On June 9th Wesley Fenlon posted an article at Tested “How PenTile Displays Are Brighter But Not Always Better.” This article was in some ways even more difficult to follow than the one we recently struggled to translate from Russian. The biggest part of the confusion was between PenTile RGBG OLED and PenTile RGBW LCDs as well as the unrelated diversion into metal oxide backplane technology.
Let me begin by differentiating these two implementations of PenTile technology:
PenTile RGBG OLED
- Value proposition: enables OLED to achieve higher resolution with equivalent brightness
- How accomplished: eliminating one-third of the subpixels reallocates the dead space around each subpixel into useful luminous areas, reducing current density and allowing high resolution panels to appear as bright as lower resolution panels without lifetime degradation.
- PenTile algorithms: subpixel rendering SPR, edge sharpening
PenTile RGBW LCD
- Value proposition: improves transmissivity of the LCD
- How accomplished:
- Adding a clear subpixel to pass white light from backlight without significant attenuation
- Eliminating one-third of the subpixels allows each subpixel to be one-third wider and have better aperture ratio
- Using dynamic backlight control (DBLC) allows images to be analyzed to add global dimming, to reduce power to the backlight , while preserving the color of high luminance saturated colors and at the same time increasing the contrast.
- PenTile algorithms: RGB to RGBW gamut mapping, subpixel rendering SPR, edge sharpening , image analysis for dynamic backlight control
Let me add another clarification: RGB stripe can be used in both OLED and LCD. PenTile can be used in both OLED and LCD. Wesley confuses things by comparing a PenTile RGBW LCD to a RGB stripe OLED like the Samsung 4.5-inch Super AMOLED Plus
So Wesley had it right when he said that the PenTile RGBW used in the WQXGA panel had a fourth subpixel (clear or white) to let more light through, “making the screen brighter as it consumes less power”. He also had it right when he said that”… backlight dimming for darker images, the 2560 x 1600 PenTile display clearly has an advantage in power efficiency…”
Wesley says that “PenTile screens have fewer subpixels overall than RGB (stripe) LCD arrays. The pixels are also bigger, which means screens for these display(s) often have to be slightly larger than (RGB stripe) LCDs, hence that 4.5” Super AMOLED Plus on the Samsung Infuse…” Let me correct and clarify this. It is true that for a given diagonal size that the subpixels are larger, than the equivalent RGB stripe panel, however the pixels are equivalent. PenTile displays use logical pixels that contain varying numbers of subpixels with luminance centers equivalent to an RGB stripe panel. RGB stripe displays are comprised of pixels that each have 3 subpixels and are always the same size. This is true for both LCD and OLED. Having fewer and larger subpixels does not cause image quality issues that were supposedly illustrated with pictures in Wesley’s blog and have previously appeared in other blogs. Shown below are images that I took myself from an RGB stripe display and from a PenTile RGBW display. You will see that PenTile images with equivalent number of pixels (not subpixels) as RGB stripe do not suffer from blurriness or lack of crispness (note that some small amount of aliasing/moire on both images is due to the digital nature of such image capture). I did nothing in Photoshop to change the quality of either image other than equivalent scaling to fit the format needed for the blog. In a future blog I will show such a comparison of B&W text to illustrate again that such text is not degraded by PenTile
Samsung has offered WVGA 4.3-inch and 4.5-inch diagonal AMOLED. These are RGB stripe LCDs. Samsung offers WVGA AMOLED that are 4.1-inch and smaller that are PenTile OLED. The reason that PenTile is not offered in these larger sizes is that it would exhibit pattern visibility at such a low dpi and offers less value to these OLED designs. If however OLED were needed in high dpi at these larger diagonal sizes thenPenTile OLED would again have utility.
In the final paragraph of Wesley’s blog he states that “…the RGBW layout, (for) the 2560 x 1600 PenTile does seem to be employing that same improvement…” I am not entirely clear which improvement he is referencing, but since it follows the reference to the Super AMOLED Plus, I think he means the use of RGB stripe. It is true that the 2560 x 1600 panel does not use RGB stripe, but it also is not AMOLED. The reason it does not use RGB stripe is that an amorphous silicon TFT backplane at 300 dpi with today’s state of the art technology would have about a 30% aperture ratio, making it impractical for power consumption for a table application. Perhaps LTPS or metal oxide transitors might improve this aperture ratio, but uniformity of these technologies for 10.1-inch diagonal has yet to be demonstrated in production. Once that technology is proven for this diagonal, it may allow practical tablet applications, but at that point PenTile technology could again be applied for an additional 40% improvement in power efficiency. When comes to saving power, saving more power is always better. I like to say that people are never too thin or too rich and likewise panels never save too much power.
One other small point is that this WQXGA demonstration panel already has a very wide viewing angle technology that is very similar in performance to IPS. PenTile can be combined with any WVA technology for wide viewing angle performance equivalent to that seen on an RGB stripe panel.
- PenTile technology does not create blurry images or black and white text.
- PenTile RGBW LCDs are more power efficient than any other technology in the market.
- PenTile RGBW LCDs can be combined with IPS or any other wide viewing technology.
- PenTile RGBW LCDs are fit for use on tablets due to the demands for high resolution and low power consumption.