Apple investigating quantum dot-enhanced displays for greater color accuracy

Apple's patent for a "Quantum dot-enhanced display having dichroic filter," published by the U.S. Patent and Trademark Office, offers a highly detailed look at both quantum dot tech and how it can be applied to portable devices like the iPhone.

Quantum dots (QDs) are technically nanocrystals - in some cases smaller than a virus - made from various semiconductor materials that exhibit special quantum mechanical characteristics. Of interest to display makers, as well as other industries in the optics field, are QDs' light-emitting properties. Specifically, the material can be "tuned," or manufactured, to emit very narrow spectrums of light.

According to the filing, the electronic properties of QDs are defined by their size and shape, meaning the colors emitted during excitation can be controlled through fabrication. Nanocrystal size and wavelength emission (more accurately bandgap energy) are inversely related, so the larger the QD, the smaller the emission frequency.

Applied to color, bigger dots will skew toward longer wavelengths like red, while smaller dots emit shorter, higher frequency wavelengths like violet. For example, a cadmium selenide quantum dot may be tuned to gradually reproducing the entire visible spectrum, ranging from approximately 460nm (blue) to 650nm (red). This ability to change color by varying nanocrystal size is unlike current phosphor- or organic-based LEDs, which emit light in a fixed spectrum.

In Apple's invention, as with other proposed implementations, a dichroic filter is disposed on a quantum dot-enhanced film (QDEF) in an red/blue/green (RGB) LCD display.

One embodiment calls for a non-QD light source that emits a blue light be combined with a light emitting layer of two QD groups, one red and one green. The layer, possibly a nano-film, is positioned above the first illumination source, thereby absorbing a portion of its blue light to emit red and green light. A second portion of the blue light is allowed to transmit through the QDEF.

The dichroic filter above the QDEF then reflects some of the transmitted blue light back down to be recycled, while allowing an even smaller portion through as white light.

In the embodiment discussed at length in patent application's body, the display also implements an LCD that controls passage of the aforementioned white light through color filters arranged in subpixels. This would provide extremely accurate color reproduction.

A less confusing way to imagine Apple's invention would be to follow light emitted by the LED backlight unit. First, a normal blue LED emits light that is sent to a light guide panel. The LGP evenly distributes the light to the back of an LCD panel with color filters arranged in subpixels.

At this point, instead of using phosphors to produce red and green light, the QDEF's quantum dots are brought to photoluminescence by the blue light, which consequently produces red and green light. The dichroic filter then reflects some of the blue light transmitted through the QDEF back to the QDEF, thereby increasing excitation and red/green output.

The filing goes on to explain how brightness can be boosted using prisms and other light forming techniques. Also noted in detail are candidate semiconductor materials from which QDs and QDEFs can be fabricated in a non-toxic manner.

It is unclear if or when Apple will use quantum dot technology in its devices, though the company has a history of delivering some of the best displays in the industry. Recently, however, the colors reproduced by the iPad mini with Retina display were found by one study to be less accurate than competing tablets, possibly leaving some room for improvement.

Apple's quantum dot-enhanced display patent application was first filed for in 2012 and credits Chenhua You as its inventor.