts as a switch, and full-bridge inverter  100 , consisting of diodes  102 - 108 , permit a selective bypassing of the capacitor in input line  30 . Operation of transistor  98  acts as a switch which shorts this portion of the circuit every full cycle of the current input. Therefore, in operation either capacitor  64  will be in series with the lamp, or the switch, defined by transistor  98  and full-bridge rectifier  100  will be in series with the lamp. 
     By passing capacitor  64 , causes the current input to the lamp to increase, whereas opening of switch  98  causes current to flow through capacitor  66  resulting in the input current being lowered. 
     It is to be appreciated the current level variation in this embodiment is very small. Particularly, this results in a decrease of approximately 14 mA out of a total of approximately 214 mA. By altering the amplitude, the present amplitude modulation circuit design disturbs the resonance occurring within the lamp. 
     The present design as shown for example in FIG.  3  and FIG. 6 does not increase the crest factor of the lamp system, and therefore does not increase the stress on the lamp. This system also does not introduce a dc bias which at certain levels is known to cause mercury within the lamp to migrate toward one end. This results in the lamp having bright spots on one end and dull spots on an opposite end. Also, the addition of the amplitude modulation circuit described in FIG. 6 will only decrease the efficiency of the lamp by approximately ½% or less. 
     In an alternative embodiment, the amplitude modulation circuit may be integrated into the ballast. In this design, it is not necessary to include the amplitude modulation power source defined by the diode bridge  68  and Zener diode  78  of FIG.  6 . Particularly, power from the ballast circuit itself is used to power electronics  90  and  94  of FIG.  6 . Therefore, when a circuit such as circuit  60  of FIG. 6 is integrated with a ballast, the current transformer  64  and signal line  82  may continue to provide the input to Schmidt trigger  90 . Using this powering sequence, results in an efficient circuit whereby the decrease in efficiency of the overall lighting system is significantly less than ½%. 
     Further, while the present embodiment is shown implementing the switching techniques through the use of Schmidt trigger  90 , along with voltage divider  94 , other design alternatives are possible. For example, a digital timer may be used to control operation of the switch  98 . Further, the switching network including switch  98  along with the full-bridge rectifier  100  may also be implemented in a variety of designs in order to obtain amplitude modulation of the input current. For example, in another embodiment, it may be appropriate to inject a signal within the system, thereby increasing the input line current rather than using capacitor  66  to decrease the input line current. It is to be understood that these designs are also considered by the inventors as being within the scope of the present invention. Further, all embodiments of the present invention may be implemented using other known electronic control devices which are capable of adjusting the amplitude of the input lamp current. 
     In this regard, and with attention to still another embodiment as shown in FIG. 7, when integrated into the ballast, amplitude modulation circuit  110  may be used. Particularly, as shown in FIG. 7, since this circuit is internal to the ballast, there is no need to generate separate power for the electronics. Rather, power  112  is supplied directly from the ballast. In this embodiment, in place of using switch  98  with full-bridge rectifier  100 , a pair of switching transistors, such as MOSFETS or other appropriate transistor,  114  and  116  are used. 
     In this design a signal is periodically applied between the connected gates and sources, with the drains placed in parallel with capacitor  118  across input line  30 . When both transistors  114  and  116  are in an “on” state, they act as resistors with very small resistances, dependent upon their RDS values. In this state, the input lamp current bypasses capacitor  118 . When the transistors are “off”, they act as a blocking mechanism forcing the lamp input current to pass through capacitor  118 . Since transistors  114  and  116  are tied together, when the voltage across the gates are at zero, and they are n-channel devices, intrinsic diodes act to block any current flow, resulting in the arrangement to be equivalent to an open switch. 
     The gates may be turned “on”, for example, by applying 5 volts between the gates and source. At this point, again, the transistors act as resistors having small values, thereby shorting out the capacitor  118 . By making the resistances of the n-channel devices low enough, the voltage drop across the channels of transistors  114  and  116  will not be high enough to turn on the intrinsic diodes resulting in transistors  114  and  116  acting simply as resistive elements. Therefore, if for example, there was 200 mA flowing in the circuit, and 2 ohm transistors are used, then there would be only 0.4 volts drop across each transistor. This results in a very low voltage system. If the current or resistance of the transistors is higher such that the intrinsic diodes are turned on, then the voltage of the system would include the diode voltage drops plus the RDS of transistors  114  and  116 . 
     An aspect of the present invention is to solve the striation problem without unnecessarily affecting efficiency of the circuit. The foregoing circuits achieve this goal. 
     Turning to FIG. 8, illustrated is a lamp  130  having an amplitude modulation circuit module  132  incorporating the design of the forgoing embodiments, integrated as part to lamp  130  via signal connection point  133 . In this design, an end user would buy the lamp without the requirement of any retrofitting of the ballast. FIG. 9 illustrates a lamp  130  where an amplitude modulation circuit module  134  is plugged into lamp  130  at connection prongs  136 . FIG. 10 depicts a design where the amplitude modulation circuit  138  is integrated within a ballast  140 . By this design, and as previously mentioned, the requirement of a power source within the amplitude modulation circuit  138 . 
     FIG. 11, illustrates a system having an amplitude modulation circuit  140  integrated at a commonly shared inverter or ballast  142  used to power a multiple number of lamps  144 ,  146 ,  148 . By this design, a single amplitude modulation circuit  140  may be used to remove visual striations from multiple lamps. 
     As previously noted, while the present invention may be implemented in numerous forms. In the forgoing embodiments, component designations and/or values for the circuits of FIGS. 6 and 7 would include: 
     Transformer Inductor  62  (2 coupled 1.0 inductors) . . . 100 uH; 1 mH 
     Capacitor  66  . . . 22 uF 
     Diodes  72 - 76  each . . . D1N4148 
     Zener Diode  78  . . . 5 volts, D1N4740 
     Resistor  84  . . . 100 K 
     Schmidt Trigger  90  . . . National Semi CD40106 
     Capacitor  91  . . . 100 mF 
     Flip-Flop  94  . . . National Semi CD4013 
     Transistor  98  . . . IRF510 
     Diode Bridge  102 - 108  each . . . D1N4148 
     Capacitor  118  . . . 22 mF 
     Transistors  114 ,  116  . . . Fairchild 6303N 
     It is to be appreciated additional balancing components may also be added to the circuits of FIGS. 6 and 7. Additionally, while a variety of lamps may be used, for the values presented, the present lamps would operate on a power supply of line 120/277 Vac at 60 Hertz cycle where the lamps may be a gas discharge lamp such as rare gas filled T 8  linear fluorescent. 
     Although the present invention is described primarily in connection with fluorescent lamps, the circuit herein described may be used to control any type of gas discharge lamp. Since certain changes may be made in the above-described circuit without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted in an illustrative and not a limiting sense.