Quantum dots are increasingly used in displays to achieve high color gamut at high efficiency. This paper describes the impact of quantum dot lifetime on long-term display performance and the methods used for predicting in-device lifetime from accelerated aging results. Test methods are described which give confidence in aging predictions. Author KeywordsQuantum Dot; LCD; QD; Quantum Dot Enhancement Film; QD lifetime; QD reliability; wide color gamut. Objective and BackgroundQuantum dots (QDs) that are implemented in Liquid Crystal Display (LCD) backlights convert a portion of the high energy blue light from a light emitting diode (LED) source to lower energy red and green light. QD emission wavelength is dependent upon the size of the QD. Because the sizes of QDs can be controlled very precisely and their emission is spectrally narrow, this enables displays with high color gamuts [1] [2] [3]. The relative mix of red and green QDs combined with their location in the backlight determine the display white point. As with LEDs, QDs have a finite lifetime and eventually stop emitting light. In the LCD application, this results in a reduction in luminance and can also result in a change in color if the relative lifetimes between the QDs and LEDs differ significantly.Ultimately, an LCD must maintain a front-of-screen luminance and color specification for the lifetime of the display under normal operating conditions. For televisions, manufacturers typically like to specify lifetimes of 20,000 to 30,000 hours. This represents approximately 2.25 to 3.5 years of continuous operation at so-called "1x" conditions. Testing at these conditions is impractical, so accelerated test methods must be developed, and the results must be translated into "1x" conditions. This paper explains the testing and models used to determine the lifetime of QDs in film for use in LCDs.Two factors dominate the lifetime of the QD based film: temperature and radiant flux.It can be shown that the temperature largely follows an Arrhenius relationship within a reasonable range. The more challenging acceleration component is the acceleration due to flux. Although it may be simplest to think of the amount of incident blue flux when determining the lifetime of the QD-film, this poses many challenges. These challenges are related to the recycling of the backlight unit and the optical path that the blue flux takes. Without fully characterizing these in the accelerated systems and the in-device system a good relationship cannot be established for the blue flux. We have found, both conceptually and practically, it is simpler to think of the amount of converted flux when determining the flux acceleration. This is because the amount of red and green light generated by the QD-film is a direct reflection of the amount of transitions the QDs are undertaking, or put another a way, the amount of work they are performing.Using this understanding we have developed measurement techniques that allow us to directly measure and then estimate the length of time our Q...
Quantum dot technology offers the promise of efficient and relatively inexpensive liquid crystal displays (LCDs) with large color gamuts (~115% NTSC u'v'). Now, for the first time, a variety of high color performance devices are on the immediate horizon for applications ranging from 3" diagonal hand held smart phones to +85" diagonal TVs . While people clearly prefer color displays, the relative value of different color reproduction systems remains an open question. Here we review how quantum dots enable high color performance and some of the advantages of large color gamut displays. Author KeywordsLCDs, quantum dots, QDEF, color, color gamut, NTSC A Brief Overview of Quantum dotsQuantum dots (QDs), molecule-sized spheres of nanosemiconductor materials, are highly efficient phosphor crystals. When pumped with blue light, they emit photons in a narrow spectral distribution with a peak wavelength ( peak ) based on the size of the quantum dot. The narrow spectral distribution and tunable peak wavelength of quantum dots make them ideal for LCD backlight systems in order to produce large color gamuts.[1] [2] The first QD enabled LCD products are coming to market. QD displays have the ability to efficiently express 50 percent more color (~115% NTSC u'v' compared to ~87% NTSC u'v') resulting in color performance approximately equal to that offered by the organic light-emitting diode (OLED) displays. [3] Quantum dots absorb relatively short wavelength light and emit a narrow spectrum of light at longer wavelengths, with the emitted peak wavelength depending on the size of the quantum dot. As examples, a 3nm quantum dot will emit saturated green light ( peak of ~535nm and FWHM of ~30nm) while a 7nm dot will produce saturated red light ( peak of ~630nm and FWHM of ~35nm). By tailoring the size of the dot, the emitted light can be closely tuned to the desired wavelength to within ~1nm.When emitted light from green and red quantum dots are combined with the non-converted blue light, the result is white light with narrow spectral peaks corresponding to the three primary R, G, B colors. When this light passes through color filters (CFs), the subpixels transmit saturated primaries that can be regulated and mixed to create a large color gamut. [2] In conventional displays employing white YAG (yttrium, aluminum and garnet) LEDs the light from the BLU has significant concentrations of non-primary, intermediate wavelengths. This intermediate flux is, at best, absorbed and reduces system efficiency or, at worst, leaks through more than one color filter and reduces color gamut. In other words, the presence of these non-primary colors prevents the LCD from efficiently producing saturated reds and greens. (See Figure 1.) Figure 1. The spectral distribution from a conventional "white" YAG LED that passes through a set of color filters is relatively broad. The spectral distribution from a QD enabled BLU has narrow peaks that can be optimized to the color filters to efficiently produce large color gamuts. Integrating Quantum Dots ...
KeywordsRec. 2020; color; display quality; colorimetry; quantum dots Objective and BackgroundThe International Telecommunications Union (ITU) published recommendations for ultra high definition television aimed at "enhancing visual experience" (ITU-R BT.2246-2 & BT.2020The recommendations include an expanded color reproduction capability; specifically, a trichromatic system with monochromatic (single wavelength) primaries. The wavelengths of the three primaries were selected to best encompass natural colors. Current mass-produced technology is unable to achieve this target. Of the existing technologies, lasers & quantum dots have demonstrated potential to come "close" to the standard. Given the demands of the standard and limitations of current technology, it is useful to ask how close would display primaries have to be considered practically equivalent to the standard?We define practically equivalent as having no perceptible impact on the displayed content. Establishing perceptual equivalence for general-purpose displays is challenging because it depends on the content, the viewer and the task the viewer is performing. For example, differences that are detectable to colorists proofing a single image for distribution may not be detectable to home viewers watching dynamic content. Thus, setting guidelines for qualifying a display as meeting the Rec. 2020 standard should involve some consideration of the display application.In this study, we measured color discrimination thresholds along the boundary of the Rec. 2020 color space. The idea is that thresholds can be used to establish guidelines: as long as a display can present colors whose differences are smaller than the threshold for discrimination along gamut boundary, then that display could be considered practically equivalent to a true Rec. 2020 display. The data we present here serve both as a starting point for discussion on setting guidelines and as a test for the validity of color difference formulae. Our analysis focuses on quantification within 2-D chromaticity metrics in large part by demand from within the industry to establish relatively simple guidelines for qualifying the color gamut of displays.Methods. Eight participants (seven males and one female, age range from 21 to 46) completed the study.We measured discrimination thresholds along the red-green Rec. 2020 boundary using a Mitsubishi L75-A96 75-Inch LaserVue television running at 60Hz and 1920x1080 resolution. The display was controlled using a Windows 7 computer and a 10-bit per color channel NVidia Quadro K-4200 video card. Stimuli were generated and controlled using the Psychtoolbox in Matlab version 2013b.Display calibration. The Mitsubishi L75-A96 has a variety of settings, all of which do different forms of color manipulation of the incoming signal. The "Brilliant" setting with the Color options set to maximum provided the best independence between color channels. The "gamma" curves were monotonic but not smooth (i.e. not a simple input-output relationship). In addition, color m...
Quantum dots have already been adopted in LCDs for higher power efficiency or higher color gamut. In this paper, we report that, by using Quantum Dot Enhancement Film, LCDs are capable of delivering a color gamut that covers more than 90% (CIE 1976) of the Rec. 2020 color gamut, which is the color standard for UHD TV broadcasting. Author KeywordsBacklight, color gamut, display, ITU BT.2020, LCD, quantum dot, Rec. 2020, 3M, Nanosys. Objective and BackgroundA focus of the display industry is to improve the end user's viewing experience. The recent publication of the International Telecommunication Union (ITU) recommendations (ITU-R BT.2246-2 & BT.2020) for ultra high definition television broadcasting is in line with this focus. The recommendations include the ability to broadcast images with a color gamut beyond that which can be shown with existing technology. Quantum dots (QDs) are one technology that has been introduced into liquid crystal displays (LCDs) as a way of increasing their color gamut. In this paper, we examine how close existing QD and LCD technology can get to the ITU's recommended color gamut standard (commonly referred to as Rec. 2020).Quantum dots are a new phosphor material that efficiently converts short wavelength light into higher wavelengths with narrow emission linewidths, in the 30-40nm range. Quantum dots are used with blue LEDs to make practical light sources for LCD displays. Over the past two years, they have seen market adoption in tablets, notebook computers, and televisions. In the case of tablets, quantum dots enabled an accurate Rec. 709 display while improving the power efficiency to yield a longer battery life for the consumer. In notebooks and TVs, quantum dots delivered high color gamuts that cover both Adobe-RGB and DCI-P3 color standards. This paper will briefly review quantum dot technology and discuss how quantum dots are being integrated into LCD systems. Modeling and measurement analysis will show how close current LCD systems can come to achieving full Rec. 2020 color gamut coverage with commercially available quantum dot films and color filters. Further, system optimization around quantum dot performance parameters (peak wavelength and spectral emission width) and color filter performance parameters will be presented to illustrate that near full coverage of Rec. 2020 is attainable with minor modifications. Quantum Dot OverviewQuantum dots convert short-wavelength light (e.g., blue light) to colors spanning the visible spectrum. Unlike conventional phosphor materials, the spectral output of a quantum dot can be controlled to give emission lines with nanometer-level precision. They can also be made with narrow emission full width at half maximum (FWHM). This combination of control over the location and width of the emission spectra allows QDs to produce the saturated colors needed for the Rec. 2020 primaries. The ITU recommends three monochromatic RGB primaries located at 630, 532 and 467nm. QDs can be produced with peaks at these locations and narrow FWHM to a...
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