Abstract:
A means of effecting colour change or improved colour rendition by altering the ratios of average lamp arc current to RMS lamp arc current in a high intensity discharge lamp typically a metal halide lamp operating at less than nominal power rating.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to an apparatus and method for improving the colour spectrum of the light emitted from a high intensity discharge (HID) lamp when operating under less than nominal power conditions (i.e. when dimmed). The invention is particularly, though not exclusively, suited to the ballasting of metal halide discharge lamps. Typically such systems can be used for highway lighting, architectural floodlighting, warehouse, retail display and industrial lighting.  
         BACKGROUND ART  
         [0002]    Traditionally, ballasting for HID lamps is by use of inductors or chokes capable of controlling the lamp current through the impedance they present in series with the mains supply voltage. With some types of HID lamp a high striking voltage, typically 4-5 kV, is required to ionize the gas filling the tube and to initiate the arc.  
           [0003]    In prior art systems for ballasting HID lamps, the lamp ballasting means and the lamp striking means are typically discrete circuit elements. Historically, HID lamps have been ballasted by using the impedance of a series connected inductor for controlling the lamp current and a separate starter or igniter module to provide the necessary high voltage to strike the lamp.  
           [0004]    More recently electronic ballasting means have been devised. Referring to FIGS. 1 and 2, a conventional power factor controller  2  is formed by transistor TR 1 , inductor L 1 , diode D 1  and capacitor C 1 . Alternating positive and negative output voltage is provided to the lamp by a full bridge arrangement of a lamp bridge  4  comprising four transistors TR 3 , TR 4 , TR 5 , TR 6 . The transistors are alternately switched on and off in complementary pairs TR 3 , TR 6  and TR 4 , TR 5  at a low frequency, typically 100-200 Hz.  
           [0005]    Connected in series with the lamp across the bridge is an igniter circuit  5  comprising pulse transformer TX 1 , a Sidac, capacitor C 3  and resistor R 1 . When the igniter circuit operates, the capacitor C 3  charges through resistor R 1  to a voltage at which the Sidac device switches on, discharging the capacitor C 3  into the primary winding of the transformer TX 1 . The voltage applied to the transformer primary is multiplied by the high turns ratio of the transformer and is sufficient to ionize the gas filling the lamp&#39;s arc tube, thereby initiating an arc.  
           [0006]    Since the voltage is AC, the arc will be extinguished when the lamp current approaches zero and the voltage applied to the tube is subsequently reversed. Therefore the igniter must operate again in the opposite voltage half cycle to re-strike the arc for the flow of current in the opposite direction. This ignition cycle is repeated until the lamp electrodes are sufficiently heated by the arc current for thermionic emission to take place. Then the arc voltage in the tube falls below the threshold voltage of the Sidac and arc current is maintained without operation of the igniter circuit.  
           [0007]    A further transistor TR 2  of a current source  3  controls the flow of current in the output bridge circuit and consequently controls the lamp current. Transistor TR 2  is turned on until the current in inductor L 2  reaches a preset threshold value, then the transistor TR 2  is turned off. Current continues to flow via a diode D 2  until the current has decayed to another preset threshold value, then the transistor TR 2  is turned on again. Typically this controlled current is further controlled by a lamp power control circuit  6  which takes account of lamp arc voltage and sets the lamp current accordingly so as to maintain the lamp power (watts) at a near constant value over a range of lamp arc voltages.  
           [0008]    Thus the lamp current is substantially square wave with a small amount of high frequency ripple current superimposed, caused by the switching of TR 2  and the consequent high frequency current modulation caused thereby. FIG. 3 illustrates schematically a typical lamp current waveform in accordance with the above prior art, with the lamp operating at nominal power.  
           [0009]    Recently electronic ballasts for metal halide lamps were introduced with the capability for dimming the lamp to approximately half power. This was done by reducing the square wave lamp current amplitude (under the control of a lamp power control circuit  6 ) so as to reduce the lamp power to a desired value. Such means whilst effective in controlling the lamp power and consequent light output to a desired reduced value are deficient in maintaining desirable colour rendition from the lamp. This is due to the lamp operating at reduced temperature and some of the various elements used in the filling (Dose) of the lamp arc tube condensing out in the arc tube. Typically the operation of a metal halide discharge lamp at reduced power results in a relative reduction in light output near the red end of the colour spectrum and a relative increase in the light output at the blue/green end of the colour spectrum. Such colour shift may be acceptable in some lighting applications but is not acceptable in particular in retail display lighting applications and other applications where the perceived colour in the lit area is important to sales or safety. FIG. 4 illustrates a typical lamp current waveform in accordance with the prior art with the lamp operating at a dimmed or reduced power.  
         SUMMARY  
         [0010]    The invention provides a circuit for improving the light spectrum emitted from a dimmed high intensity discharge lamp by superimposing a high frequency waveform onto a substantially square wave lamp current derived from a high intensity discharge lamp ballast, characterized in that the frequency and mark to space ratio of the high frequency waveform are such that for each half cycle of a resultant waveform, the magnitude of a RMS lamp current is greater than the magnitude of an average lamp current.  
           [0011]    The invention further provides a method for improving the light spectrum emitted from a dimmed high intensity discharge lamp wherein a high frequency waveform is superimposed on a substantially square wave lamp current derived from a high intensity discharge lamp ballast, characterized in that the frequency and mark to space ratio of the high frequency waveform are such that for each half cycle of a resultant waveform, the magnitude of a RMS lamp current is greater than the magnitude of an average lamp current.  
           [0012]    Within both aspects of the invention, the modulation of lamp arc current increases the RMS value of the lamp arc current whilst maintaining an average lamp arc current commensurate with dimmed or reduced power operation. Substantially the average value of light output from a typical metal halide lamp or other high intensity discharge lamp is controlled by the average current whilst, substantially the operating temperature of a typical metal halide lamp or other high intensity discharge lamp arc tube and contents thereof is controlled by the RMS arc current. Thus by increasing the RMS content of the lamp arc current whilst maintaining the average current at a value commensurate with dimmed operation an increase in arc tube and contents thereof operating temperature may be affected. This increased operating temperature of the arc tube and contents keeps the dose elements in the gaseous state and improves the colour rendering of the lamp nearer to that which is expected of a lamp operating at nominal power.  
         BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    By way of example only, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings in which:  
           [0014]    [0014]FIG. 1 illustrates a lamp ballast circuit according to the prior art;  
           [0015]    [0015]FIG. 2 illustrates a lamp ballast block diagram according to the prior art;  
           [0016]    [0016]FIG. 3 illustrates schematically a typical lamp current waveform in accordance with the prior art with the lamp operating at nominal power (the rise and fall times of the waveform have been exaggerated for clarity);  
           [0017]    [0017]FIG. 4 illustrates a typical lamp current waveform in accordance with the prior art with the lamp operating at a dimmed or reduced power (the rise and fall times of the waveform have been exaggerated for clarity);  
           [0018]    [0018]FIG. 5 illustrates a lamp ballast block diagram according to a preferred embodiment of the invention; and  
           [0019]    [0019]FIG. 6 illustrates a typical lamp current waveform produced by the lamp ballast of FIG. 5 with the lamp operating at a dimmed or reduced power (the rise and fall times of the waveform have been exaggerated for clarity).  
           [0020]    [0020]FIG. 7 illustrates a lamp current waveform produced by an electronic ballast operating at a high frequency with the lamp operating at a dimmed or reduced power. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0021]    Referring to FIGS. 5 and 6 which represent a lamp ballast operating according to the invention, the square wave fundamental operating frequency lamp current waveform is modulated in this case by a higher frequency square wave. By altering the amplitude and mark to space ratio of this modulating waveform it is possible to alter the RMS current content of the overall lamp arc current waveform relative to the average current value of the lamp arc current waveform. In this way the operating temperature of the lamp&#39;s arc tube and its contents may be controlled in accordance with the invention. The formula for evaluating the values of the average current and the RMS current in the illustrated waveform in FIG. 6 is as follows:  
         I   ave     =               I   1          t   1       +       I   2          t   2             t   1     +     t   2                       and                   I     r                 m                 s         =             I   1   2          t   1       +       I   2   2          t   2             t   1     +     t   2                                   
 
         [0022]    where:  
         [0023]    I 1  is the current value at the maximum current plateau;  
         [0024]    t 1  is the time during which the waveform is at the maximum current plateau;  
         [0025]    I 2  is the current value at the minimum current plateau; and  
         [0026]    t 2  is the time during which the waveform is at the minimum current plateau.  
         [0027]    Care must be taken in the choice of modulating frequency and wave shape so as to avoid the possibility of stimulating acoustic arc resonance effects. Thus, the choice of frequency, mark to space ratio and modulating frequency waveform may be dictated by the lamp type or other factors such as audible noise caused by the modulating frequency. In some cases it may be desirable to use modulation frequency waveforms other than square wave, such as sinusoidal, triangular or other waveforms so as to minimize the problems above whilst maximizing the overall colour change effect in the lamp. Care must also be taken to minimize the damage that may be caused to the lamps electrodes by excessive high peak arc currents.  
         [0028]    The relationships of II, t 1 , I 2  and t 2  varies as a function of specific lamps or by type of lamp. For example, the thermal time constant of the lamp may influence the relationships. Other values of I 1 , I 2 , t 1  and t 2  than shown in FIG. 6 may be used, such as having t 1  longer or the same as t 2  and/or the difference between I 1  and I 2  being greater or lesser. In one embodiment, the lamp current square wave operates within 50-200 Hz, the higher frequency modulation waveform (coulor control waveform) operates within 500 Hz-2 kilz, and the ripple current is about 100 kHz. Other frequencies, frequency ranges and relative frequencies between these three waveforms may be used.  
         [0029]    [0029]FIG. 5 shows the added lamp current modulator  7  for modulating the ratio of the I 1  to I 2  higher frequency current and controlling the t 1  to t 2  ratio. A typical ballasting circuit for HID lamps consists of a power factor controller circuit  2  which controls the power factor of the ballast presented to the mains input voltage. This power factor circuit  2  generates a DC voltage controlled bus to which a switch mode current source circuit  3  is connected. The current source circuit  3  controls the lamp current by means of a reference voltage signal (A) which is connected to one input of a comparator. The other input of the comparator is provided with a voltage proportional to lamp current signal (B). When the signal voltage (A) is less than the signal voltage (B), the comparator causes the current source transistor to turn on. The current source transistor is connected to an inductor as per FIG. 1 (prior art). The current in this inductor and therefore the lamp bridge and lamp increases at a rate dependent on lamp voltage, DC bus voltage and the inductance value of the inductor. When the lamp current and therefore the lamp current proportional voltage signal (B) exceeds the voltage of the reference voltage signal (A), the comparator causes the current source transistor to turn off. The current in the inductor is then commutated to the diode as shown in FIG. 1- The current reduces at a rate dependent on lamp voltage and inductance value until the lamp current proportional voltage signal (B) is below the voltage of the reference voltage signal (A) when the  5  comparator again switches on the current source transistor. The comparator is provided with a fixed hysteresis such that the current is switched on and off between two preset values around the desired current value thus limiting the switching frequency of the current source transistor to a reasonable frequency (approx. 50-200 kHz).  
         [0030]    The lamp current modulator  7  modulates the voltage of the reference voltage signal (A) so that the lamp current is similarly modulated in sympathy with the voltage of the reference voltage signal (A). A programmed microprocessor, application specific integrated circuit, transistors, switches, analog devices, digital devices or similar device now known or later developed is so programmed to drive a digital to analogue converter. The digital to analogue converter adds to or subtracts from the voltage of the reference voltage signal (A) to modulate the lamp current with the higher frequency square wave. In alternative embodiments, analog circuits or other devices for adding or subtracting from the voltage reference signal (A) are used.  
         [0031]    In a further embodiment, HID lamps, such as metal halide lamps, are operated at high frequency (in the range 20 kHz to 2 MHz) from electronic ballasts at a reduced (dimmed) power. The frequency, amplitude and mark to space ratio of the lamp current modulation waveform is controlled such that as discussed above in reference to FIG. 6 and shown in FIG. 7 such that the average lamp current is lower than the RMS and peak lamp currents. The temperature of the arc tube and the contents thereof are maintained thereby, keeping the arc tube contents (dose) in a gaseous state and therefore improving the color rendering of the lamp nearer to that which is expected of a lamp operating at nominal power.  
         [0032]    The high frequency lamp current may be substantially triangular, sinusoidal, square or other in shape. The amplitude modulation of the high frequency lamp current may be effected by modulating the frequency of the waveform or by other means, such as hardware current limiting, altering the lamp series connected impedance or DC bus voltage in response to a signal generated by a software program in a microcontroller or by hardware signal generation.  
         [0033]    While the invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made without departing from the scope of the invention. It is therefore intended that the foregoing detailed description be understood as an illustration of the presently preferred embodiment of the invention, and not as a definition of the invention. It is only the following claims, including all equivalents, that are intended to define the scope of this invention.