Abstract:
A method for efficient lighting includes supplying power to a light source to control the intensity of light emitted from the light source according to an intensity waveform. The amplitude of the waveform over one period is at a high level for a first time interval and at or below a low level for a second time interval. The method includes selecting durations of the first time interval according to a first characteristic of human visual perception and selecting the second time interval according to a second characteristic of human visual perception.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is related to U.S. application Ser. No. 11/413,513, titled “EFFICIENT LIGHTING,” which is being filed concurrently with the present application, and which is also incorporated herein by reference. 
   BACKGROUND 
   The invention relates to efficient lighting, including design of energy-saving LED lighting. 
   Various approaches to powering a light source, such as a light emitting diode (LED), include applying a time varying signal (e.g., a voltage or current square wave) to power the source. In some light flashing circuits, the time varying signal is slow enough to generate a perceptible variation in light intensity, such as for flashing warning lights. Various studies of human visual perception suggest that for flashing light to be perceived as discrete flashes, the flash rate should be below the “flicker-fusion” frequency of approximately 20-30 Hz, above which a flashing light appears as a steady light. In some light dimming circuits, the duty cycle is reduced to provide a perception of a dimmed light source, and the frequency is fast enough (e.g., &gt;100 Hz) to prevent perceptible flicker. 
   SUMMARY 
   In a general aspect, efficient lighting or energy-saving lighting, in particular for LED-based lighting, is based on a design approach that recognizes an interrelationship between two factors: the characteristics of the light source (e.g., an LED) and characteristics of human visual perception. Some types of light sources are able to provide fast transitions to a full brightness level, or to a complete dark level. For example, in LEDs, a quantum-well can light up to full brightness in less than 0.1 milliseconds, and can turn off in less than 0.1 milliseconds, and thus without circuit delay effects, some LEDs can be considered an immediate constant intensity light source when turned on, and can be considered immediately dark when turned off. Circuit delay can affect how quickly a light source can be turned on. For example, parasitic capacitance of an LED is one cause of circuit delay. The amount of parasitic capacitance of an LED can be on the order of above 100 micro-farad (e.g., on the order of 1 farad) for a package with a 1 millimeter square LED chip. The associated circuit delay can be taken into account when selecting what kind of waveform to use for driving the LED circuit. 
   Human visual perception is associated with characteristic response times. For example, in human visual perception, the human visual system can retain images (i.e., retain the perception of intensity of past brightness) for as long as 30-50 milliseconds (“retention time”), and also has a short response time to perceive the full brightness, e.g., about 1-3 milliseconds (“response time”). The retention time is on the order of the inverse of the flicker-fusion frequency. A design approach for efficient or energy-saving lighting takes advantage of the fast response of LEDs and the large ratio of retention time to response time in the human visual system. 
   In one aspect, in general, the invention features a method for efficient lighting. The method includes supplying power to a light source to control the intensity of light emitted from the light source according to an intensity waveform. The amplitude of the waveform over one period is at a high level for a first time interval and at or below a low level for a second time interval. The method includes selecting durations of the first time interval according to a first characteristic of human visual perception and selecting the second time interval according to a second characteristic of human visual perception. 
   In another aspect, in general, the invention features an apparatus, comprising: a light source; and circuitry coupled to the light source configured to supply power to the light source to control the intensity of light emitted from the light source according to an intensity waveform. The amplitude of the waveform over one period is at a high level for a first time interval and at or below a low level that is less than half of the high level for a second time interval that is at least half of the period. The circuitry is configured to supply the power so that the perceived brightness of the light source is substantially the same as the perceived brightness of the light source when the light source is powered continuously to emit light at the high level. 
   Aspects can include one or more of the following features. 
   Selecting the duration of the first time interval according to the first characteristic comprises selecting the first time interval to be longer than a response time to perceive full brightness of the light source. 
   Selecting the duration of the first time interval according to the first characteristic comprises selecting the first time interval to be longer than the response time to perceive full brightness plus a time delay between application of a power signal supplied to the light source and emission of light from the light source. 
   Selecting the duration of the second time interval according to the second characteristic comprises selecting the second time interval to be shorter than a retention time of perception of past brightness of the light source. 
   The method further comprises selecting the period according to characteristics of human visual perception to be shorter than the inverse of a flicker-fusion frequency. 
   The low level corresponds to a level of a power signal supplied to the light source being approximately equal to zero. 
   The low level corresponds to a level of a power signal supplied to the light source being below a threshold value associated with light emission from the light source and greater than half of the threshold value. 
   The waveform comprises a substantially rectangular waveform. 
   The period of the waveform is between about 3 ms and 50 ms. 
   The period of the waveform is between about 20 ms and 30 ms. 
   The duty cycle of the waveform is between about 6% and 90%. 
   The duty cycle of the waveform is between about 10% and 50%. 
   The lighting element comprises at least one light emitting diode. 
   Aspects can have one or more of the following advantages. 
   With an LED that is driven to full brightness in less the response time of the human visual system, energy savings can be achieved by using a duty cycle that has an on time that exceeds the response time and an off time that is less than the retention time of the human visual system. 
   One factor associated with powering a light source is circuit delay between a time a signal (e.g., a voltage step) is applied and the time the light source (e.g., a quantum well of an LED) receives the full power provided by the signal. In some circuits, the frequency of the signal used to power an LED is high, such that, in the presence of circuit delay, the LED on time is shorter than the circuit delay time plus the response time. In these cases, the circuit provides a dimming effect. By selecting the frequency and duty cycle such that the LED on time is at least as long as the circuit delay time plus the response time and the LED off time is shorter than the retention time, a circuit can provide the perceived brightness of an LED that is always on with lower energy expended in a given time period. In some cases, a circuit controls a group of lighting elements arranged so that each element illuminates a different region of visual perception. The regions correspond to different parts of a lighting area such as a room. The lighting elements (e.g., LEDs) are selectively illuminated to scan over the lighting area in a “cycle time.” To save energy, the signals powering the LEDs fulfill at least the following criteria: (1) the cycle time is shorter than the retention time; (2) the LED on time of each LED is longer than the circuit delay time plus the response time. Other relevant criteria, described in more detail below, enable a power supply circuit to reduce the twinkling of the LEDs to a level that human visual system cannot detect. 
   An approach in which the LED on time is shorter than the circuit delay time plus the response time may expend less energy in a given time period relative to an LED that is always on, but does not save energy while providing the same perceived brightness as an LED that is always on. Approaches described herein can achieve energy efficient lighting with at least the same perceived brightness as compared to DC driven light source. 
   Other features and advantages of the invention are apparent from the following description, and from the claims. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic diagram of a control circuit for powering an LED. 
       FIGS. 2A and 2B  are plots of electric signal waveforms. 
       FIG. 3  is a plot of a detector intensity reading. 
       FIGS. 4A and 5  are schematic diagrams of lighting systems including multiple lighting elements. 
       FIGS. 4B and 4C  are plots of electrical signal waveforms. 
       FIG. 6  is a table showing a sequence in which subsets of lighting elements are powered. 
       FIG. 7  is a schematic diagram of an array of LEDs backlighting an LCD panel. 
   

   DESCRIPTION 
   Referring to  FIGS. 1 and 2A , a control circuit  100  controls the supply of power to an LED  102  by applying a control waveform  200 , a voltage v(t), to input terminals of a switch  104  (e.g., a transistor). When the control waveform  200  closes the switch, current flows to power the LED  102 .  FIG. 2B  shows a resulting intensity waveform  202  that represents intensity I(t) of light emitted from the LED  102 . The control waveform  200  is a square-wave with a period T, and a duty cycle D≈25%. The resulting intensity waveform  202  has an “on time” of T on ≈TD, during which the LED is emitting light, and an “off time” of T off ≈T(1−D), during which the LED is not emitting light. The on and off times of the intensity waveform are approximately determined by the duty cycle of the control waveform, but the times may deviate somewhat since the characteristics of the intensity waveform  202  are not necessarily the same as those of the control waveform  200  due to circuit effects and parasitic capacitance and/or inductance of the LED. For example, the waveform  202  is delayed with respect to the waveform  200  by a circuit delay time T cd , and the shape of the intensity waveform  202  is not an exact square-wave. 
   The control circuit  100  can apply other shapes of control waveforms to obtain an intensity waveform that has a shape closer to that of a square-wave. For example, the control circuit  100  takes into account the current-voltage (I-V) characteristic of the light source. In this example, the LED has an I-V characteristic of a diode with negligible current when an applied voltage is below a threshold voltage V c . When the applied voltage (controlled by the control waveform  200 ) is above V c , the current through the LED increases approximately exponentially. 
   In one approach, the control circuit and control waveform are configured such that the voltage across the LED during the “off time” is closer to a value of V c  than to a value of zero. The circuit delay (e.g., due to parasitic capacitance) between an “off” voltage just below V c  and an operating “on” voltage of V o  at full light emission, can be reduced compared to a circuit delay between an “off” voltage of zero and on “on” voltage of V o . Other, approaches can be used to produce a substantially rectangular intensity waveform, including the use of waveform shaping circuitry, for example, to generate an intensity waveform that has short rise and fall times and short delay between application of a control waveform and the resulting intensity waveform. 
   A procedure for configuring a control circuit to provide power to a light source, such as an LED, includes selecting on and off times of the waveform representing power supplied to the light source according to characteristics of human visual perception. For example, without intending to be bound by theory, the following description of a light detector provides an example of a model of human visual perception that can be used for selection of waveform characteristics. 
     FIG. 3  shows a plot  300  of an intensity reading of the detector modeling human visual perception. In this model, the detector receives a constant intensity I 0  light flux via the opening of a very fast shutter (which takes no significant time) at time t=0. Before the shutter opens the intensity reading of the detector is I=0. After the shutter opens, as time goes on, the reading of the light flux will increase (approximately linearly) and stabilize at t=T u  to a reading of I=I 0 . The time T u  represents the visual response time (or time to saturation). When the shutter is closed at t=T s &gt;T u , the detector reading remains I=I 0  for a time T b  and starts to decrease (approximately linearly) at t=T s +T b . The detector reads I=0 after a time period T d  beyond t=T s +T b . The time T b  represents the visual retention time (or persistence time), and T d  is the decay time. 
   Under this model, as shown in plot  302 , if the shutter is open at t=0 and closed at t=T m &lt;T u , the detector reading will not rise from I=0 to I=I 0  by t=T m , since the shutter was open for less than the response time T u . Instead, the detector will read I=I m &lt;I 0  at t=T m , and will maintain this reading until t=T m +T c , where T c  is not greater than T b . The detector will read I=0 at t=T m +T c +T e , where T e  is not greater than T d . 
   The following two cases demonstrate the effect on the detector of repeatedly opening and closing the shutter to represent a light source controlled according to a periodic waveform, for example. 
   In a first case, if the shutter is repeatedly opened (for a time T m &lt;T u ) and closed (for a time T x &lt;T c ) resulting in an open/close shutter cycle with a period T p =T m +T x  the detector will eventually achieve a steady state intensity reading of I&lt;I 0 . This case corresponds to a model for a lower perceived intensity (or “dimming”) of a light source. In this case, the “off time” T x  is shorter than the retention time T c  to provide a constant perceived intensity without flicker. 
   In a second case, if the shutter is repeatedly opened (for a time T s &gt;T u ) and closed (for a time T y &lt;T b ) resulting in an open/close shutter cycle with a period T p =T m +T y  the detector will eventually achieve a steady state intensity reading of I=I 0 . This case corresponds to a model for achieving a full perceived intensity of a light source, even though the light source has been turned on and off periodically. In this case, in order to ensure the full intensity is perceived, the light source on/off time intervals (modeled by the shutter open/close times) are selected such that: (1) the “on time” T s  longer than the response time T u , and (2) the “off time” T y  is shorter than the retention time (to provide a constant perceived intensity without flicker). 
   Although an LED can be turned on or off with a short switching time (T LED ) less than 1 ms (e.g., approximately 0.1 ms), the circuit delay (T cd ) between the application of an electrical signal to a circuit powering the LED and the full light emission from the LED can be greater than 1 ms, and depending on the circuit and parasitic capacitance and/or inductance of the LED, can be as long as 3 ms, 5 ms, 10 ms, or even longer. 
   If the circuit delay T cd  is longer than or comparable to the “on time” of the waveform powering the LED, then the voltage across LED may not reach a full operating voltage, causing the LED to have a lower brightness than it has from the full operating voltage. In some cases, the light flux (and resulting brightness) from the LED is a strong function of the voltage across the LED beyond a threshold voltage (e.g., 3.3 volts). 
   If the LED switching time T LED  is 1 ms, and the circuit delay T cd  is in the range of 3 to 5 ms, it would take T LED +T cd =4 to 6 ms for the LED to reach full intensity after the circuit switches the LED on. If the modeled human visual response time Tu is in the range of 1 to 3 ms, it would take T LED +T cd +T u =5 to 9 ms for the full brightness to be perceived. In such a case, the “on time” of the waveform powering the LED at a given voltage level should be at least 9 ms to ensure the perceived brightness of the LED is substantially the same as the perceived brightness of an LED continuously powered at the same voltage level. A shorter “on time” could cause a lower perceived brightness by (1) not allowing enough time for the voltage across LED from reaching a full operating voltage, and/or (2) not allowing enough time for human visual response to perceive the full brightness. 
   For a given set of on and off times for a waveform powering an LED, another technique for increasing the perceived brightness level includes increase the high voltage level of the waveform. For example, an increased voltage helps to overcome the effect of parasitic inductance and capacitance to achieve an operating voltage across LED in a shorter time. An increased voltage also helps to achieve a higher steady state perceived brightness. However, increasing the voltage level reduces the energy savings that are achieved, and may even lead to higher energy consumption. 
   Power savings can also be achieved in a distributed light source with multiple lighting elements arranged to illuminate different regions of visual perception. Referring to  FIG. 4A , a control circuit  400  supplies power to a first lighting element  402 A illuminating a first room (Room A), and to a second lighting element  402 B illuminating a second room (Room B). For example, a lighting element can include an LED or array of multiple interconnected LEDs. The control circuit  400  supplies power to the first lighting element  402 A according to a first waveform and to the second lighting element  402 B according to a second waveform out of phase with the first waveform. 
   For example, the control circuit  400  drives the first lighting element  402 A from a pair of electrical terminals with a sine wave  404 A ( FIG. 4B ) alternating between +12 volts and −12 volts derived from a 60 Hz power line voltage source. The control circuit  400  drives the second lighting element  402 B with a sine wave  404 B ( FIG. 4C ) from the same terminals with opposite polarity. During one lighting cycle T in Room A, the first lighting element  402 A emits light for a time T on , corresponding to the sine wave  404 A being above a threshold V th . During one lighting cycle T in Room B, the second lighting element  402 B emits light for a time T on , corresponding to the sine wave  404 B being above the threshold V th . Since one lighting cycle is one period of the 60 Hz sine wave (about 16.7 ms), the off time of the lighting elements is less than the retention time of the human visual system (about 30-50 ms). The on time T on , of the lighting elements depends on the threshold V th , but is approximately 5-8 ms when the circuit delay is kept small (e.g., less than a few milliseconds), which is greater than the response time of the human visual system (about 1-3 ms). 
   This exemplary “AC lighting” approach can save energy compared to a “DC lighting” approach in which a 60 Hz power line voltage source is converted to a constant DC voltage to power the lighting elements. The AC lighting approach can provide comparable perceived brightness with lower consumed power since the power supply does not need to convert from AC to DC. The power savings is higher compared to power supplies that generate large current (for example &gt;3 A) since large current conversion efficiency is lower (e.g., typically less than 60% efficiency). 
   The different regions of visual perception can correspond to different spaces such as the rooms in the previous example, or upper and lower cabinets of a show-case, for example, or can correspond to different overlapping regions of visual perception. 
   Referring to  FIG. 5 , a control circuit  500  supplies power to a group of lighting elements  502 A- 502 G arranged to illuminate different overlapping regions of visual perception (or “lighting zones”) within an illumination area (e.g., a room). The control circuit  500  powers subsets of 3 lighting elements at a time in a sequence shown in  FIG. 6 . The rows A-G correspond to lighting elements  502 A- 502 G, and the columns  1 - 7  correspond to seven time slots in a repeated sequence for powering the lighting elements. The control circuit  500  illuminates lighting elements  502 A- 502 C during the first time slot, lighting elements  502 B- 502 D during the second time slot, and so on as shown in  FIG. 6 . The control circuit  500  scans over the illumination area over a time period T sc  that is less than the retention time of the human visual system. During each time slot, the control circuit  500  powers on the corresponding subset of lighting elements for a time longer than the response time of the human visual system. By selecting the phases of the waveforms that power the subsets of lighting elements according to the table in  FIG. 6 , the power consumption level is essentially constant in time and only three lighting elements need to be powered at any given time. 
   Another aspect of arranging lighting elements to efficiently illuminate different regions of visual perception is controlling the beam shapes and resulting footprint of the respective illuminated areas. At a given distance from a lighting element, the intensity of light at the illuminated area is higher when the beam divergence (and the footprint) is smaller. 
   For example,  FIG. 7  shows a two-dimensional array of LEDs  700  to provide backlight for a liquid crystal display (LCD) panel  702 . A small lighting footprint can be achieved in at least two ways: (1) the LEDs can be placed a short distance from the panel (e.g., shorter than 5 cm), and (2) the angle of illumination from the LEDs can be made small (e.g., by choice of the numerical aperture of an optical enclosure for the LED). If the illumination footprint of each LED at the panel  700  is reduced by a factor of α (in diameter), the number of LEDs used to illuminate the panel can be increased by approximately a factor of 1/α 2  to cover the same area with a brighter backlight. By powering subsets of LEDs with waveforms that are out of phase, as described above, the amount of power used to backlight the panel can be reduced compared to a panel backlit by fewer continuously powered LEDs. For example, a control circuit  704  powers a first set of rows  706 A according to a first waveform, and a second set of rows  706 B according to a second waveform out of phase with the first waveform. 
   Other embodiments are within the scope of the following claims.