An Overview of the Problem of Chromaticity Variations

While LEDs retain several advantages over past technologies such as cold cathode fluorescent lamps (CCFLs), due to the manner in which certain LEDs are fabricated, particularly phosphor-coated LEDs, chromaticity variations may be present in the light emitted from an LED over a range of angles (e.g., a "viewing angle") relative to an optical axis. In certain backlight units, such as edge-lit backlight units, these variations in chromaticity may negatively affect the color uniformity of the light emitted from a light guide of the backlight unit, such as by causing a color shift along an axis of the backlight unit.

As such, it may be beneficial to provide a technique for reducing color shift caused by chromaticity variations of LED lighting, thereby improving color uniformity in LCD displays that utilize LEDs as a light source.

Apple's Technique for Reducing Chromaticity

Apple's patent generally relates to an edge-lit backlight unit for a display device, such as an LCD display. In one embodiment, the edge-lit backlight unit includes a light guide configured to receive light from a light source along a first lateral edge and to propagate the received light towards a second opposite lateral edge.

The backlight unit may include a light-extracting layer having multiple light-extracting elements configured to extract a portion of the propagating light and to allow the remaining portion to reach the second lateral edge. For instance, the light-extracting layer may generally be understood to be an arrangement of the light-extracting elements and may be formed on a rear surface of the light guide. The light-extracting surface area provided by the light-extracting elements on the rear surface may be less relative to the arrangement of light-extracting elements on rear surfaces of light guides in conventional backlight units.

In one embodiment, a specular reflector disposed at the second lateral edge may cause retro-propagation of the light reaching the second lateral edge back towards the first lateral edge. The retro-propagating light may be extracted by the multiple light-extracting elements and mixed with the extracted propagating light to provide improved color uniformity along an axis of the display. In certain embodiments, the retro-propagating light may be between approximately 5 to 35 percent of the total light received by the light guide.

Overview of a Backlit LED Display

Following along in Apple's patent FIG. 4 representing an example of an LCD display (noted collectively as patent point 34) – we see that it includes an LCD panel (46) and a backlight unit (48) which may be assembled within a frame or enclosure (50).

The LCD panel may include numerous pixels configured to selectively modulate the amount and color of light passing from the backlight unit through the LCD panel. For example, the LCD panel may include a liquid crystal layer, one or more thin film transistor layers configured to control orientation of liquid crystals of the liquid crystal layer via an electric field, and polarizing films, which cooperate to enable the LCD panel to control the amount of light emitted by each pixel. The LCD panel may be an in-plane switching (IPS) panel (which we know Apple uses), a fringe-field switching (FFS) panel or other variant not revealed.

The All Important Light Guide

Throughout Apple's patent claims, the "light guide" is one of the key attributes of Apple's backlighting system and therefore deserves a little more attention to its detailing.

In the illustrated embodiment, the backlight unit is depicted as being an edge-lit backlight unit, and includes a light guide (52), such as a light guiding plate, one or more optical films (54), such as one or more brightness enhancement films, and a light source (56) having a number of lighting elements (58), such as LEDs.

A Light or Air Guide

As shown, the light source is positioned to provide light to a lateral edge (60) of the light guide. In certain embodiments, the light guide may be formed using polymethyl-methacrylate ("PMMA"), an acrylic glass. Further, in some embodiments, the light guide may be an air guide (e.g., rather than using a material like PMMA, the propagation medium within the light guide may be air.

Light from the light source generally propagates through the light guide towards an opposite lateral edge via total internal reflection at forward (64) and rear surfaces (66) of the light guide and may ultimately be emitted therefrom towards the optical film(s) and LCD panel.

Chromaticity Variations Cause Undesirable Color Shift

Due to the nature in which certain LEDs are fabricated, chromaticity variations may exist in the light emitted therefrom. Accordingly, patent FIGS. 5, 8, and 9 shown below are provided herein to more clearly illustrate how such chromaticity variations may result in an undesirable color shift being present along an axis of a backlight unit – along with Apple's solution (in FIG. 10).

Apple's patent FIG. 5 is a cross-sectional view showing the operation of a light-emitting diode (LED); FIG. 8 is a simplified cross-sectional view of an edge-lit backlight unit and depicts how the propagation of light emitted from the LED of FIG. 5 may result in color shift along an axis of the edge-lit backlight unit; FIG. 9 depicts a rear surface of the edge-lit backlight unit shown in FIG. 8; FIG. 10 is a simplified cross-sectional view of an edge-lit backlight unit configured to provide for retro-propagation of light emitted from the LED of FIG. 5 to reduce color shift along an axis of the edge-lit backlight unit.

As you could clearly see by comparing Apple's patent FIG. 8 to FIG. 10 that Apple's proposed solution will be able to add a lot more light to the display by adding a unique "Speculator Reflector" as indicated in FIG. 10. The additional light captured in the backlight unit should nullify Chromaticity Variations. 

Apple credits their Sr. Optical Display Engineer Chenhua You and Shengminas Wang as the inventors of patent application 20110090142. Believe it or not, the patent was only filed on February 22, 2010 or two months ago.

So Why Does this Matter?

So why does it matter if Apple improves on their already superior backlit LED Displays? Because Apple is spending a lot of research dollars on advancing the camera technology that is going into both iOS and OS X hardware. The cameras in turn support Apple' strategically important applications such as FaceTime, iPhoto, Aperture, iMovie, and Final Cut.

And let's not kid ourselves here: Apple is eyeing the television market. There are a number of patents to support this position and the noted patent above reminds us of this fact once again ever so subtly. In Apple's patent-point number 004 Apple states that LED's are for use in "a wide variety of electronic devices, including such consumer electronics as televisions, computers, and handheld devices…" So while an Apple branded television may be a little down the road, to be sure, the fact remains that Apple keeps on improving their own display technologies to outshine the competition on multiple fronts for current and future applications.

Additionally, Apple belted out seven important patents yesterday dealing with a number of quality refinements to such software as FaceTime video conferencing under patent 20110090303 (the'303 patent). The remaining patents dealt with sharpening image data and image signal processors: 20110090242, 20110090370, 20110090371, 20110090381 and 20110091101.

In Apple's '303 patent, they even described a set of one or more warp parameters that would mathematically represent a transformation of an image from the perspective at which it was captured to a target perspective. They clarify that as meaning transforming "an image with respect to one, two, or three dimensions." The topic of 3D imaging is one that Apple has discussed in serveral patents over the last few years covering everything from avatars to personalized shopping avatars to coming 3D internet experiences to next generation interfaces and even OS X in 3D. Is that reason enough to want even brighter and sharper displays? 

While OLED based displays are likely to play some future role in Apple's iMac or Cinema Display lineups – in the shorter term, advancing backlit LED displays is likely the way that Apple will go.

Just when we thought it was impossible for computer displays to get any brighter, the Crazy Ones in Cupertino have once again turned it up a notch. Yes, that's brilliant in more ways than one.

Random Patent Figures from Apple's Sharpening Image Data Patent

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Also see Related Material: (1)  Is Apple Preparing for a Next Generation Display Standard? This discusses Apple's patent titled "Edge-lit Backlight Unit with Thin Profile." (2) Apple Patent May Point to LED iMac's Later this Year - Apple's first LED patent.  

Update: Just in - Samsung now Sues Apple. See an overview here.