At last week’s SMPTE Annual Technical Conference and Exhibition, the 3M quantum-dot demo was very colorful. For those who aren’t familiar with this technology, quantum dots are microscopic spheres of semiconductor material that emit visible light when exposed to blue or ultraviolet light due to quantum-mechanical effects.
The wavelength of the emitted light depends on the size of the dots, which are typically less than 20 nanometers in diameter—the smaller the dot, the shorter the wavelength of emitted light. Also, the emitted light occupies a very narrow range of wavelengths, making it almost monochromatic, much like a laser. Finally, the size of the dots can be very precisely controlled during their fabrication, making it possible to “tune” them to emit specific wavelengths.
In the photo above, several vials of liquid with suspended quantum dots are being exposed to ultraviolet light. In response, the material glows with a color that corresponds to the size of the QDs in each vial.
Among other applications, quantum dots are used in the backlight system of LCD TVs. Blue LEDs replace the white ones used in most sets, while red and green quantum dots are placed in front of them. The blue light from the LEDs causes the quantum dots to glow red and green, which, when combined with the blue light that isn’t absorbed by the quantum dots, creates white light that passes through the LCD layer and its color filters to illuminate the image.
This technology can be used in FALD (full-array local-dimming) sets as well as edgelit models. In a FALD set, the LEDs are mounted in an array behind the screen, and the quantum dots are embedded in a film that sits between the LEDs and the diffusion layer, which diffuses the resulting white light evenly across the LCD panel; this is the approach being taken by 3M. In an edgelit set, a thin tube of red and green QD material is placed in front of the LEDs along the edges of the screen, and the resulting white light is directed to the LCD panel as usual in such TVs; QD Vision is emphasizing this approach.
Because the colors emitted by quantum dots are essentially monochromatic, they can be used to create displays capable of reproducing a color gamut very close to BT.2020 and its monochromatic primaries, one of the holy grails of display manufacturers in the age of 4K/UHD TV. At the 2015 SMPTE tech conference, 3M was demonstrating the difference between BT.709 (the current HD color gamut), P3D65 (the digital-cinema gamut with a D65 white point), and BT.2020 on a single display, which was a prototype with FALD backlighting that the company claimed reached 94% of BT.2020.
In the video loop of three undoctored photos below, you can hopefully see the difference in color between BT.709, P3D65, and BT.2020 as the colors become more saturated from one to the next:
TV manufacturers are starting to incorporate quantum dots in their high-end models. For example, the Vizio RS65 uses quantum dots in a FALD configuration, though the RS120 does not because it’s not yet practical to make a QD film that large. All of Samsung’s SUHD models use quantum dots, called Nano Crystals, while TCL has announced QD-based edgelit models, and Hisense showed its version of this technology, called ULED, at the 2015 CES. It will be very interesting to see if QD-illuminated LCD TVs can be a viable alternative to OLED. Perhaps QDs can even become another direct-emissive technology for large-screen televisions—though I’d wager that’s a few years away.