News Column

Patent Issued for Color Temperature Tunable White Light Source

July 23, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- A patent by the inventors Li, Yi-Qun (Danville, CA); Dong, Yi (Tracy, CA); Xu, Xiaofeng (Fremont, CA), filed on May 6, 2011, was published online on July 8, 2014, according to news reporting originating from Alexandria, Virginia, by VerticalNews correspondents.

Patent number 8773337 is assigned to Intematix Corporation (Fremont, CA).

The following quote was obtained by the news editors from the background information supplied by the inventors: "This invention relates to a color temperature tunable white light source and in particular to a light source based on light emitting diode arrangements. Moreover the invention provides a method of generating white light of a selected color temperature.

"As is known the correlated color temperature (CCT) of a white light source is determined by comparing its hue with a theoretical, heated black-body radiator. CCT is specified in Kelvin (K) and corresponds to the temperature of the black-body radiator which radiates the same hue of white light as the light source. Today, the color temperature from a white light source is determined predominantly by the mechanism used to generate the light. For example incandescent light sources always give a relatively low color temperature around 3000K, called 'warm white'. Conversely, fluorescent lights always give a higher color temperature around 7000K, called 'cold white'. The choice of warm or cold white is determined when purchasing the light source or when a building design or construction is completed. In many situations, such as street lighting, warm white and cold white light are used together.

"White light emitting diodes (LEDs) are known in the art and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As is known white light generating LEDs ('white LEDs') include one or more phosphor materials, that is a photo luminescent material, which absorbs a portion of the radiation emitted by the LED and re-emits radiation of a different color (wavelength). Typically, the LED die or chip generates blue light in the visible part of the spectrum and the phosphor re-emits yellow or a combination of green and red light, green and yellow or yellow and red light. The portion of the visible blue light generated by the LED which is not absorbed by the phosphor mixes with the yellow light emitted to provide light which appears to the eye as being white in color. The CCT of a white LED is determined by the phosphor composition incorporated in the LED.

"It is predicted that white LEDs could potentially replace incandescent, fluorescent and neon light sources due to their long operating lifetimes, potentially many 100,000 of hours, and their high efficiency in terms of low power consumption. Recently high brightness white LEDs have been used to replace conventional white fluorescent, mercury vapor lamps and neon lights. Like other lighting sources the CCT of a white LED is fixed and is determined by the phosphor composition used to fabricate the LED.

"U.S. Pat. No. 7,014,336 disclose systems and methods of generating high-quality white light, that is white light having a substantially continuous spectrum within the photopic response (spectral transfer function) of the human eye. Since the eye's photopic response gives a measure of the limits of what the eye can see this sets the boundaries on high-quality white light having a wavelength range 400 nm (ultraviolet) to 700 nm (infrared). One system for creating white light comprises three hundred LEDs each of which has a narrow spectral width with a maximum spectral peak spanning a predetermined portion of the 400 nm to 700 nm wavelength range. By selectively controlling the intensity of each of the LEDs the color temperature (and also color) can be controlled. A further lighting fixture comprises nine LEDs having a spectral width of 25 nm spaced every 25 nm over the wavelength range. The powers of the LEDs can be adjusted to generate a range of color temperatures (and colors as well) by adjusting the relative intensities of the nine LEDs. It is also proposed to use fewer LEDs to generate white light provided each LED has an increased spectral width to maintain a substantially continuous spectrum that fills the photopic response of the eye. Another lighting fixture comprises using one or more white LEDs and providing an optical high-pass filter to change the color temperature of the white light. By providing a series of interchangeable filters this enables a single light fixture to produce white light of any temperature by specifying a series of ranges for the various filters.

"The present invention arose in an endeavor to provide a white light source whose color temperature is at least in part tunable."

In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventors' summary information for this patent: "According to the invention a color temperature tunable white light source comprises: a first light emitting diode LED arrangement operable to emit light of a first wavelength range and a second light emitting diode LED arrangement operable to emit light of a second wavelength range, the LED arrangements being configured such that their combined light output, which comprises the output of the source, appears white in color; characterized in that the first LED arrangement comprises a phosphor provided remote to an associated first LED operable to generate excitation energy of a selected wavelength range and to irradiate the phosphor such that it emits light of a different wavelength range, wherein the light emitted by the first LED arrangement comprises the combined light from the first LED and the light emitted from the phosphor and control means operable to control the color temperature by controlling the relative light outputs of the two LED arrangements. In the context of this patent application 'remote' means that the phosphor is not incorporated within the LED during fabrication of the LED.

"In one arrangement the second LED arrangement also comprises a respective phosphor which is provided remote to an associated second LED operable to generate excitation energy of a selected wavelength range and to irradiate the phosphor such that it emits light of a different wavelength range, wherein the light emitted by the second LED arrangement comprises the combined light from the second LED and the light emitted from the phosphor and wherein the control means is operable to control the color temperature by controlling relative irradiation of the phosphors.

"The color temperature can be tuned by controlling the relative magnitude of the drive currents of the respective LEDs using for example a potential divider arrangement. Alternatively, the drive currents can be dynamically switched and the color temperature tuned by controlling a duty cycle of the drive current to control the relative proportion of time each LED emits light. In such an arrangement the controls means can comprise a pulse width modulated (PWM) power supply which is operable to generate a PWM drive current whose duty cycle is used to select a desired color temperature. Preferably, the light emitting diodes are driven on opposite phases of the PWM drive current. A particular advantage of the invention resides in the use of only two LED arrangements since this enables the color temperature to be tuned by controlling two relative drive currents which can be readily implemented using simple and inexpensive drive circuitry.

"In one arrangement the first and second LED arrangements emit different colors of light which when combined these appear white in color. An advantage of such an arrangement to generate white light is an improved performance, in particular lower absorption, as compared to an arrangement in which the LED arrangements each generate white light of differing color temperatures. In one such arrangement the phosphor emits green or yellow light and the second LED arrangement emits red light. Preferably, the first LED used to excite the phosphor is operable to emit light in a wavelength range 440 to 470 nm, that is blue light.

"In a further arrangement light emitted by the first LED arrangement comprises warm white (WW) light with a color temperature in a range 2500K to 4000K and light emitted by the second LED arrangement comprises cold white (CW) light with a color temperature in a range 6000K to 10,000K. Preferably, the WW light has chromaticity coordinates CIE (x, y) of (0.44, 0.44) and the CW light has chromaticity coordinates CIE (x, y) of (0.3, 0.3).

"In another arrangement the first phosphor emits green light with chromaticity coordinates CIE (x, y) of (0.22, 0.275) and the second phosphor emits orange light with chromaticity coordinates CIE (x, y) of (0.54, 0.46). Preferably, the LED used to excite the phosphors is operable to emit light in a wavelength range 440 to 470 nm.

"In a further arrangement the phosphors share a common excitation source such that the second LED arrangement comprises a respective phosphor provided remote to the first LED and wherein the first LED is operable to generate excitation energy for the two phosphors and the source further comprises a respective light controller associated with each phosphor and the control means is operable to select the color temperature by controlling the light controller to control relative irradiation of the phosphors. Preferably, the light controller comprises a liquid crystal shutter for controlling the intensity of excitation energy reaching the associated phosphor. With an LCD shutter the control means is advantageously operable to select the color temperature by controlling the relative drive voltages of the respective LCD shutter. Alternatively, the control means is operable to dynamically switch the drive voltage of the LCD shutters and the color temperature is tunable by controlling a duty cycle of the voltage. Preferably the control means comprises a pulse width modulated power supply operable to generate a pulse width modulated drive voltage.

"To increase the intensity of the light output, the source comprises a plurality of first and second LED arrangements that are advantageously configured in the form of an array, for example a square array, to improve color uniformity of the output light.

"Since the color temperature is tunable the light source of the invention finds particular application in street lighting, vehicle headlights/fog lights or applications in which the source operates in an environment in which visibility is impaired by for example moisture, fog, dust or smoke. Advantageously, the source further comprises a sensor for detecting for the presence of moisture in the atmospheric environment in which the light source is operable and the control means is further operable to control the color temperature in response to the sensor.

"According to the invention a method of generating white light with a tunable color temperature comprises: providing a first light emitting diode LED arrangement and operating it to emit light of a first wavelength range and providing a second light emitting diode LED arrangement and operating it to emit light of a second wavelength range, the LED arrangements being configured such that their combined light output appears white in color; characterized by the first LED arrangement comprising a phosphor provided remote to an associated first LED operable to generate excitation energy of a selected wavelength range and to irradiate the phosphor such that it emits light of a different wavelength range, wherein the light emitted by the first LED arrangement comprises the combined light from the first LED and the light emitted from the phosphor and controlling the color temperature by controlling the relative light outputs of the two LED arrangements.

"As with the light source in accordance with the invention the second LED arrangement can comprise a respective phosphor provided remote to an associated second LED operable to generate excitation energy of a selected wavelength range and to irradiate the phosphor such that it emits light of a different wavelength range, wherein the light emitted by the second LED arrangement comprises the combined light from the second LED and the light emitted from the phosphor and controlling the color temperature by controlling the relative irradiation of the phosphors.

"The method further comprises controlling the color temperature by controlling the relative magnitude of the drive currents of the respective LEDs. Alternatively, the drive currents of the respective LEDs can be dynamically switched and a duty cycle of the drive current controlled to control the color temperature. Advantageously the method further comprises generating a pulse width modulated drive current and operating the respective LEDs on opposite phases of the drive current.

"Where the second LED arrangement comprises a respective phosphor provided remote to the first LED and wherein the first LED is operable to generate excitation energy for the two phosphors the method further comprises providing a respective light controller associated with each phosphor and controlling the color temperature by controlling the light controller to control relative irradiation of the phosphors. The color temperature can be controlled by controlling the relative drive voltages of the respective light controllers. Alternatively the drive voltage of the light controllers can be switched dynamically and the color temperature controlled by controlling a duty cycle of the voltage."

URL and more information on this patent, see: Li, Yi-Qun; Dong, Yi; Xu, Xiaofeng. Color Temperature Tunable White Light Source. U.S. Patent Number 8773337, filed May 6, 2011, and published online on July 8, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8773337.PN.&OS=PN/8773337RS=PN/8773337

Keywords for this news article include: Electronics, Light-emitting Diode, Intematix Corporation.

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Source: Electronics Newsweekly


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