Improving Picture Quality: Beyond the Pixel

August 5, 2014, Emerging Display Technology Conference, San Jose, CA—A panel looked at the other system-level issues for video images. Pete Ludé from Mission Rock Digital moderated the group. Panelists were Julian Carey from Intematix, Veeral Hardev from Nanosys, Seth Coe-Sullivan fro QD Vision, Jimmy Kim from NPD DisplaySearch, and Roland Vlaicu from Dolby.

Ludé stated that technology migrates from cinema to broadcast. In ’05, most theaters in the US converted to 4k digital projection, but suffered from the many versions in place. Viewers can distinguish the difference between 2k and 4k in any setting as a result of the retina physiology. Sensing the advantage of 4k depend on the individual’s visual acuity, ambient light, depth of field, motion and blur, and context. The optics and computer and image processing can affect all of the image parameters.

Spatial resolution averages about 60 cycles per degree, the various resolutions on a Snellen eye chart, with 20/20 vision at 30 cycles per degree. For a 4k image, the optimum viewing distance is 1.5 screen height or about 3 feet for a 60 inch TV. In comparison, an HD image is best viewed at 3 screen heights, so an HD and UHD image would be roughly equivalent for viewing distances of 1.5 height.

In many movies, the added resolution doesn’t matter, since most CGI are rendered at 2k. Beyond spatial resolution are HDR and HFR technologies that come closer to real life in terms of temporal resolution. Screen brightness needs to increase to accommodate the high-dynamic range images as even relatively dim objects like the moon measure 3000 nits (candelas per m2). HDR images are usually in the 0.005 to 3000 nit range so the displays need at least that brightness to handle the full dynamic range.

The eye can detect 60 Hz flicker so the displays need to be run at a minimum of 120 Hz. Color gamut is on a display is much smaller than the eye capability and the new standards are trying to address the discrepancies.

In addition to better displays, sound is changing from simple stereo to object-based high-order acoustics and wavefield synthesis. The SMPTE UHDTV ecosystem repost details the evolution and also indicates the directions for next-generation standards like ATSC 3.0. Many other standards need to be put in place for 4k t become viable.

Vlaicu agreed with the necessity for changes and the fact that improved picture quality is not just spatial resolution. Brightness has to increase, since the eye has a total dynamic range of over 109 nits within sub ranges. Most displays today are only capable of a range of 0.01 to 100 nits. Wider dynamic range enables better specular highlights and more detail in the dark areas.

Test show that most users prefer much brighter images with total intensities over 200 times that currently available. The problem with low intensity and dynamic range is that the images have to be compressed to 8 bits to fit into the useable range. It is possible to get a TV with a wider dynamic range, but the rest of the ecosystem doesn’t exist.

Really good 4k TV needs to have HDR, HFR, and wider color gamut to approach the eye’s capabilities. In addition, the industry needs to develop the complete content pipeline to start with a high-quality master image that is re-mapped to the parameters available on the end device.

Kim continued on the color issue with comments on quantum dots. The typical new light source is matched to the color filter absorption windows. The color filters have to be designed for minimal overlap and matched to the light sources with no extra emission peaks to reduce color missing at the filter tails. For example, AMOLED uses separate RGB to achieve 100 percent of NTSC, but at low levels the colors bleed due to leakage.

An alternative is to use quantum dots to re-emit colors with the proper excitement light. The colors can be tuned and are fairly narrow bandwidth. The technology is drawing more new companies into the field.

The challenges for wide color LCD are reliability, cost, and the need for a color sensor to adjust the drive to the different colors. Even with better color filters, the colors are transformed at the broadcast and aliased to the 8-bit standard. The combination of AMOLED for backlight and LCD is too expensive. High efficiency displays can use pure color to enable thinner color filters. A cost analysis shows that luminance efficiency is improved with thinner color filters, and this translates to higher power efficiency.

The estimated market for quantum dots is about one third of the LCDs and 10 percent of the AMOLED displays. Adding quantum dots and using thinner color filters is cost competitive to other display technologies. The market is for TVs and tablets, although other apps are possible. The potential for better color and energy savings due to the quantum dots makes this technology a good potential replacement for AMOLED displays.

Consumers want better visual experiences since the average user has 5 devices and looks at them about 4.4 hours a day. A better user experience sells and color can be a differentiator. Picture quality using customizable solutions of LCD and quantum dots can have the lowest energy of any display. The quantum dots being tunable light emitting transistors can enable all standards for color.

The materials are highly reproducible an can be used in various form factors including optic edge, film, and on chip. Current focus is on the large LCD for TVs with Sony, TPV, and BDE making Adobe RBG capable monitors. The next market will be in all-in-one computers as standards and competition drive the market adoption. The full gamut enables accuracy and colorfulness and can be downgraded to other standards.

Coe-Sullivan added more on quantum dots. His company makes a high-efficiency film with spectrum engineering to achieve less than 30nm bandwidths and 95 percent quantum efficiency. The key is to put energy into the areas of the spectrum where it is needed and have little light in the filter crossovers.

Hardev talked about making compelling and realistic color that is much greater than the sRBG standard which only addresses one third of detectable colors. Other technologies use a white light and red and green phosphors to generate the full spectrum, but quantum dots have much greater efficiency. The market forecasts indicate quantum dots growing at a CAGR of 125 percent and should overtake OLED in ’15.

Carey offered an alternative to quantum dots with narrow-band phosphors. These materials are used in LED components as remote phosphors and part of the optical delivery systems. Most phosphors are used in lighting to change the blue light to more white. The problem is that the broadband phosphors are lower efficiency and have high losses in the color filters in displays. The broadband red and green show the greatest losses and both have extra intensity spikes outside their nominal emission bands.

In comparison, narrow band phosphors can have a 10nm spectral envelope and can be mapped to the transmission and rejection band of the filters, which allows a 30 percent higher transmission. The materials can be integrated into existing process flows with no changes for the red colors. The green materials need more work because it has a 25nm bandwidth at 520nm peak. The biggest problem is that the phosphors need to be excited at 365nm, in the ultraviolet range. Future work will improve efficiency and reliability while changing the excitation to a common blue light from the UV.