Patent Application
20140103812
The present disclosure relates to an adjustable color light source in the illumination arts, light arts, and related arts. More particularly, the present disclosure relates to an adjustable light emitting diode (LED) illumination device that varies the off time for each of multiple light emitting diode (LED) chip colors in succession in order to produce white light and to stabilize the color-shifting or degradation that gradually occurs in LEDs.
In solid state lighting devices, including a plurality of LEDs of different colors, control of both intensity and color is commonly achieved using pulse width modulation (PWM). Such PWM control is well-known, and indeed, commercial PWM controllers have long been available specifically for driving LEDs. See, e.g., Motorola Semiconductor Technical Data Sheet for MC68HCO5D9 8-bit microcomputer with PWM outputs and LED drive (Motorola Ltd., 1990). In PWM, a train of pulses is applied at a fixed frequency, and the pulse width (that is, the time duration of the pulse) is modulated to control the time-integrated power applied to the light emitting diode. Accordingly, the time-integrated applied power is directly proportional to the pulse width, which can range between 0% duty cycle (no power applied) to 100% duty cycle (power applied during the entire period).
Known PWM illumination control has certain disadvantages. In particular, known systems and methods introduce a highly non-uniform load on the power supply. For example, if the illumination source includes red, green, and blue illumination channels and driving all three channels simultaneously consumes 100% power, then at any given time the power output may be 0%, 33%, 66%, or 100%, and the power output may cycle between two, three, or all four of these levels during each pulse width modulation period. Such power cycling is stressful for the power supply, and dictates using a power supply with switching speeds fast enough to accommodate the rapid power cycling. Additionally, the power supply must be large enough to supply the full 100% power, even though that amount of power is consumed only part of the time.
Power variations during PWM may be avoided by diverting current of each “off” channel through a “dummy load” resistor. However, the diverted current does not contribute to light output and hence introduces substantial power inefficiency.
Known PWM control systems are also problematic as relating to feedback control. To provide feedback control of a color-adjustable illumination source employing known PWM techniques, the power level of each of the red, green, and blue channels must be independently measured. This typically dictates the use of three different light sensors each having a narrow spectral receive window centered at the respective red, green, and blue wavelengths. If further division of the spectrum is desired, the problem becomes very expensive to solve. If, for instance, a five channel system has two colors that are very close to one another, only a very narrow band detector is able to detect variations between the two sources.
In order to overcome these problems, one known illumination system utilizes a multi-channel light source having different channels that generate illumination of different colors corresponding to the different channels. The system includes a power supply that selectively energizes the channels by utilizing time division multiplexing (TDM) to generate illumination of a selected time-averaged color. However, this system was designed to cover a large color space. In order to achieve this large color space, the system uses TDM to selectively vary the “on” time of one individual LED color at a time for a specified duration. Therefore, because only one color of LED is used at a time, a large number of LEDs are required to produce some colors, particularly white light. Further, while this approach can provide any color within the full range of available LED chips, it has a low utilization of LEDs. This large quantity of LEDs provides a large Gamut, but does not make efficient use of LEDs.
Therefore, there remains a need for an illumination system that economically and effectively produces white light by concurrently utilizing a majority of the LED chips in the system. There also remains a need for an illumination system that quickly and efficiently stabilizes the color-shifting or degradation that gradually occurs in LEDs.
In at least one aspect, the present disclosure provides an adjustable color light source including a light source having different channels for generating illumination of different colors corresponding to the different channels, and a set of light emitting diodes associated with each of the different channel. In operation, the different channels are selectively energized to maintain all but one of the different channels in the operational state at any given time in order to produce a selected time-averaged color such as white light. In at least a further aspect, the present disclosure provides an electrical power supply that selectively energizes the different channels using time division multiplexing to generate illumination of a selected time-averaged color. The electrical power supply includes a power source that generates a substantially constant root-mean-square drive current on a timescale longer than a period of the time division multiplexing, and circuitry that time division multiplexes the substantially constant root-mean-square drive current into selected ones of the different channels.
In at least another aspect, the present disclosure provides an adjustable light source including a light source having different sets of LEDs wherein each set of LEDs is formed of a single unique color. The sets of LEDs each form channels that generate illumination of different colors corresponding to the different channels, and an electrical power supply selectively energizing the channels using time division multiplexing to generate illumination of a selected time-averaged color. The light source includes solid state lighting devices grouped into N channels, wherein the solid state lighting devices of each channel are electrically energized together when the channel is selectively energized. The electrical power supply includes switching circuitry that, in operation, energizes all but one of the channels at any given time, and a color controller that causes the switching circuitry to operate over a time interval in accordance with a selected time division of the time interval to generate illumination of the selected time-averaged color.
In yet another aspect, the present disclosure provides a method for generating adjustable color including generating a drive electrical current and energizing selected channels of a multi-channel light source using the drive electrical current, wherein the selected channels include all but one of the channels of the multi-channel light source. The method further includes rotating the energizing amongst the selected channels of the multi-channel light source fast enough to substantially suppress visually perceptible flicker. The method further includes controlling a time division of the rotating to generate a selected time-averaged color, wherein the selected time-averaged color is white light.
The present disclosure may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The present disclosure is illustrated in the accompanying drawings, throughout which, like reference numerals may indicate corresponding or similar parts in the various figures. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the disclosure. Given the following enabling description of the drawings, the novel aspects of the present disclosure should become evident to a person of ordinary skill in the art.
The following detailed description is merely exemplary in nature and is not intended to limit the applications and uses disclosed herein. Further, there is no intention to be bound by any theory presented in the preceding background or summary or the following detailed description. While embodiments of the present technology are described herein primarily in connection with light emitting diodes (LEDs), the concepts are also applicable to other types of lighting devices including solid state lighting devices. Solid state lighting devices include, for example, LEDs, organic light emitting diodes (OLEDs), semiconductor laser diodes, and the like. While adjustable color solid state lighting devices are illustrated as examples herein, the adjustable color control techniques and apparatuses disclosed herein are readily applied to other types of multicolor light sources, such as incandescent light sources, incandescent, halogen, other spotlight sources, and the like.
In at least one embodiment, a system and method is provided, which provides an adjustable LED illumination device that utilizes multiple colors of LED chips to create a desired color temperature. In at least one embodiment, the system and method varies the “off” time of each LED and deduces the light output from that LED by subtraction. The system, in one or more embodiments, includes a control system that utilizes the light output information to vary the output of the individual LEDs to compensate for variations in light output due to, for example, degradation and the like. By varying the “off” time, the system concurrently utilizes the majority of the LEDs, thus enabling the production of stable white light with fewer LEDs. In one or more embodiments, the system allows for a wide choice of chip colors and quantities in order to produce a wider and more even spectral distribution of color (when compared to traditional LED white methods) thereby providing superior color rendering.
A constant current power source 112 drives the light source 118 via a R/B/G switch 114. The constant current power source 112 outputs a “constant current” or constant rms (root-mean-square) current. In some embodiments, the constant rms current is a constant direct current. However, the constant rms current can be a sinusoidal current with a constant rms value, or the like. The “constant current” is optionally adjustable, but should be understood that the current output by the constant current power source 112 is not cycled rapidly as is the case for PWM. The output of the constant current power source 112 is input to a RIB/G switch 114. The R/B/G switch 114 functions as a demultiplexer (demux) or one-to-three switch to channel the constant current into two of the three color channels R, G, B at any given time. The RIB/G switch 114 of the present embodiment ensures that only one of the total available colors is “off” at any given time, i.e., only one of the three colors is “off” at any time. It should be noted that while the present embodiment has been described in terms of a three channel switch that ensures that two and only two colors are concurrently “on” while the third color is simultaneously “off”, other embodiments are envisioned that utilize different numbers of colors including but not limited to, for example, four and five colors without departing from the disclosure. In embodiments that employ four colors, three of the four colors will be concurrently “on” at any given time while the fourth color is simultaneously “off”. Similarly, in embodiments that employ five colors, four of the five colors will be concurrently “on” at any given time while the fifth color is simultaneously “off”.
Each of the three fractional time periods T1, T2, and T3 corresponds to two selected color channels being concurrently “on” during that time. Alternatively stated, each of the three fractional time periods T1, T2, T3 corresponds to one selected color channel being “off” during that time. Specifically, fractional time period T1 corresponds to the red color channel R1 and the green color channel G1 being “on”, i.e., T1=R1+G1. Fractional time period T2 corresponds to the green color channel G1 and the blue color channel B1 being “on”, i.e., T2=G1+B1. Fractional time period T3 corresponds to the blue color channel and the red color channel R1 being “on”, i.e., T3=B1+R1. During the first fractional time period T1 the R/G/B switch 114 is set to flow the constant current from the constant current power source 112 into two of the color channels, i.e., into the red color channel R1 and the green color channel G1. As a result, the light source 118 generates only red and green light during the first fractional time period T1, i.e., the red and green lights are maintained in the “on” state. During this time, no power is supplied to the blue lights and the blue lights are maintained in the “off” state. During the second fractional time period T2 the R/G/B switch 114 is set to flow the constant current from the constant current power source 112 into a second pair of the color channels, i.e., into the green color channel G1 and the blue color channel B1. As a result, the light source 118 generates only green and blue light during the second fractional time period T2, i.e., the green and blue lights are maintained in the “on” state. During this time, no power is supplied to the red lights and the red lights are maintained in the “off” state. During the third fractional time period T3 the R/B/G switch 114 is set to flow the constant current from the constant current power source 112 into a third pair of the color channels, i.e., into the blue color channel B1 and the red color channel R1. As a result, the light source 118 generates only blue and red light during the third fractional time period T3, i.e., the blue and red lights are maintained in the “on” state. During this time, no power is supplied to the green lights and the green lights are maintained in the “off” state. This cycle continues to repeat with the time period T.
The time period T is selected to be shorter than the flicker fusion threshold, which is defined herein as the period below which the flickering caused by the light color switching becomes substantially visually imperceptible, such that the light is visually perceived as a substantially constant blended color. That is, T is selected to be short enough that the human eye blends the light output during the fractional time periods T1, T2, and T3 so that the human eye perceives a uniform blended color. For example, the period T should be below about 1/10 second, and preferably below about 1/24 second, and more preferably below about 1/30 second, or still shorter. A lower limit on the time period T is imposed by the switching speed of the R/G/B switch 114, which can be quite fast since its operation does not entail changing current levels.
The color can be computed quantitatively, as follows. The total energy of the red light and green light output by the red and green LEDs during the first fractional time period T1 is given by E1=T1(R1+G1). The total energy of the green light and blue light output by the green and blue LEDs during the second fractional time period T2 is given by E2=T2(G1+B1). The total energy of the blue light and red light output by the blue and red LEDs during the third fractional time period T3 is given by E3=T3(B1+R1). If the fractional time period had proportionality P1:P2:P3=1:1:1 then the light output would be visually perceived as an equal blending of red, green, and blue light, which would produce a light output that is in the center of the gamut. The generation of white light is thus dependent on the choices of the LEDs and the ratios of P1 to P2 to P3.
The current output by the constant current power source 112 into the light source 118 remains substantially constant at all times. That is to say that the constant current power source 112 outputs a substantially constant current to the load comprising the components 114, 118.
In some embodiments, the switching between fractional time periods performed by the color controller 116 is done in an open-loop fashion, i.e., without reliance upon optical feedback. In these embodiments, stored information, e.g., a look-up table, stored mathematical curves, or other stored information, associates the values of the fractional ratios with various colors. For example, if a1=a2=a3 then the values P1=P2=P3=1/3 may be suitably associated with the “color” white.
In other embodiments, the color is optionally controlled using optical feedback. With further reference to
Instead of measuring one color at a time for a specified time duration, the R/G/B control circuit 110 ensures that two and only two sets of LEDs of different colors are energized to be operational (“on”) at any given time. Utilizing two sets of operational (“on”) LEDs of different colors at a time allows the color controller 116 to calculate the color output and changes in the color output of each color phase by varying the “off” time of the third set of LEDs, and then deducing the light output by subtraction. This allows the system to stabilize and compensate for the small color-shifting that occurs in the LEDs over time due to degradation and the like. Utilizing two sets of concurrently operational (“on”) LEDs allows the system to produce a white light with far fewer LEDs and more even spectral distribution of color when compared to systems that utilize only one set of operational (“on”) LEDs at a time, thereby providing a more efficient and economical system. Further, utilizing two sets of concurrently operational (“on”) LEDs also allows for more rapid and accurate correction of color-shifting due to degradation and the like, thereby producing superior color rendering and providing the ability to track color to maintain a color temperature within one ellipse over the life of the system.
The color controller 116 uses the measured color energies E1, E2, E3 to provide feedback color control. In operation, the photosensor 120 measures various light outputs from the light source 118 in rapid sequence, i.e., at a rate that a person cannot perceive changes in light intensity due to inherent human persistence of vision. The photosensor 120 measures the change in light output for each pair of LED channels. The color controller 116 uses the output information and compares it to a baseline to deduce the light output of that particular set of LEDs. For example, the color controller 116 may utilize an algorithm to calculate the light output for each pair of LEDs of the R/G/B light source 118. Since two pairs of LEDs or sources are on simultaneously, the system utilizes subtraction to determine the light output for each pair of LEDs.
Assuming that P1, P2, and P3 correspond to photosensor measurements during T1, T2, and T3, respectively (i.e., P1=photo sensor during T1; P2=photo sensor during T2; and P3=photo sensor during T3), calculation of the energy output for each of the red, green, and blue sets of LEDs is respectively provided by the following:
R(T1)=(P1+P3−P2)/2 (1)
G(T2)=(P2+P1−P3)/2 (2)
B(T3)=(P3+P2−P1)/2 (3)
The term “color” as used herein is to be broadly construed as any visually perceptible color. The term “color” is to be construed as including white, and is not to be construed as limited to primary colors. The term “color” may refer to, for example, an LED that outputs two or more distinct spectral peaks (for example, an LED package including red and yellow LEDs to achieve an orange-like color having distinct red and yellow spectral peaks). The term “color” may also refer to, for example, an LED that outputs a broad spectrum of light, such as an LED package including a broadband phosphor that is excited by electroluminescence from a semiconductor chip. An “adjustable color light source” as used herein is to be broadly construed as any light source that can selectively output light of different spectra. An adjustable color light source is not limited to a light source providing full color selection. For example, in some embodiments an adjustable color light source may provide only white light, but the white light is adjustable in terms of color temperature, color rendering characteristics, and the like.
While the present embodiment discloses a set of three series-connected strings of five LEDs each, other embodiments are contemplated without departing from the disclosure. The set of LEDs may be of a number other than three and may include, for example, four or five strings of LEDs of different colors. In each embodiment, the control circuit 110 operates to maintain one and only one string of LEDs in the “off” state at any time while all other strings of LEDs are concurrently in the operational or “on” state. Similarly, while the present embodiment discloses five LEDs per string, the number of LEDs may be selected based on the use and technical requires of the adjustable color light source, e.g., desired light output and the like. Therefore, each string may include any number of LEDs without departing from the disclosure. Further, while LEDs of particular wavelengths are disclosed herein these wavelengths have been selected for simplicity (e.g., to fall within the ranges of red light, green light, and blue light, respectively) and should not be deemed as limiting. LEDs of varying wavelengths may be utilized without departing from the disclosure. Further still, each string of LEDs may also include LEDs of different wavelengths, e.g., multiple LED within the same or similar color range, without departing from the disclosure.
Referring further to
Alternative embodiments, examples, and modifications which would still be encompassed by the disclosure may be made by those skilled in the art, particularly in light of the foregoing teachings. Further, it should be understood that the terminology used to describe the disclosure is intended to be in the nature of words of description rather than of limitation.
Those skilled in the art will also appreciate that various adaptations and modifications of the preferred and alternative embodiments described above can be configured without departing from the scope and spirit of the disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the disclosure may be practiced other than as specifically described herein.
