The present invention relates generally to the field of electronic displays, and more particularly to liquid crystal displays (LCDs) with backlighting provided by light emitting diodes (LEDs).
LCDs are used in a variety of electronic devices, including computers, smart phones, e-book readers, and televisions. Transmissive LCDs are typically used to achieve high brightness, contrast and color saturation. Such LCDs have an internal light source, known as a backlight, located at the back of the LCD. The LCD backlight usually comprises an array of LEDs, which can be white LEDs and/or tri-color RGB (red, green, blue) LEDs.
When forward biased, LEDs emit light, which is the sole purpose they serve in a conventional LCD backlight array. But, when exposed to light and reverse biased, LEDs are also capable of photovoltaic (PV) behavior, acting as photodiodes to generate electrical current. Given the increasing power demands of portable electronic devices with LCDs and the limitations of battery power sources, the present invention is designed to leverage the PV behavior of backlight LEDs as a secondary power source for portable LCDs.
In certain respects, LCDs are better configured for the use of LEDs in a PV/photodiode mode as compared to direct-lighted LED displays, in which the LEDs themselves form the image on the upper part of the display. Because each LED in such direct-lighted displays operates as a distinct pixel which must be individually biased, it's not practical to wire them in series and/or parallel so as to aggregate voltage and/or current from their PV behavior. In an LCD, on the other hand, the articulation of display pixels occurs in the liquid crystal module, allowing the LED backlight array to be wired in series and/or parallel, so as to collect and aggregate the voltage and/or current generated by the individual LEDs in the PV mode.
On the other hand, the multi-layered structure of a backlighted LCDs presents several technical challenges in the PV-LED mode that do not affect direct-lighted LED displays. In the latter, the LEDs are typically on the uppermost layer of the display, separated from ambient light by only the protective glass of the screen. In an LCD, however, the backlight LEDs are beneath/behind a liquid crystal (LC) module, which in turn is usually sandwiched between two orthogonally oriented polarizers. On a pixel-by-pixel basis, the passage of LED light through the LC module is controlled by the voltage applied to each element of the LC, which rotates the polarization angle of the incoming LED light between 0° and 90°. When the LC twists the polarization angle by 90°, the outgoing LED light passes freely through the upper orthogonal polarizer, and the corresponding pixels have maximum brightness. When the LC does not twist the incoming polarization angle at all, the outgoing LED light is totally blocked by the upper orthogonal polarizer, and the corresponding pixels are completely dark.
The same interaction of the LC module with its bracketing orthogonal polarizers acts to block incoming ambient light from reaching the LED backlight array when the display is completely dark—i.e., when the display is off/asleep. Ambient light that passes through the upper polarizer will acquire a polarization angle orthogonal to that of the lower polarizer behind the LC module. But in the dark screen mode, the LC module does not twist the incoming ambient light to enable it to pass through the lower polarizer, so the ambient light cannot reach the LED backlight array to activate its PV behavior.
Another technical challenge associated with the multi-layered LCD structure relates to energy loss caused by the passage of incoming ambient light through the polarizers. Even when the LC module is twisting the polarization angle of the incoming ambient light to allow it to pass through the lower polarizer, the incoming ambient light, in passing through the upper polarizer, has already given up half of its energy, thereby reducing the PV energy-harvesting efficiency of the LEDs by 50%.
As will be explained herein below, the present invention is designed to address the foregoing technical challenges involved in leveraging the PV behavior of backlight LEDs in an LCD display.
The foregoing summarizes the general design features of the present invention. In the following sections, specific embodiments of the present invention will be described in some detail. These specific embodiments are intended to demonstrate the feasibility of implementing the present invention in accordance with the general design features discussed above. Therefore, the detailed descriptions of these embodiments are offered for illustrative and exemplary purposes only, and they are not intended to limit the scope either of the foregoing summary description or of the claims which follow.
Referring to
In
The reoriented polarized light 212 next passes through a color filter 204 (used with white backlight LEDs), which does not change its polarization, but instead controls the color of each pixel by adjusting the amount of red, green and blue light that makes up each blended pixel color displayed by the LCD 200. The color-filtered light 213 then passes through a second (upper) polarizer 205, which permits only vertical components of the light 214 to pass through. Thus, for example, if the LC module 203 reorients the light's polarization 212 by 90°, all of it will be in the vertical plane and will pass through the second polarizer 205, thereby creating a bright spot in the display 206. But if the light's reoriented polarization 212, after passing through the LC module 203, is 0° (i.e., unchanged), then all of it will remain in the horizontal plane and be blocked by the second polarizer 205, thereby creating a dark spot in the display 206.
It will be appreciated by a person of ordinary skill in the art that, while a P-channel MOS transistor is illustrated for the first control transistor 304, and an N-channel MOS transistor is illustrated for the second control transistor 308, this is but one particular representative embodiment. Other mechanisms may be used for these control transistors, and only one of them may be used at the discretion of the circuit designer. In
In accordance with the preferred embodiment of the present invention, the LED backlight circuit is operated in the light-emitting mode 300 when the LCD is on/active, i.e., displaying images. Conversely, when the LCD is off/asleep, i.e., displaying a dark screen, the LED backlight circuit is operated in the PV energy-harvesting mode 400, with the LEDs functioning as photo diodes. However, a person of ordinary skill in the art will appreciate that the present invention can also be configured to harvest energy from specific sections of the LED array 404 during intervals when such LED array sections are turned off by the control transistors 304308 to control brightness while the display is otherwise active.
In accordance with the preferred embodiment of the present invention, when operated in the PV energy harvesting mode, the LC module 203 is biased to re-orient the polarization angle of incoming ambient light 402 so that this light 402 will pass through the first (lower) polarizer 202 to the LED array 201. But, since the ambient light must first pass through the second (upper) polarizer 205, before it gets to the LC module 203, half of the ambient light 402 has already been filtered out, and its recoverable energy reduced proportionally. In order to compensate for this energy loss, in one embodiment of the present invention, at least the second (upper) polarizer 205, or optionally both polarizers 202205 are polarizing organic photovoltaics (ZOPVs), as described in the paper “Polarizing Organic Photovoltaics,” by Dr. R. Zho, A. Kumar and Prof. Y. Yang of UCLA, published in Advanced Materials, 2011, XX, 1-6, which is incorporated herein by reference. In this embodiment, the energy harvested by the ZOPVs in the form of electric current is directed to the battery 409 to supplement the recharging current 402 generated by the LED array 404 operating in the energy-harvesting mode 400.
Although the preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that many additions, modifications and substitutions are possible, without departing from the scope and spirit of the present invention as defined by the accompanying claims.
This application claims the benefit of the filing date of U.S. Provisional Application No. 62/406,084, filed on Oct. 10, 2016, the entire content of which is incorporated herein by reference.
Number | Name | Date | Kind |
---|---|---|---|
6664744 | Dietz | Dec 2003 | B2 |
9041697 | Soto | May 2015 | B2 |
9646544 | Soto | May 2017 | B2 |
20080248837 | Kunkel | Oct 2008 | A1 |
20110069094 | Knapp | Mar 2011 | A1 |
20120194493 | Soto | Aug 2012 | A1 |
20130040707 | Metcalf | Feb 2013 | A1 |
20130084919 | Glynn | Apr 2013 | A1 |
20130133736 | Van Bommel et al. | May 2013 | A1 |
20140028957 | Yang et al. | Jan 2014 | A1 |
20150221259 | Soto | Aug 2015 | A1 |
20150301380 | Baldo et al. | Oct 2015 | A1 |
20160218553 | He et al. | Jul 2016 | A1 |
20170005235 | Chou et al. | Jan 2017 | A1 |
Number | Date | Country |
---|---|---|
WO2013089554 | Jun 2013 | WO |
Entry |
---|
Zhu et al. (Polarizing Organic Photovoltaics; 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim), 2011. |
PCT International Search Report for PCT/US2017/046045, dated Oct. 23, 2017. |
Zhu, Kumar & Yang, “Polarizing Organic Photovoltaics,” Advance of Materials, 2011, XX, pp. 1-6, Wiley-VCH Verlag GmbH & Co., K G & A, Weinheim. |
Number | Date | Country | |
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20180101035 A1 | Apr 2018 | US |
Number | Date | Country | |
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62406084 | Oct 2016 | US |