Who amongst us knew that Apple had a secret retardation solution for their media players? Surely not I. But there it is in black and white, a granted patent for one of Apple's super secrets of how their display stays so crisp under the most demanding of circumstances. In case you didn't know, Apple's Engineering Team responsible for display and touch technology is as fanatical about their craft as Jonathan Ive is about Industrial Design. Two of the senior display engineer's on this team include Cheng Chen who has a PhD in Liquid crystal physics/optics and is behind Apple's upcoming iPad as is John Zhong who won an honorary award in 2009 from The Society for Information Display (SID) for his exceptional contributions to display technology. Today's granted patent presents us with a basic overview of this team's use of retardation film on media displays so that we could continue to enjoy crisp imagery even when we're wearing sunglasses. At the end of the day, Apple's granted patent may even provide us with a little reasoning behind Apple's recent move to ban protective screen film from the Apple Store.
Explosive growth in the popularity of mobile communication and computing devices has created a burgeoning demand for low-power displays. Liquid crystal displays (LCDs), which are based on polarization optics and typically employ linear polarizers on their front surfaces, are frequently used in portable devices. LCDs emit linearly-polarized light, with an electric field that vibrates only in one direction.
The use of portable computing devices outdoors or in other bright environments can result in users viewing such devices through polarized sunglasses, which typically only allow through light with an electric field that vibrates in the vertical direction. Hence, a user looking at the LCD display of a portable device, may see a distorted image in the display when viewed through polarized sunglasses, due to the polarized filters in the sunglasses blocking the light when the display is viewed from some angles.
Depending on the angle at which the device is held or viewed, the image might be clear, completely dark, or somewhere in-between. An image might be further distorted when a lens cover is placed in front of the display for protection or industrial design, because such lens plastics are typically manufactured without good control of optical birefringence, which can result in non-uniform optical retardation. As a result, when viewed through polarized sunglasses, the image may appear to include numerous color- and gray-shade artifacts.
Apple's patent covers their secret use of retardation films on current media players to combat the distortions and negative effects described above.
Apple's patent FIG. 2 noted above illustrates a quarter-wave retardation film 200. The thickness of the retardation film (also known as an "A-plate") could be tuned to circularly polarize light that passes through. When used in conjunction with a display that linearly polarizes light, this film emits circularly-polarized light with consistent image quality even when viewed at different angles through a polarizing sunglass.
Quarter-Wave Retardation Film in Proximity to a Liquid Crystal Display
In Apple's patent FIG. 3 illustrated above we see a quarter-wave retardation film in proximity to a liquid crystal display. The quarter-wave retardation film 200 is in proximity to the front polarizer 302 of the LCD. Internal A-plates 304 may also be used in the LCD, for instance to enlarge the viewing angle of the display, and may be above, below, or both above and below a liquid crystal layer 306 sandwiched between two glass layers 308. The LCD typically also includes a bottom polarizer 310. Additional display components 312 may vary depending on the type of display. For instance, the additional display components may include circuitry to control the transparency of the liquid crystal layer, a backlight that generates transmissive light 314, reflective structures that incorporate reflected light 316 in the display image, or structures that support both transmissive and reflective capabilities. Note that the technique described in the present invention can be applied to transmissive displays, reflective displays, and trans-flective displays.
In a further embodiment of the present invention, the optical axis of the retardation film is aligned 45 degrees with respect to the transmission direction of the linearly-polarized light emitted by the front polarizer of the LCD display.
In one embodiment of the present invention, the retardation film can be: laminated on top of the front polarizer 302; formed as a separate film in front of the display screen; and/or incorporated into a display cover which could be attached to the display mechanism. For instance, in the third case, the film could be incorporated into a protective plastic cover, such as a protective sleeve fit over a portable music player. The film could be laminated onto a lens cover or formed through a special process during lens formation, for instance using flow-induced birefringence while injection-molding a poly-carbonate material with a directional control.
Note that addition of the film or plastic cover onto a typical mobile LCD does not adversely affect display luminance or image quality under normal (e.g. without polarized sunglasses) viewing conditions.
Apple's patent FIG. 4 illustrates perceived LCD brightness as a function of the angle between the display and the polarized filter transmission direction 404, with the perceived brightness normalized to that of an LCD without a retardation film viewed without sunglasses. When viewed through a polarized filter without a retardation film 406, the perceived display brightness 402 varies widely, e.g. from no light received to full brightness on this angle, depending on this angle. When viewed through a polarized filter with a retardation film 406, the perceived display brightness is substantially regular across the range of angles.
In summary, the present invention uses a retardation film to circularly polarize light emitted by a display that emits linearly-polarized light. By circularly polarizing light, the retardation film allows the display to be viewed at different angles through polarized filters, such as those found in polarizing sunglasses, without perceived distortion.
Apple credits John Zhong, Wei Chen Cheng Chen, Victor Yin and Shawn Gettemy as the inventors of Granted Patent 7,683,983, originally filed in Q3 2006.
Authors Side Note
At the end of the day, Apple's retardation film solution patent may have in fact provided us with a basic understanding of why Apple moved to ban protective films from the Apple Store last week.
We clearly see that the science behind this retardation film process is so precise - that by adding a third party protective overlay on top of their film may in fact be creating an unexpected adverse affect that Apple went to great lengths to negate.
Whether Apple is working with third party developers behind the scenes to correct this issue or is developing their own solution or forever banning such protective film is unknown at this time. In the meantime, the patent certainly provides us with some form of educated explanation and rationale behind Apple's recent actions.
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