Abstract:
A constant lumen output control system for providing a constant lumen output throughout the life of a lamp at the mean or preset lumen level. The lumen con system ( 315 ) coupled to a lamp driver ( 310 ) initially reduces the power to the lamp ( 330 ) to prevent the lamp from being operated at power levels that result excess mean or preset lumen levels. With increased lamp usage, the lumen control system gradually increases power to the lamp to compensate for lamp lumen depreciation due to light-reducing mechanisms. By compensating for lamp lumen depreciation the lamp is operated at a constant mean or preset lumen output throughout the life of the lamp.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to lumen output control of a light source. More particularly, the invention provides a method and system for increasing and decreasing a ballast output power, which is connected a light source, to provide a constant light output during the life of the light source. 
     2. Description of the Related Art 
     Over time, the lumen output of a lamp continually decreases Lumen output can be defined as a unit of luminous flux equal to the light emitted in a unit solid angle by a uniform point source of one candle intensity. As related to power, a lumen is 1/683 watts of radiant power at a frequency of 540×10 12  Hertz. The lumen output degradation in the lamp can occur for a variety of reasons, for example, lamp lumen depreciation, the lamp&#39;s interaction with a ballast, supply voltage variations, dirt or dust on the lamp, and the ambient temperature in a fixture.  FIG. 1  illustrates a lumen degradation curve for a typical quartz metal halide high intensity discharge (HID) lamp that uses a conventional ballast.  FIG. 1  is a chart  100  illustrating two curves in relation to an X-axis  102  (lamp operating hours) and a Y-axis  104  (lumens per lamp watt). The curve  106  illustrates the degradation curve for a magnetic constant wattage autotransformer (CWA) lamp and the curve  108  illustrates a degradation curve for a Prismatron™ lamp. As lamp operating hours increase for the lamp, the lumen output of the lamp decreases. 
     The decrease in lumen output occurs due to a variety of processes that occur within the lamp. One factor contributing to this decrease is a loss of chemicals that contributing to light output. These chemicals can be lost through portions of the lamp structure, for example, an arc container Another factor contributing to light degradation is metal being deposited on an arc tube wall of the lamp. An HID lamp is started by applying a very high voltage across an arc tube to break down high pressure gasses within the lamp into a conduction state. Following this breakdown, high current normally flows across a relatively low-voltage arc that heats the electrodes, which subsequently enter into thermionic emission. This tends to eject molecules of the metal electrode material that eventually condense on the wall of the arc tube, causing “blackening” and lowering the light transmission of the arc tube. 
     Due to such degradation in lumen output, many lighting applications are designed using a mean light level. The mean light level, or lamp&#39;s lumen, is defined when a HID lamp is at forty percent of its rated life. Typically to achieve a minimum light level emission, a lighting system designer will design a lighting system at the mean light level. Once the lamp is at a point past the mean light level, replacement of the lamp is usually necessary to maintain a desired light output level. 
     In HID applications, a ballast is used to control the operating power delivered to a lamp.  FIG. 2  is a block diagram  200  illustrating a typical ballast  202 . The ballast  202  regulates the power to the lamp  204  which is received as an input voltage from a power source (not shown). The ballast  202  also provides proper starting conditions for the lamp  204  at start-up 
     Some ballast designs use magnetic transformers. As a result, the output level of a lamp cannot be varied and is limited to an output of full power or some fixed output level lower than full power. Other ballast designs, such as electronic ballasts, provide for continuous variation of lamp voltage between full power and a predetermined lower limit. 
     However, a problem with conventional ballast systems, using the mean light level to set a desired lamp output, is that the ballast initially consumes additional power for the time period prior to achieving the mean light level. Powering the lamp at full output prior to achieving the mean light level causes an output higher than is necessary which consumes more power than necessary to provide the desired light output. 
     Accordingly, there is a need and desire for a ballast having a power regulation technique for outputting power to a lamp, which will create a constant lumen output from the lamp, thereby decreasing the power consumption of the lamp system. 
     SUMMARY 
     The present invention provides a constant output lumen control system that has the ability to provide a continuous lumen output from a lamp over the lifetime of the lamp. The lighting system initially reduces the power to the lamp, and subsequently varies the power delivered to the lamp to compensate for light-reducing mechanisms that will affect the lumen output of the lamp over time. By properly adjusting the power delivered to the lamp, the lighting system provides a constant light output from the lamp. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other advantages and features of the invention will become more apparent from the detailed description of exemplary embodiments of the invention given below with reference to the accompanying drawings. 
         FIG. 1  is a chart illustrating a lumen degradation curve for a typical standard metal halide HID lamp, 
         FIG. 2  is a block diagram illustrating a typical ballast design, 
         FIG. 3  is a block diagram illustrating a ballast design including lumen control circuitry in accordance with an embodiment of the invention, 
         FIG. 4  is a chart illustrating lamp output degradation as a function of the number of lamps starts, 
         FIG. 5  is a chart illustrating a re-lamp cycle for an HID lamp for lamp replacement detection, 
         FIG. 6  is a flow chart illustrating the process steps of an embodiment of the control circuitry of the invention, 
         FIG. 7  is a block diagram of an illumination system for implementing a first exemplary embodiment of the present invention, 
         FIG. 8  is a chart illustrating power consumption of a conventional ballast and a ballast according to an embodiment of the invention, 
         FIG. 9A  is a chart illustrating a re-ignition peak voltage as the lamp voltage vanes with time, and 
         FIG. 9B  is a chart illustrating the relationship between a voltage crest factor and lamp life. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and which is shown by way of illustration of specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical, and programming changes may be made without departing from scope of the present invention. 
       FIG. 3  is an exemplary illumination control system  300  employed in a ballast  302 . The ballast  302  includes a power factor correction circuit  304 , a power supply  306 , a ballast control circuit  308 , a lamp driver  310 , sense circuits  312 , and an illumination control system  315 . The illumination control system  315  includes a computational control circuit  314  and a non-volatile storage device  316 . Non-volatile storage device  316  may use any comparable non-volatile memory format, for example, dynamic random access memory (DRAM), flash memory, magneto-resistive random access memory (MRAM), etc. Computational control circuit  314  may utilize a microprocessor or any other comparable processing device to conduct mathematical processing for adjusting power supplied to the lamp  330  to achieve a constant lumen output from the lamp  330 . Non-volatile storage device  316  provides storage for various computational equations, mathematical constants, ballast operational software  318 , timers  317 , counters  319  and information regarding various lamp types, and their specific operational requirements which are used by the computational control circuit  314  during processing. The lamp  330  can be any type of high intensity discharge lamp (HID), such as HID lamps that use high pressure mercury, high pressure sodium, or some other suitable gas. 
     The ballast control circuit  308  adjusts the power received from the power supply  306  for use by the lamp  330 . The ballast control circuit  308  receives a lamp power setting signal and a lamp operational control signal from the computational control circuit  314 . The ballast control circuit  308  also receives a lamp feedback signal from the sense circuits  312  and provides operating power to the lamp driver  310 . The lamp driver  310  starts the lamp  330 , receives operating power from ballast control circuit  308 , and provides operating power to the sense circuits  312 . The lamp driver  310  receives a lamp on/off control signal from the computational control circuit  314  for use in discontinuing power being supplied to the lamp  330 . The sense circuits  312  monitor the supply power input to the lamp  330  and provide feedback about the operation of the lamp  330  to the computational control circuit  314  and the ballast control circuit  308 . The sense circuits  312  send a lamp current feedback signal and a lamp voltage feedback signal to the computational control circuit  314 . The sense circuits  312  also send a lamp feedback signal to the ballast control circuit  308  to monitor other important lamp operational parameters. 
     The illumination control system  315  utilizes various factors and parameters to determine a rate of degradation for a particular type of lamp  330 . The parameters and factors are used to control the output of the lamp  330  over its lifecycle. For example, illumination control system  315  may utilize operating hours (total hours the lamp has been operating) and lamp starts (total number of starting sequences for the lamp) to determine a rate of degradation of the lumen output of the lamp  330 . Other parameters may be considered in determining the degradation rate. For example, a stabilized lamp operating voltage, lamp re-ignition voltage, voltage crest factors, current crest factors, or combination thereof may be used. Based upon the rate of degradation of the lamp  330 , the illumination control system  315  adjusts the power supplied to the lamp  330  to provide a constant lumen output from the lamp  330 . 
     The ballast operational software  318  resides in non-volatile storage  316  and provides a variety of timers  317 . For example, the timers  317  include an accumulated lamp timer for measuring the number of operating hours for the lamp  330 , and a lamp warm-up timer for determining when the lamp  330  has achieved a stable state after starting for use by the computational control circuit  314 . The ballast operational software  318  also provides counters  319  for measuring the number of lamp starts for the lamp  330 . The ballast operational software  318  also controls the operation of the ballast  302  and the power output by the ballast  302 . 
       FIG. 4  illustrates a diagram  400 , which compares the number of lamp starts to a percentage of lamp output power for the lamp  330 . The X-axis  402  represents a number of lamp starts for the lamp  330  and the Y-axis  404  represents a percentage of output of the lamp  330 . The output of the lamp  330 , which is illustrated using curve  406 , degrades due to lamp lumen depreciation as the number of starts for the lamp  330  increases. 
     In calculating degradation due to the number of hours that the lamp  330  is in operation, the computational control circuit  314  uses what is referred to as a burnloss equation to determine lamp degradation due to operating hours for use in calculating a dim level setting for the lamp  330 . The following second order polynomial equation determines the value for burnloss
 
Burnloss= A ×Hours 2   +B ×Hours+ C   Eq 1
 
     The burnloss equation is stored in the non-volatile storage device  316  along with constants A, B and C which are associated with the particular type of lamp  330  being powered by the ballast  302 . The constants A, B &amp; C are derived from a least squares curve fitting using experimental data, based on light loss due to the number of operating hours of the lamp  330 . The process of deriving the constants A, B and C could also be done using a look-up table relating the variables, but such an approach would require additional storage space in non-volatile storage device  316 . 
     In calculating degradation due to the number of lamp starts, the computational control circuit  314  uses what is referred to as a startloss equation to determine lamp degradation due to the number of lamp starts for use in calculating a dim level setting for the particular type of lamp  330 . The following second order polynomial equation determines the value for startloss.
 
Startloss= D ×Hours 2   +E× Hours+ F   Eq 2
 
     The startloss equation is stored in non-volatile storage device  316  along with constants D, E and F which are associated with a particular type of lamp  330  being powered by the ballast  302 . The constants D, E and F are derived and stored in non-volatile storage device  316  in a similar manner as constants A, B and C. 
     The burnloss and startloss values for the lamp  330  are combined to calculate an overall expected level of light loss at a given point in the lifecycle of the lamp  330 . A ratio is then calculated using the expected level of light loss at a given point in the lifecycle of the lamp  330  and a predetermined lumen output target is stored in non-volatile storage  316 . For example, an expected lamp output for a given point (2000 hours) may be 95% of the initial lamp output, while the predetermined lumen output target is 85%. Thus, the output wattage to the lamp  330  is decreased by an appropriate amount to reduce the light output of the lamp  330  to the predetermined lumen output target. Although the target lumen output of the lamp  330  may be set to any reasonable lumen output, two meaningful output settings which may be used are an end of life lumen output and a mean lumen output. The mean lumen output is typically the average light output after 40% of the expected life of the lamp  330  has elapsed and is usually set by the manufacturer of the lamp  330 . 
     By using the ratio of expected lumen output to current lumen output, the power supplied to the lamp  330  may be adjusted by the illumination control system  315  to set an appropriate source wattage for the lamp  330 . For example, if the lamp  330  is a quartz metal halide HID lamp, a lumen output for the illumination control system  315  would be varied 1 8 times a change in wattage due to the relationship between the lamp wattage and the delivered light output for the particular type of lamp  330 . Therefore, the wattage from the ballast  302  to the lamp  330  is changed by a ratio of 1/18 to obtain a desired constant lumen output. Thus, as the number of operating hours and lamp starts accumulate, the illumination control system  315  continually evaluates the degradation of the lamp  330  to compensate for lamp lumen degradation by increasing the wattage output supplied from the ballast  302  to the lamp  330 . When the lamp  330  degrades to a point at which the lamp  330  requires more power than its maximum power rating (100%) to maintain the desired lumen output level, the illumination control circuit  315  will limit the power output by the ballast  302  to the maximum power rating of the lamp  330 . By limiting the lamp  330  to its maximum power rating, safety is improved because the lamp  330  is not overdriven which could damage the circuitry within the ballast  302  and the lamp  330 . Once the lifecycle of the lamp  330  is completed, the lamp  330  is subsequently replaced. 
     After the lamp  330  is replaced, values such as the number of operating hours and the number of lamp starts stored in the non-volatile storage device  316  are reset. Although it is possible to reset the non-volatile storage device  316  manually, a reset means using a form of lamp replacement detection may be employed. The lamp replacement detection technique may be employed using software included in ballast operational software  318  which is stored in the non-volatile storage device  316  for use by the computational control circuit  314 . By comparing the measured lamp voltage of the lamp  330  to the lamp voltage stored in memory, the computational control circuit  314  determines if a change in lamp voltage has occurred which would indicate that the lamp  330  has been replaced. 
     Thus, a lamp replacement detection technique may utilize the fact that as a lamp ages, many electrical variables associated with the lamp change. For example, a root mean squared (RMS) voltage across the lamp  330  and a re-ignition voltage for the lamp  330  change over time. The lamp replacement detection technique uses the software included in ballast operational software  318  to store these voltages and other variables in the non-volatile storage device  316 . Each time the lamp  330  is started, a stabilized lamp voltage is compared to a stored stabilized lamp voltage setting. If a step in voltage is greater than a predetermined threshold level stored in the non-volatile storage device  316 , then it is determined that the lamp  330  has been replaced. For example, if a decrease of 5 volts in lamp voltage is determined by the computational control circuit  314  after the lamp voltage has stabilized, the lamp  330  is determined to have been replaced. After such a determination, the number of operating hours and the number of lamps starts are reset in the non-volatile storage device  316 . 
       FIG. 5  illustrates the above described replacement technique using the comparison of lamp start voltages. The chart  500  graphs a percent relamp cycle  502  versus a lamp start voltage  504  using curve  506 . During each start, the voltage of the lamp  330  is obtained and compared to a lamp voltage stored in the non-volatile storage device  316  from the previous lamp start. If the lamp voltage step between starts is greater than the predetermined threshold, for example, a step from 160 volts ( 508 ) to 100 volts ( 510 ), the illumination control system  315  determines that the lamp  330  has been replaced since the stabilized lamp voltage is reduced by 60 volts from a previous lamp operation. Subsequently, the number of operating hours and the number of lamp starts stored in the non-volatile storage device  316  are reset. Those skilled in the art will recognize there are many other comparable means to perform the lamp replacement detection described above. 
       FIG. 6  is flow diagram  600  of process steps implemented by the illumination control system  315 . The blocks in the flow diagram  600  may be performed in the order shown, out of the order shown, or may be performed in parallel. At step  602 , power is applied to the ballast  302  turning on the lamp  330 . Next, at step  604 , the lamp  330  is adjusted to full power At step  606 , ballast  302  obtains a variety of constant lumen output control (CLO) values, for example, total lamp starts, historic lamp voltage and lamp life constants based on the particular type of lamp  330  used from the non-volatile storage device  316 . At step  608 , the ballast  302  starts a lamp warm-up timer having a predetermined warm-up time setting, for example, 20 minutes. At step  610 , the accumulated lamp timer is started. The lamp warm-up timer and accumulated lamp timer are created using the timers  317  which are stored in the non-volatile storage device  316  for use by the computational control circuit  314 . Next, at step  612 , the ballast  302  increments the counter  319  ( FIG. 3 ) measuring the number of lamp starts and stores the new lamp start value in the non-volatile storage device  316 . At step  614 , the ballast  302  determines whether the predetermined warm-up time period has elapsed to assure the lamp wattage and voltage has stabilized. If the warm-up time period has not elapsed, the process returns to step  614  At step  616 , if the warm-up time period has elapsed, the ballast  302  determines whether the lamp  330  has been replaced using the technique described in  FIG. 5 . 
     If the lamp  330  has been replaced, then, at step  618 , the ballast  302  resets the number of operating hours and the number of lamp starts to their predetermined reset values. For example, operating hours are assigned a value of 10 and the number of starts is assigned a value of 1. If the lamp  330  has not been replaced, the process proceeds to step  620  where the ballast  302  writes the current value for the number of operating hours, the number of lamp starts and a lamp start voltage being used by the lamp  330  into the non-volatile storage device  316 . 
     At step  622 , the ballast  302  determines the projected lamp lumen output for the lamp  330  based on the degradation curve stored in the non-volatile storage device  316  for the particular lamp type. Subsequently, at step  624 , the degradation of the lamp due to the number of starts is derived from the stored compensation curve for the particular type of lamp  330  being utilized At step  626 , the target output lumens of the lamp  330  is ratioed to the calculated current lumens to adjust the power supplied to the lamp  330  to maintain a constant lumen output from the lamp  330  At step  628 , the ballast  302  determines the actual power setting, in watts, to which the lamp  330  should be adjusted to provide the target lumens by converting output lumens to watts. The conversion is calculated from a light output versus power curve for the lamp type  330  being utilized. At step  630 , the ballast  302  adjusts the output wattage to the lamp  330  by setting an internal reduced power level setting. 
     Thus, by using the ballast  302  which can adjust power input to the lamp  330 , an illumination system may be implemented which is efficient and cost-effective. 
     As mentioned above, the ballast  302  may also utilize the stabilized lamp operating voltage to maintain a constant lumen output for the lamp  330 . Instead of combining the results of the burnloss and startloss equations, the computational control circuit  314  calculates a value for what is referred to as Slov, and combines the Slov and startloss equations to maintain a constant lumen output for the lamp  330 . Slov represents the stabilized lamp operating voltage and could be determined by using the following second order polynomial equation
 
Slov= G ×Hours 2   +H ×Hours+ I  
 
     The value for Slov is stored in non-volatile storage device  316  along with constants G, H and I which are associated with a particular type of lamp  330  being powered by the ballast  302 . The constants G, H and I are derived and stored in non-volatile storage device  316  in a similar manner as constants A, B and C. 
       FIG. 7  illustrates an illumination system  700  using multiple ballasts  302 . Illumination system  700  includes multiple ballasts  302  each connected to power supply  702  for controlling the lumen output of a lamp  330  connected to each ballast  302 . Thus, illumination system  700  utilizes multiple ballasts  302  and lamps  330  to illuminate larger areas which could be used in a variety of lighting applications. 
       FIG. 8  is a diagram  800  illustrating power consumption of a lamp  330  using a conventional ballast and the ballast  302 . In  FIG. 8 , a time component (X-axis  802 ) and a percent lamp power component (Y-axis  804 ) are used to compare a constant light output  806  produced by the lamp  330  using supply power from the ballast  302  versus light output  808  from the lamp  330  using supply power from a conventional ballast. Because a conventional ballast cannot adjust power input to the lamp  330 , the conventional ballast provides full power to the lamp  330  when full power is not needed. The area indicated at  810  between curves  806  and  808  illustrates power wasted when a lamp  330  is conventionally controlled. Thus, power consumed by a lamp  330  that is controlled by a conventional ballast exceeds the power consumed by a lamp  330  that is controlled by the ballast  302 . By adjusting the power output from the ballast  302 , the lamp  330  is provided with only enough power to maintain an established lumen output level. Thus, power costs are reduced since the ballast  302  does not overdrive the lamp  330  by supplying more power than is required. 
     As mentioned with reference to  FIG. 3 , another alternative to burning hours and lamp starts utilizes the re-ignition voltage, or more specifically the voltage crest factor (VCF). The re-ignition of the lamp discharge occurs each time the lamp current changes polarity. As a result, the arc and electron flow must be re-established, which takes a finite amount of time. This time creates a resultant arc impedance change, which results in an instantaneous rise in lamp voltage that is limited by the instantaneous open circuit voltage of the ballast. The time and voltage necessary to re-establish the arc is dependent on the ability of the electrode to supply electrons and continue the recombination process. As the HID lamp  330  ages, the ability of the electrode and fill gas to provide and transport electrons decreases. The resultant magnitude of the voltage peak, measured at zero current crossing, is called the re-ignition voltage, which subsequently increases. Turning now to  FIG. 9A , the peak re-ignition voltage for a new HID lamp is shown at reference numeral  910 . After some time, the peak re-ignition voltage for this aged HID lamp is shown at reference numeral  920 . Hence the peak re-ignition voltage is a factor that vanes with lamp age. 
     The VCF is defined using the peak re-ignition and rms lamp operating voltage that can be used for monitoring lamp life. More specifically, the VCF is the ratio of the peak re-ignition voltage to the rms voltage of the lamp operating voltage. Because the VCF changes as the peak re-ignition voltage changes with lamp age, the VCF vanes with lamp age. The graph  930  in  FIG. 9B  illustrates the variation of the VCF with lamp life. Thus, monitoring of the VCF can be used as a parameter to estimate the burning hours of the lamp  330  and provide data to the computational control  314  to adjust the power to the lamp  330  for maintaining constant lumen output. 
     While the invention has been described in detail in connection with an exemplary embodiment, it should be understood that the invention is not limited to the above-disclosed embodiment. Rather, the invention can be modified to incorporate any number of variations, alternations, substitutions, or equivalent arrangements not heretofore described, but which are commensurate with the spent and scope of the invention. In particular, the specific embodiments of the constant lumen output control system described should be taken as exemplary and not limiting. For example, the ballast  302  may also determine lumen degradation of lamp  330  by measuring the change in the RMS voltage, voltage and current crest factors, re-ignition voltage or combination of these parameters of lamp  330  or by monitoring the lumens emanating from the lamp  330 , by lumens received at a task being illuminated by the lamp  330 . Accordingly, the invention is not limited by the foregoing description or drawings, but is only limited by the scope of the appended claims.