News Column

Patent Issued for LED Backlighting System with Closed Loop Control

June 25, 2014



By a News Reporter-Staff News Editor at Journal of Engineering -- According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventor Brukilacchio, Thomas J. (Reading, MA), filed on October 24, 2008, was published online on June 10, 2014.

The assignee for this patent, patent number 8746943, is Innovations In Optics, Inc. (Woburn, MA).

Reporters obtained the following quote from the background information supplied by the inventors: "Previous backlight illumination sources for flat panels include cold cathode fluorescent lamp (CCFL), hot cathode fluorescent lamp (HCFL), electro luminescent (EL), organic light emitting diode (OLED) which is generally a broad area emitter, and tungsten and tungsten halogen filament lamps and arc lamps in conjunction with woven fiber optics.

"In comparison to these other backlight technologies, LEDs combine the advantages of low voltage operation, high reliability, long life, wide color gamut, high contrast, shock and vibration safe, contain no mercury, have high luminous efficiency, operate over a wide ambient temperature range between -40 and +85.degree. C., rapid switching speed, dimmable by pulse width modulation (PWM), reduced blur (owing to short response time relative to LCD switching time), and are particularly well suited for edge lighted illumination. While these advantages make the use of LEDs very attractive for LCD backlighting applications, a number of challenges still remain. These challenges include luminous efficiency, cost, system design complexity, including the need for new power supplies and controllers, independent closed loop intensity control of red, green, blue (RGB) based systems, and thermal control requirements associated with the use of RGB based systems.

"The prior art shows an approach replacing CCFLs by the use of red, green, and blue LEDs in combination. While this approach has the benefits of improved color gamut and contrast relative to CCFLs, it suffers from thermal instability, primarily due to the strong red, and to a lesser but still significant extent, the green dependence on the LED intensity on temperature.

"The prior art uses a number of photodiodes distributed throughout the lighting system to address this problem, but the photodiodes are separate from the LED source to control the LED intensity. However, this approach adds considerable cost and complexity to the overall system.

"This temperature dependence is addressed by the present invention by use of closed loop feedback control, which maintains a constant brightness both as a function of time and temperature. The present invention can work with either LED excited phosphors or with standard red, green and blue LED die.

"In view of the many shortcomings of the prior art, it is a principal object of the present invention to provide an improved backlighting source employing LEDs for use in illuminating LCD flat panel screens and the like.

"It is another object of the present invention to provide an LED based source of illumination having improved color gamut and contrast.

"It is yet another object of the present invention to provide an LED based linear light source.

"It is still another object of the present invention to provide an LED based illumination source having relaxed thermal management needs and simplified control requirements.

"It is yet another object of the present invention to provide an improved LED board architecture by which closed loop intensity control is implemented and high thermal performance achieved.

"Another object of the present invention to use multiple emitters on a single LED board to significantly reduce cost over the approach of using prepackaged devices.

"Yet another object of the present invention is to provide an optical system in a narrow and short package which allows for low divergence output in a direction normal to an LCD panel.

"Other objects of the present invention will be obvious and will appear hereinafter when the following detailed description is read in connection with the accompanying drawings."

In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventor's summary information for this patent: "The invention herein generally relates to an LED based illumination system for generating one or more line sources that have particular utility in backlighting liquid crystal display (LCD) panels to achieve improved color gamut and contrast or can be used for other applications requiring bright, uniform line sources.

"More particularly, the invention is an Illumination apparatus for producing high intensity line sources of uniform intensity and spectral content, comprising an elongated support member having formed thereon a linear array of regularly spaced LED modules each of which comprises one or more LED emitting areas having a predetermined spectral output that is emitted over a predetermined solid angle. An elongated collection optic is provided with an array of non-imaging concentrators the individual non-imaging concentrators of which are optically coupled in one-to-one correspondence with the LED modules. Each non-imaging concentrator in the array of non-imaging concentrators operates to collect radiation emitted by each of the LED modules and to re-emit substantially all of the collected radiation as a diverging beam having a solid angle smaller than the predetermined solid angle over which radiation is emitted by each of said LED modules. The diverging beam is spatially and spectrally uniform in the near field of the exit aperture and propagates in a direction along an optical axis of the apparatus. Finally, an elongated converging optical element is positioned to receive the diverging beam and converge it in a vertical plane to form a line source of predetermined dimensions.

"The LED modules have LED emitting areas that are selected from the group comprising R,G,B LEDs, Blue LEDs used to excite red and green phosphor, and Blue and Green LEDs used to excite red phosphor.

"The non-imaging concentrators are selected from the group comprising compound parabolic, elliptical, hyperbolic concentrators, straight tapers, and concentrators having curvatures described by higher order polynomial functions.

"In a preferred embodiment, the non-imaging concentrators are rectangular in cross section to control the divergence of the diverging beams in vertical and horizontal planes mutually perpendicular to the optical axis of the apparatus.

"In another aspect of the invention, the illumination apparatus of the collection optic further includes an elongated mixing section having a single continuous surface for receiving the diverging beams from the non-imaging concentrators, mixing their colors for more spectral and spatial uniformity, and re-emitting them along an exit face thereof for travel to the elongated converging optical element.

"In another aspect of the invention, the illumination apparatus collection optic further includes an elongated .theta. by .theta. compound parabolic concentrator section following the mixing section and a second elongated truncated compound parabolic concentrator section following the .theta. by .theta. compound parabolic concentrator section. The .theta. by .theta. compound parabolic concentrator section is structured to receive the light emerging from the exit face of the mixer section and direct it into the elongated truncated compound parabolic concentrator section after which the light emerging from the second elongated truncated compound parabolic concentrator section strikes the elongated converging optical element. Preferably the elongated converging optical element comprises a cylindrical lens having one or more aspheric surfaces but may also be an off-axis parabolic reflector.

"Further included is a planar wedge light guide for receiving light from the line source and propagating it along the length of planar wedge guide. The planar wedge guide has small optical features on one surface thereof to deflect light upwards out of the wedge guide through an exit face thereof opposite to the surface having the small optical features.

"Additionally included a diffuser positioned opposite the exit face of the planar wedge guide to receive light therefrom to improve its homogeneity and control its angular direction for subsequent use with an LCD display panel, individual output of said LED modules to provide a signal for feedback control of the intensity of light output from said LED modules.

"A further attribute of the invention is the use of a high thermal performance LED board to which the LED die are directly attached. This increases the light that can be extracted from the die under high current density conditions, increases LED die lifetime, decreases system complexity, and decreases system cost, as fewer LED die are required to achieve a particular light output."

For more information, see this patent: Brukilacchio, Thomas J.. LED Backlighting System with Closed Loop Control. U.S. Patent Number 8746943, filed October 24, 2008, and published online on June 10, 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=8746943.PN.&OS=PN/8746943RS=PN/8746943

Keywords for this news article include: Innovations In Optics Inc.

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Source: Journal of Engineering


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