Abstract:
A lighting system is disclosed, including a first lighting module and a second lighting module connected parallel to the first lighting module. The first lighting module, with a first activation voltage, generates light at a first color temperature and the second lighting module, with a second activation voltage, generates light at a second color temperature. The two lighting modules generate light when current flows through them. When input voltage is changed, both the amount of current flowing through the two modules changes and the ratio of current flowing through the two lighting modules changes. The change in ratio changes the color temperature of the light produced by the lighting system resulting from combination of the light produced by the two modules. The combined output brightness and color temperature each change with applied power in such a way to emulate the lighting profile of an incandescent lamp.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation-in-part of a non-provisional patent application filed May 28, 2009 having application Ser. No. 12/455,127, which has since issued as U.S. Pat. No. 8,354,800 dated Jan. 15, 2013. The entirety of both application Ser. No. 12/455,127 and U.S. Pat. No. 8,354,800 are each incorporated herein by reference. Additionally, this patent application claims the benefit of priority under 35 USC sections 119 and 120 of a provisional patent application filed Oct. 19, 2009 having Application Ser. No. 61/279,317, the entirety which is incorporated herein by reference. 
    
    
     BACKGROUND 
     This invention relates to LED lighting systems. More particularly, the present invention relates to LED light output color temperature control and dimming. 
     Incandescent Light, Luminance, and Dimming 
     For over a century, incandescent lamps reigned supreme as the most used devices to provide light to humanity. When electrical power (measured in Watts, W) is applied to an incandescent lamp, the incandescent lamp produces light. 
     The total amount of light (luminance) generated by an incandescent lamp depends upon the amount of electrical power applied to the lamp. That is, increases in the input electrical power to an incandescent lamp causes the lamp to produce greater luminance (brighter light) until a threshold is reached where the incandescent lamp fails due to the high input electrical power, duration of time such power is applied to it, or both. 
     Likewise, decreases in the input electrical power to an incandescent lamp causes the lamp to produce lesser luminance (dimmed light) until the input electrical power is decreased to a threshold value below which no light is produced by the lamp. Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. The international system of units (SI) unit for luminance is candela per square meter (cd/m 2 ). In the drawings and in this document, luminance is represented by capital letter L. 
       FIG. 1  is a diagram illustrating, inter alia, relationship between the input power levels (the X-axis) and corresponding output luminance (the first Y-axis) of an incandescent lamp. As illustrated by luminance curve  10  (sold line curve), at a first threshold input power, W TH1 , the incandescent lamp begins to produce light at some minimal luminance, L MIN1 . The luminance  10  increases as the input power increases until at a second threshold input power, W TH2 , the lamp produces light at its maximum luminance, L MAX1 . Further increases in the input power beyond the second threshold level, W TH2 , would cause the incandescent lamp to fail prematurely and this is not illustrated in the Figures. The luminance curve  10  is a generalized and simplified representation of the relationship between the input power and the output luminance of an incandescent lamp; the curve  10  is used for illustrative purposes only and as an aid to understanding the relationship. For example, the luminance curve  10 , as illustrated, may appear to indicate a mostly linear relationship between the input power and the output brightness. However, typically the relationship is closer to logarithmic. Here, the Output Brightness scale (the first Y-axis) may be in logarithmic scale. In any scale, the discussed relationship of increasing power leading to increased luminance output is valid. 
     Accordingly, the amount of light produced by an incandescent lamp can be controlled by a dimming switch. The dimming switch controls the input power to the incandescent lamp, which, in turn, controls the luminance of the produced light. This dimming effect is useful for many applications including, for example only, ambient mood lighting. 
     In addition to the dimming effect, changes in the input power level (to the incandescent lamp) change the color temperature of the produced light. 
     Color Temperature 
     Color temperature is a characteristic of light that may be defined and understood in a number of different ways. Light is electro-magnetic radiation at a range of frequencies. The perceived color of light depends on the frequency (or wavelength) of the radiation. Most light, especially ambient light such as the light produced by incandescent lamp is a mixture of, or combination of, light have at a range of frequencies (or, differently expressed, at different wavelengths, or “colors”). 
     Color temperature of light can be understood as the spectral distribution and content of the light. More simply, color temperature is the relative amounts of different “colors” present in the light. Color temperature is measured using a scale having Kelvin (K) units. 
     For example, a burning candle typically generates light having a wide spectrum of colors; however, in the candle light, the dominant light components have yellow and orange color. Accordingly, overall, candle light is typically characterized as having a color temperature below 1,900 degrees Kelvin. An incandescent lamp typically generates light having a wide spectrum of colors; however, here, overall, incandescent light is typically characterized as having color temperature ranging approximately from 2,500 to 3,500 degrees Kelvin. These two examples are of comparatively low color temperature light having comparatively more yellow to red light components. Such light is generally referred to as being “warm” or “soft” light. 
     Higher color temperature light has comparatively more white to blue components and is generally referred to as being “cold” or “harsh” light. For example, “white” fluorescent lighting often found at retail spaces and offices is characterized as having color temperature ranging approximately from 3,500 to 4,500 degrees Kelvin. The sunlight at mid summer day has color temperature ranging approximately from 5,500 to 6,000 degrees Kelvin. 
     Color Temperature Changes During Dimming of Incandescent Lamps 
     Changes in the input power level to an incandescent lamp not only change the output luminance, but also change the color temperature of the light produced by the incandescent lamp. 
       FIG. 1  also illustrates relationship between the input power levels (the X-axis) and the color temperature (the second Y-axis) of the light produced by an incandescent lamp at various power levels. As illustrated by color temperature curve  12  (dashed line curve), at relatively higher power levels (and correspondingly higher luminance), the produced light has a comparatively higher color temperature indicated in  FIG. 1  as temperature K HIGH . Also illustrated by the color temperature curve  12 , as the input power level is decreased (and the luminance reduces as illustrated by curve  10 ) the color temperature of the produced light also decreases toward a lower color temperature indicated in  FIG. 1  as temperature K LOW . That is, the incandescent light has a color temperature range  14  as illustrated. In some applications, lower color temperature light is preferred because the lower color temperature light may be perceived as a warmer, softer light. 
     For residential ambient lighting applications, the low and the high color temperature values K LOW  and K HIGH  may range approximately 2,500 to 3,500 degrees Kelvin, respectively. However, the actual values of the color temperature may vary widely outside of these values depending on many factors. The color temperature curve  12  is a generalized and simplified representation of the relationship between the input power and the color temperature of the produced light of an incandescent lamp; the curve  12  is used for illustrative purposes only and as an aid to understanding the relationship. 
     Incandescent Dimming Effect in Both Luminance and Color Temperature 
     As discussed above, for incandescent lamps, when input power is dimmed, both the output luminance and the color temperature of the output light are reduced. The result of the dimming is softer, warmer, and more pleasing light. For many lighting applications, this is a desirable characteristic of incandescent lamps. 
     Incandescent Dimming Effect in Both Luminance and Color Temperature 
     Even with such desirable operating characteristics, the use of incandescent lamps is being discouraged. In its place, light emitting diodes (LEDs) are being used to provide lighting in many applications. LEDs are much more energy efficient compared to the energy efficiencies of incandescent lamps. 
     Similar to the incandescent lamps, the luminance of the light produced by LEDs can be varied by varying the input power to the LEDs. However, variations in the input power to the LEDs do not lead to any significant changes of the color temperature of the light produced by an LED. In “white” LEDs that have a blue semiconductor and yellow phosphor, there may reach a point on overdriving the LED that the phosphor would be saturated and only blue light would increase upon further energy input. This would not be good for the longevity of the LED, however. Additionally, there may be a thermal effect that at higher temperatures the spectrum changes slightly, but again this is not good for the LED lifetime. 
       FIG. 2  is a diagram illustrating, inter alia, relationship between the input power levels (the X-axis) and corresponding output luminance (the first Y-axis) of an LED. As illustrated by luminance curve  20  (sold line curve), at a third threshold input power, W TH3 , the LED begins to produce light at some minimal luminance, L MIN2 . The luminance  20  increases as the input power increases until at a fourth threshold input power, W TH4 , the LED produces light at its maximum rated luminance, L MAX2 . The luminance curve  20  is a generalized and simplified representation of the relationship between the input power and the output luminance of an LED; the curve  20  is used for illustrative purposes only and as an aid to understanding the relationship. Accordingly, the amount of light produced by an incandescent lamp can be controlled by a dimming switch. However, changes in the input power level do not result in significant change in the color temperature of the light produced. This is illustrated by color temperature curve  22  (dashed line). Increased input power may cause slight changes in the color temperature of the light from an LED. This may be due to phosphor saturation, thermal changes, or both causing change in the color temperature. This is illustrated as a color temperature range  24 . In the Figure, the range  24  is illustrated in exaggerated matter to more clearly indicate the slight range. This color temperature range is not significant and is typically not even perceptible for standard operating range for ambient temperature. In fact, the color temperature range  24  is orders of magnitude lower than the color temperature range  14  (of  FIG. 1 ). Applied power beyond W TH4  is not recommended for the longevity of the device. In the range above W TH4 , though there may be phosphor saturation or thermal effects affecting the color temperature, again, this is at the risk of shortening LED life. 
     That is, dimming of (reducing the input power to) an LED lamp over its recommended operating range results in a dimmer light but not softer or warmer light. In this way, the LED lamp lacks a desired operating characteristic compared to the incandescent lamp. In addition, LEDs present a nonlinear current load to applied electrical voltage, especially when alternating current (AC) power is applied. This may create a high total harmonic distortion (THD). This is an undesirable characteristic of LED lamps. 
     Accordingly, the need remains for LED based lighting systems having color temperature properties similar to incandescent lighting while maintaining low THD values and high efficiency. 
     SUMMARY OF THE INVENTION 
     The need is met by the apparatus and methods of the present invention. In a first embodiment of the present invention, a lighting system includes a first lighting module and a second lighting module. The first lighting module includes at least one light emitting element, the light emitting element of the first lighting module generating, when power is applied, light at a first color temperature. The second lighting module includes at least one light emitting element, the light emitting element of the second lighting module generating, when power is applied, light at a second color temperature. The first lighting module activates at a first activation voltage. The second lighting module activates at a second activation voltage. The lighting elements of these lighting modules can be light emitting diodes (LEDs) or any other electrically activated lighting device. 
     In one embodiment of the present invention, a lighting system includes a first lighting module and a second lighting module. The first lighting module includes at least one light emitting element, the light emitting element of the first lighting module generating, when power is applied, light at a first color temperature. The second lighting module includes at least one light emitting element, the light emitting element of the second lighting module generating, when power is applied, light at a second color temperature. The first lighting module activates at a first activation voltage. The second lighting module activates at a second activation voltage. The lighting elements of these lighting modules can be light emitting diodes (LEDs) or any other electrically activated lighting device. A first capacitor is connected in series with the first lighting module, the first capacitor connected in parallel to said second lighting module. A second capacitor is connected in series with both said first lighting module and the second lighting module. The second lighting module is electrically connected in parallel to the first lighting module. When electrical power is applied to the lighting system, the first lighting module conducts electrical current during a first conduction period within each power cycle and the second lighting module conducts electrical current during a second conduction period within each power cycle. 
     In a third embodiment of the present invention, a method of generating light is disclosed. At application of electrical energy, a first lighting module is activated at a first activation voltage and a second lighting module is activated at a second activation voltage. The first lighting module includes at least one light emitting element, which when activated, generates light at a first range of color temperature. The second lighting module includes at least one light emitting element, which when activated, generates light at a second range of color temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph illustrating various characteristics of a prior art incandescent lamp; 
         FIG. 2  is a graph illustrating various characteristics of a prior art LED lamp; 
         FIG. 3  is a lighting system in accordance with one embodiment of the present invention; 
         FIGS. 4 and 5  are graphs illustrating various electrical characteristics of the embodiment of  FIG. 3 ; 
         FIG. 6  is a lighting system in accordance with another embodiment of the present invention; 
         FIGS. 7 and 8  are graphs illustrating various electrical characteristics of the embodiment of  FIG. 6 ; 
         FIG. 9  is a lighting system in accordance with yet another embodiment of the present invention; and 
         FIG. 10  is a lighting system in accordance with yet another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the present invention, a lighting system includes a first lighting module including at least one light emitting element, and a second lighting module including at least one light emitting element. The lighting elements may be, for example, LEDs. The second lighting module is electrically connected in parallel to the first lighting module. The first lighting module has a first activation voltage. When activated, the first lighting module generates light at a first color temperature. The second lighting module has a second activation voltage. When activated, the second lighting module generates light at a second color temperature. 
     Because the lighting modules have different activation voltages, they are activated for different durations during each power cycle. Furthermore, because the lighting modules generate light at different color temperatures, the color temperature of light generated by the combined light from these two modules is a third color temperature (combined light color temperature). 
     Utilizing these factors, by adjusting the amount of light generated by each of the two lighting modules, the color temperature of the combined light can be changed. Finally, the amount of light generated by each of the two lighting modules can be varied by adjusting the input voltage. 
     That is, by varying (increasing or decreasing (“dimming”)) the input voltage, the ratio of light generated by each of the lighting modules can be changed. Because each of the lighting modules generates light having different color temperature (compared to the color temperature of the other lighting module), when the ratio changes, the color temperature of the resulting combined light changes, and the desired effect is achieved. This is illustrated in the Figures and discussed in more detail below. 
     In the Figures, various graphs and curves are generalized and simplified representation of the relationship between various electrical voltages, currents, and responses used for illustrative purposes only and as an aid to the disclosure and for even better understanding of the present invention. 
     First Embodiment 
       FIG. 3  illustrates a lighting system  500  in accordance with one embodiment of the present invention. Referring to  FIG. 3 , the lighting system  500  includes a first lighting module  530  and a second lighting module  540 . The first lighting module  530  is adapted to connect to an alternating current (AC) electrical power source  120  via a dimming device  420 . In the U.S., the AC power  120  provides a cyclical voltage of approximately 120 volts RMS (root mean square) with a peak voltage value ranging from approximately positive 170 volts (V) to approximately negative 170 volts. In Europe and other countries, the available AC power is approximately 240 volts RMS. Other countries may use a different frequency, for example, 50 Hz. Other platforms (for example, aircraft avionics) may use another frequency such as 400 Hz. The same principles apply to the following discussion regardless of applied oscillatory voltage or frequency. There are a number of dimmers in the marketplace that can be used for the dimming device  420 . 
     Although the AC electrical power source  120  provides the power to drive the lighting system  500 , the lighting system  500  is not directly connected to the power source  120 . Rather, the lighting system  500  is directly connected to the dimming device  420  and operates on the electrical power that the dimming device  420  allows through to the lighting system  500 . In fact, often, dimming devices include switches that open the circuit thus separating the power source  120  from the lighting system  500 . For this reason, in this document, “electrical power”, “input power” and similar terms and phrases indicate the power applied to the lighting systems of the present invention from the dimming device  420 . 
     The dimming device  420  provides alternating current (AC) electrical power, the electrical power having power cycles. In the U.S., the AC power provides a cyclical voltage of approximately 120 volts RMS (root mean square) with a peak voltage value ranging from approximately positive 170 volts to approximately negative 170 volts. In Europe and other countries, the available AC power is approximately 220 volts RMS. The first lighting module  530  defines a first current path and the second lighting module  540  defines a second current path. Further, the dimming device  420  provides electrical power at different voltages. Often, that is the function of the dimming device  420 . There are many prior art devices and techniques for providing variable power source to the lighting system  500 . 
       FIGS. 4 and 5  are graphs illustrating various electrical characteristics of the embodiment of  FIG. 3 . In the graphs of  FIGS. 4 and 5 , the X-axis represents time flowing from left to right; the first Y-axis (solid line) represents electrical voltage applied to the lighting system  500 ; and the second Y-axis (dashed line) represents current flowing in the lighting system  500 . 
     Referring to  FIGS. 3 ,  4 , and  5 , the input AC power from the dimming device  420  is cyclical in that the AC power typically has an oscillation frequency of approximately 60 Hertz (Hz).  FIGS. 4 and 5  illustrate a single oscillation of the input AC power voltage as represented by solid line graphs  420 A and  420 B where graph line  420 A is the input AC power at some higher voltage swing (compared to the voltage swing of  420 B) and, correspondingly,  420 B at some lower voltage swing (compared to the voltage swing of  420 A). Each complete oscillation of voltages is considered a complete power cycle and includes 360 degrees. 
     As illustrated, a single power cycle, in this example, lasts approximately 16.7 milliseconds (ms) which is one second divided by 60 cycles. For convenience of discussion herein, a single power cycle period  425  is used to discuss the operations of the lighting system  500 . As for the beginning and the ending of the power cycle period  425 , it is arbitrary where the power cycle is deemed to begin and to end as long as the power cycle period  425  includes a complete oscillation, the entire 360 degrees. The single power cycle  425  can be divided into a positive swing cycle  421  and the negative swing cycle  423 . 
     The first lighting module  530  includes at least one light emitting element, for example, an LED. The LED of the first lighting module  530  generates, when sufficient electrical power is applied, light at a first color temperature. This can be any color temperature depending on the desired application. For ambient lighting, the LEDs of the first lighting module  530  generates light having color temperature of about 3,500 degrees Kelvin. 
     Further, the first lighting module  530  has a first activation voltage. That is, the first lighting module  530  has a threshold voltage, V TH530 , necessary for the LEDs of the first lighting module  530  to conduct electricity and to generate light. Some LEDs have a turn-on voltage of about 2.5 volts. The first activation voltage can be achieved using various techniques including, for example only, serially connecting a number of LEDs, and optionally connecting resistor elements. 
     In the illustrated example embodiment, the first lighting module  530  includes a plurality of pairs of LEDs, and each pair including a first lighting element connected in a first electrical direction and a second lighting element connected in a second electrical direction, the second electrical direction opposite the first electrical direction. 
     The second lighting module  540  includes at least one light emitting element, for example, an LED. The LED of the second lighting module  540  generates, when sufficient electrical power is applied, light at a second color temperature. This can be any color temperature depending on the desired application. For ambient lighting, the LEDs of the second lighting module  540  generates light having color temperature of about 4,100 degrees Kelvin. Further, the second lighting module  540  has a second activation voltage. That is, the second lighting module  540  has a threshold voltage, V TH540 , necessary for the LEDs of the second lighting module  540  to conduct electricity and to generate light. The second activation voltage can be achieved using various techniques including, for example only, serially connecting a number of LEDs, along with optionally connecting resistor elements. The second lighting module  540  is electrically connected in parallel with respect to the first lighting module  530 . In the present example, the first activation voltage V TH530  is lower than the second activation voltage V TH540 . 
     In the illustrated example embodiment, the second lighting module  540  includes a plurality of pairs of LEDs, each pair including a first lighting element connected in a first electrical direction and a second lighting element connected in a second electrical direction, the second electrical direction opposite the first electrical direction. 
     The actual numbers of LEDs for the lighting modules  530  and  540  are implementation dependent and can vary widely. The lighting modules  530  and  540  may have the same number of LEDs or different number of LEDs. In the present example embodiment, the first lighting module  530  includes a first predetermined number of LEDs, for example 12 LED pairs (for a total of 24 LEDs), and the second lighting module  540  includes a second predetermined number of LEDs, for example 21 LED pairs (for a total of 42 LEDs), wherein the first predetermined number is less than the second predetermined number. 
     Operation of the Lighting System  500  with a Comparative High Input Voltage 
     Referring to  FIGS. 3 and 4 , when a comparatively high input voltage power  420 A is applied to the system  500 , during its positive swing cycle  421 , the forward biased LED grouping  431  of the first lighting module  530  conducts current (and generates light) when its activation threshold voltage V TH530  is exceeded at time T 1H . The first lighting module current is represented by dashed curve  530 A. The first lighting module  530  continues to conduct current (and generate light)  530 A until the applied voltage  420 A fails to exceed its activation threshold voltage V TH530  at time T 2H . In the Figures, this period of time is indicated as duration  532 A. 
     Additionally, in response to the comparatively high voltage of the positive swing cycle  421  of the input power  420 A, the forward biased LED grouping  521  of the second lighting module  540  conducts current (and generates light) when its activation threshold voltage V TH540  is exceeded at time T 5H . The second lighting module current is represented by dash-dot curve  540 A. The second lighting module  540  continues to conduct current (and generate light)  540 A until the applied voltage  420 A fails to exceed its activation threshold voltage V TH540  at time T 6H . In the Figures, this period of time is indicated as duration  542 A. 
     A current limiting element  510  is connected in series with the first lighting module  530  but in parallel with the second lighting module  540 . The current limiting element  510 , in combination with the first lighting module  530 , provides for sufficient resistance, reactance, or both, at time T 5H , to allow the voltage V TH540  to be applied across the second lighting module  540 . Depending on the application, the current limiting element  510  may be implemented using a resistor, capacitor, inductor, transistor, diode, or any combination of these electrical components. 
     Note that the duration  542 A is slightly less than the duration  532 A. This is because the second activation voltage V TH540  is higher than the first activation voltage V TH530  and that the high voltage input AC power  420 A takes slightly longer to reach and exceed the higher activation voltage V TH540  than it takes to reach the first activation voltage V TH530 . 
     Similarly, during the negative swing cycle  423  of the applied voltage  420 A, the reverse biased LED grouping  433  of the first lighting module  530  conducts current (and generates light) when its activation threshold voltage V TH530  is exceeded at time T 3H . The first lighting module  530  continues to conduct current (and generate light) until the applied voltage  420 A fails to exceed its activation threshold voltage V TH530  at time T 4H . In the Figures, this period of time is indicated as duration  534 A. 
     Additionally, in response to the comparatively high voltage of the negative swing cycle  423  of the input power  420 A, the reverse biased LED grouping  533  of the second lighting module  540  conducts current (and generates light) when its activation threshold voltage V TH540  is exceeded at time T 7H . The second lighting module  540  continues to conduct current (and generate light) until the applied voltage  420 A fails to exceed its activation threshold voltage V TH540  at time T 8H . In the Figures, this period of time is indicated as duration  544 A. 
     Operation of the Lighting System  500  with a Comparative Low Input Voltage 
     Referring to  FIGS. 3 and 5 , when a comparatively low voltage power  420 B is applied to the system  500 , during its positive swing cycle  421 , the forward biased LED grouping  431  of the first lighting module  530  conducts current (and generates light) when its activation threshold voltage V TH530  is exceeded at time T 1L . Here, the first lighting module current is represented by dashed curve  530 B. The first lighting module  530  continues to conduct current (and generate light)  530 B until the applied voltage  420 B fails to exceed its activation threshold voltage V TH530  at time T 2L . In the Figures, this period of time is indicated as duration  532 B. 
     Additionally, in response to the comparatively high voltage of the positive swing cycle  421  of the input power  420 B, the forward biased LED grouping  531  of the second lighting module  540  conducts current (and generates light) when its activation threshold voltage V TH540  is exceeded at time T 5L . Here, the second lighting module current is represented by dash-dot curve  540 B. The second lighting module  540  continues to conduct current (and generate light)  540 B until the applied voltage  420 B fails to exceed its activation threshold voltage V TH540  at time T 6L . In the Figures, this period of time is indicated as duration  546 . 
     Note that the duration  542 B is significantly less than the duration  532 B. This is because the activation voltage V TH540  is higher than the first activation voltage V TH530  and that the lower voltage input AC power  420 B takes significantly longer to reach and exceed the second activation voltage V TH540  than it takes to reach the first activation voltage V TH530 . 
     Similarly, during the negative swing cycle  423  of the applied voltage  420 B, the reverse biased LED grouping  433  of the first lighting module  530  conducts current (and generates light) when its activation threshold voltage V TH530  is exceeded at time T 3L . The first lighting module  530  continues to conduct current (and generate light) until the applied voltage  420 B fails to exceed its activation threshold voltage V TH530  at time T 4L . In the Figures, this period of time is indicated as duration  534 B. 
     Additionally, in response to the comparatively high voltage of the negative swing cycle  423  of the input power  420 B, the reverse biased LED grouping  533  of the second lighting module  540  conducts current (and generates light) when its activation threshold voltage V TH540  is exceeded at time T 7L . The second lighting module  540  continues to conduct current (and generate light) until the applied voltage  420 B fails to exceed its activation threshold voltage V TH540  at time T 8L . In the Figures, this period of time is indicated as duration  544 B. 
     Luminance of the Lighting System  500  at Differing Input Voltages 
     Referring to  FIGS. 3 ,  4 , and  5 , during the illustrated complete cycle of the high input voltage  420 A, the input voltage may swing between maximum value of +V MAX     —     H  and −V MAX     —     H . The current in the lighting modules may reach a maximum value of +I MAX     —     H  and −I MAX     —     H . Actual numbers for these values depend on the implementation. In one example, using the common AC power available in the U.S., the V MAX     —     H  may swing between +170 volts and −170 Volts. 
     In contrast, with the low input voltage  420 B, the input voltage may swing between maximum value of +V MAX     —     L  and −V MAX     —     L . The current in the lighting modules may reach a maximum value of +I MAX     —     L  and −I MAX     —     L . Actual numbers for these values depend on the implementation. In one example, using the common AC power available in the U.S., the V MAX     —     L  may swing voltages less than +170 and −170 Volts. 
     The exact numerical value and the exact shape of these curves are implementation dependent; however, the maximum positive and negative currents, +I MAX     —     H  and −I MAX     —     H  may range between plus and minus 670 mA (peak of the AC waveform). As for +I MAX     —     L  and −I MAX     —     L  these values would be less than +I MAX     —     H  and −I MAX     —     H  values. 
     Note that with the higher input voltage  420 A, the more current  530 A and  540 A flows through the two modules compared to the current  530 B and  540 B flowing through the two modules in response to the lower input voltage  420 B. That is, as illustrated by the graphs, electrical currents  530 A and  540 A have greater positive and negative values compared to the values of electrical currents  530 B and  540 B. Moreover, currents  530 A and  540 A flow for greater periods of time ( 532 A and  542 A, respectively) compared to the periods of time ( 532 B and  542 B) than currents  530 B and  540 B. This means that, with the higher input voltage  420 A, the lighting system  500  generates more light (greater luminance), and that with the lower input voltage  420 B, the lighting system  500  generates less light, lower luminance. This is a desired response. 
     Color Temperature of Light Generated by the Lighting System  500  at Differing Input Voltages 
     As already discussed above, generally, LED lighting elements generate light having the same color temperature independent of the input voltage level. While the lighting system  500  utilize these LED lighting elements, the lighting system  500  of the present invention allows for changes in the color temperature of the light in response to changes in the input voltage level by using two lighting modules, each lighting module generating light having different color temperature. 
     In the present example, the first lighting module  530  generates light having color temperature of about 3,500 degrees Kelvin, and the second lighting module  540  generates light having color temperature of about 4,100 degrees Kelvin. Combined, light from these two modules would result in light having color temperature between these two values. If the two lighting modules were generating the same luminance relative to each other, then the combined light color temperature would have been 3,800 degrees Kelvin, the average of 3,500 and 4,100. 
     Continuing to refer to  FIGS. 3 ,  4 , and  5 , when the higher input voltage  420 A is applied, the duration  542 A in which the second lighting module conducts current (generates light) is only slightly less than the duration  532 A in which the first lighting module conducts current (generates light). That is, the ratio between the luminance of the second lighting module  540  to the luminance of the first lighting module  530  is close to one (1). Since the second lighting module contributes slightly less luminance (compared to the luminance of the light generated by the first lighting module), the combined light color temperature is likely to be slightly below 3,800 degree Kelvin and can be, for example only, 3,750 degrees Kelvin. 
     When the lower input voltage  420 B is applied, the duration  542 B in which the second lighting module conducts current (generates light) is significantly less than the duration  532 B in which the first lighting module conducts current (generates light). That is, the ratio between the luminance of the second lighting module  540  to the luminance of the first lighting module  530  is significantly less than one. Since the second lighting module contributes significantly less luminance (compared to the luminance of the light generated by the first lighting module  530 ), the combined light color temperature is likely to be significantly below the average of 3,800 degree Kelvin and can be, for example only, 3,600 degrees Kelvin. 
     This means that, with the higher input voltage  420 A, the lighting system  500  generates light having higher color temperatures, and that with the lower input voltage  420 B, the lighting system  500  generates having a lower color temperature (softer, warmer light). This is a desired response. 
     Second Embodiment 
     The lighting system  500  of  FIG. 3  may suffer from some level of undesired harmonic distortions because total current drawn by the system  500  from its input power source  420  may not represent a linear response to the sinusoidal shape of the input power. Total harmonic distortions (THD) and the techniques of reducing THD are disclosed in more detail in U.S. application Ser. No. 12/455,127, which has since issued as U.S. Pat. No. 8,354,800, the entirety of which both are incorporated herein by reference. 
       FIG. 6  illustrates a lighting system  600  in accordance with another embodiment of the present invention. Referring to  FIG. 6 , the lighting system  600  includes a first lighting module  530  and a second lighting module  540 . The lighting modules  530  and  540  are configured similarly to those of  FIG. 3  and discussed above. Other portions of the lighting system  600  that are similar to the lighting system  500  include the variable input power source  420 . 
     In the lighting system  600 , a first capacitor  650  is connected in series with the first lighting module  530 . The first capacitor  650  is connected in parallel to the second lighting module  540 . In the illustrated embodiment, the first capacitor  650  has value of approximately 2.7 microfarad (μF). 
     A second capacitor  652  is connected in series with both the first lighting module  530  and the second lighting module  540  as illustrated. Further, the second capacitor  652  is connected in series with the first capacitor. In fact, the second capacitor  652  connects to the power source  420  on the one side, and on its other side, the second capacitor  652  connects to the first capacitor  650  and to the second lighting module  540 . In the illustrated embodiment, the second capacitor  652  has a value of approximately 3.3 μF. 
       FIG. 7  is a graph illustrating various electrical characteristics of the embodiment of  FIG. 6 . As with the graphs of  FIGS. 4 and 5 , the X-axis represents time flowing from left to right; the first Y-axis (solid line) represents electrical voltage applied to the lighting system  600 ; and the second Y-axis (dashed line) represents current flowing in the lighting system  600 . In  FIG. 7 , for the input power, the lower voltage  420 B curve is used for illustrative purposes. 
     Referring to  FIGS. 6 and 7 , when the voltage power  420 B is applied to the system  600 , during its positive swing cycle  421 , the first lighting module  530  conducts current (and generates light) when its activation threshold voltage V TH530  is exceeded at time T 1L . Here, the first lighting module current is represented by dashed curve  530 C. The first lighting module  530  continues to conduct current (and generate light)  530 C until the voltage applied across the first lighting module  530  fails to exceed its activation threshold voltage V TH530 . Here, because of the effects of the capacitors  650  and  652 , the voltage applied across the first lighting module  530  falls below the activation threshold voltage V TH530  at time T 2C . This is different than the operations of the lighting system  500  (of  FIGS. 3 through 5 ) where the first lighting module current  530 B stops at time T 2L . 
     In fact, for the lighting system  600 , the first lighting module current  530 C trails off after reaching its peak until it stops flowing at time T 2C . Accordingly, the duration  532 C of the first lighting module current  530 C is greater than the duration  532 B (of  FIG. 5 ) of the first lighting module current  530 B. 
     Similarly, when the voltage power  420 B is applied to the system  600 , during its positive swing cycle  421 , the second lighting module  540  conducts current (and generates light) when its activation threshold voltage V TH540  is exceeded at time T 5L . Here, the second lighting module current is represented by dash-dot curve  540 C. The second lighting module  540  continues to conduct current (and generate light)  540 C until the voltage applied across the second lighting module  540  fails to exceed its activation threshold voltage V TH540 . Here, because of the effects of the capacitors  650  and  652 , the voltage applied across the second lighting module  540  falls below the activation threshold voltage V TH540  at time T 6C . This is different from the operations of the lighting system  500  (of  FIGS. 3 through 5 ) where the second lighting module current  540 B stops at time T 6L . 
     In fact, for the lighting system  600 , the second lighting module current  540 C trails off after reaching its peak until it stops flowing at time T 6C . Accordingly, the duration  542 C of the second lighting module current  540 C is greater than the duration  542 B (of  FIG. 5 ) of the second lighting module current  540 B. During the negative swing cycle  423 , the lighting system  600  has similar operating characteristics but only in reverse electrical direction. This is indicated by the graph of  FIG. 7 . 
     For the purposes of clarity of illustration and discussion, the input AC power ( 420 A and  420 B) and the current graphs are illustrated as being in synch with each other. However, due to the capacitors  650  and  652 , the current typically leads voltages. This is illustrated in  FIG. 8 .  FIG. 8  illustrates the input power voltage  402 B and a combined current curve  550 C that is combined value of the two current curves  530 C and  540 C of  FIG. 7 . In  FIG. 8 , multiple cycles of the input power voltage  402  is illustrated to more clearly illustrate the leading nature of the current  550 C. 
     These capacitors  650  and  652  present capacitance and capacitive reactance to the input voltage  420 A and  420 B. In the present example, the power cycle of the input voltages  420 A and  420 B is delayed by almost approximately 15.1 ms. As for the beginning and the ending of the cycle period  425 , it is arbitrary where the cycle period is deemed to begin and to end as long as the cycle period includes a complete oscillation, the entire 360 degrees. 
     As is apparent from  FIG. 8 , the shape of the combined current curve  550 C is similar to the shape of the power supply voltage provided by the dimming device  420 . That is, the shape of the combined current curve  550 C is only slightly distorted compared to the shape of the power supply voltage  420 A (same as applied to  420 B). Accordingly, the total harmonic distortion (THD) generated by the lighting system  600  of  FIG. 6  when connected to the input AC power  420  is comparatively low. 
     Third Embodiment 
       FIG. 9  illustrates yet another embodiment of the present invention. Referring to  FIG. 9 , a lighting system  700  includes a first lighting module  730  including at least one light emitting element. In the illustrated embodiment, the first lighting module  730  includes a plurality of light emitting diodes serially connected in a forward direction. Again, the designation of forward or reverse is arbitrary. A first rectifier  732  is connected to the first lighting module  730 . A first capacitor  650  is connected to the first rectifier  732 . For the first lighting module  730 , each light emitting element can be a light emitting diode (LED) such as, for example LED model LW540A which operate generally between three to four forward volts. LW540A and similar LEDs are available in the marketplace. In the illustrated embodiment, the first lighting module  730  includes 12 serially connected LEDs. The first rectifier  732  can have any known rectifier configuration. In the illustrated embodiment, the first rectifier  732  is a diode-bridge type rectifier having the illustrated configuration, each diode being, for example, a 1N4004 rectifier diode available in the marketplace. The first capacitor  650  can be, for example, a 1.47 μF 100V Polyester type capacitor. The actual model, value, and type of these diode and capacitor components and the number of LEDs in the first lighting module  730  may vary depending on application. The first lighting module  730  has a first activation voltage and generates, upon activation, light having a first color temperature. 
     In the illustrated embodiment, the second lighting module  740  includes a plurality of light emitting diodes connected in a forward direction. Again, the designation of forward or reverse is arbitrary. A second rectifier  742  is connected to the second lighting module  740 . For the second lighting module  740 , each light emitting element can be a light emitting diode (LED) such as, for example type LW540A discussed above. In the illustrated embodiment, the second lighting module  740  includes 23 serially connected LEDs. The second rectifier  742  can have any known rectifier configuration. In the illustrated embodiment, the second rectifier  742  is a diode-bridge type rectifier having the same configuration and components as the first rectifier  732 . The actual model, value, and type of these diode and capacitor components and the number of LEDs in the second lighting module  740  may vary depending on application. The second lighting module  740  and the second rectifier  742  are connected to the first lighting module  730  and the first rectifier  732  in parallel. Continuing to refer to  FIG. 9 , a second capacitor  652  is connected in series with both the first rectifier  732  and the second rectifier  742 . The second capacitor can be  652 , for example, a 3.75 μF 250V Polyester type capacitor. The second lighting module  740  has a second activation voltage and generates, upon activation, light having a second color temperature. 
     The lighting system  700  may include the supporting circuit  190  illustrated in more detail in  FIG. 10  and discussed below. The supporting circuit  190  includes one or more components to protect the lighting system  700 , to support the operations of the lighting system  700 , or both. For example, the supporting circuit  190  is used to limit in-rush current at turn-on. If the in-rush current is not limited, the in-rush current may charge the capacitors  650  and  652  too rapidly, potentially damaging power switches used to activate the lighting system. Again, the supporting circuit is useful in many implementations but not absolutely necessary for the operations of the lighting system  700 . 
     The operations of the lighting system  700  are mostly similar to the operations of the lighting system  600  of  FIG. 6  and discussed above but there are some minor differences. Again, the dimming device  420  provides input AC voltage  420 A or  420 B as in  FIGS. 4 ,  5 ,  7 , and  8 . The input AC power passes through the supporting circuit  190 , passes through the capacitors  650  and  652 . However, here, prior to reaching the lighting modules,  730  and  740 , the input AC power is rectified (converted into direct current (DC) power) by rectifiers  732  and  642  respectively. Actually, the resultant DC power is a pulsed-DC voltage. The pulsed-DC voltage across the first lighting module  730  is smoothed by a third capacitor  754  connected in parallel with the first lighting module  730 . The third capacitor  754 , for example only, can be a 1.0 μF 200V electrolytic type capacitor. The third capacitor  754  reduces ripples of the pulsed-DC voltage applied to the first lighting module  730 . Such ripple reduction may be useful for some types of light emitting elements, for some application, or both. 
     Similarly, the pulsed-DC voltage across the second lighting module  740  is smoothed by a fourth capacitor  756  connected in parallel with the second lighting module  740 . The fourth capacitor  756 , for example only, can be a 1.0 μF 200V electrolytic type capacitor. The fourth capacitor  756  reduces ripples of the pulsed-DC voltage applied to the second lighting module  740 . Such ripple reduction may be useful for some types of light emitting elements, for some application, or both. 
     Fourth Embodiment 
       FIG. 10  illustrates another embodiment of the present invention. Referring to  FIG. 10 , a protected lighting system  800  includes the lighting system  810 . The lighting system  810  may be configured similarly to the lighting system  500 , the lighting system  600 , or the lighting system  700  of  FIGS. 3 ,  6 , and  9 , respectively. The supporting circuit  190  is connected between the dimming device  420  and the lighting system  810 . The supporting circuit  190  includes one or more components to protect the lighting system  810 , to support the operations of the lighting system  810 , or both. For example, the supporting circuit  190  is used to limit in-rush current at turn-on. If the in-rush current is not limited, the in-rush current may charge the capacitors  650  and  652  too rapidly, potentially damaging power switches used to activate the lighting system. 
     In the illustrated embodiment, a thermistor  198  specifically provides in-rush current limiting when first powering the circuit. In case the mains voltage is at the peak of its waveform when first applied to the circuit, there would be a relatively fast voltage surge across capacitive elements, leading to a large in-rush or surge current that could harm the LEDs or other components. When cold, the thermistor  198  acts as a resistor to minimize surge current. When heated (due to the operation of the protected lighting system  800 ) the thermistor  198  offers decreased resistance so as minimize the resistive effects against the flow of current through the protected lighting system  800 . Additionally, a fuse  194  may briefly experience a large current that could cause it to fail open, were it not for the thermistor  198 . 
     The supporting fuse  194  is connected in series with the lighting system  810 . The fuse  194  protects the lighting system  810  by opening the circuit (thereby disconnecting the lighting system  810  from the power source  120 ) in case of excessive current flows. Rating of the fuse  194  varies depending on the implementation. In the illustrated embodiment, as an example only, the fuse  194  may have a rating in the order of one or two amperes. 
     Another protective device is a spark gap  196  that protects the lighting system  810  from excessive input voltage. When excessive voltage is applied to the lighting system  810 , the current jumps the spark gap  196  rather than being directed to the lighting system  810  thereby protecting the lighting system  810  from the excessive voltage. Rating of the spark gap  196  varies depending on the implementation. In the illustrated embodiment, as an example only, the spark gap  196  may have a rating on the order of one kilo-volts. 
     In the illustrated embodiment, the supporting circuit  190  includes a transient voltage suppressor  192  such as, for example, a metal oxide variable (MOV) resistor  192  to prevent a voltage spike on lighting system  810  when transient voltage surges appear on the power source  120 . The MOV resistor  192  can be, for example, MOV resistor known as part VE13M00151K in the marketplace. The MOV resistor  192  is connected in parallel with the lighting system  810 , through the fuse  194 . 
     The supporting circuit  190  need not include all the components illustrated in  FIG. 10 . For example, the supporting circuit  190  can be as simple as including only the MOV resistor  192  and still be within the scope of the present invention. The supporting circuit  190  may include any one or more of the components illustrated, in any combination. Furthermore, the supporting circuit  190  may include additional components not illustrated therein and still be within the scope of the present invention. 
     CONCLUDING REMARKS 
     Note that although the invention has been described in terms of LEDs, the invention and embodiments described herein are not limited to LEDs but may be used with other light emitting devices such as, for example only, Organic Light Emitting Diode (OLED), Light Emitting Polymer (LEP), and Organic Electro Luminescence (OEL), or any other lighting element that generates or causes total harmonic distortion at a level that is higher than desired. The present invention is applicable to and includes regions where the supplied AC power is at 240 volts such as in Europe or other parts of the world. The present invention is applicable to and includes regions where the supplied AC power is at 50 Hz such as in Europe or 400 Hz such as on board an aircraft. The present invention is applicable to and includes use of rectifiers other than the illustrated example rectifiers which are used only for the purposes of disclosing the invention. The lighting system of the present invention can be, for example, a light bulb, a lighting surface, a light wall, a projection system, and the like that includes a plurality of light emitting elements such as LEDs.