All posts tagged smartphones

Why PenTile RGBW Improves LCD Efficiency

This blog is being written in response to some misinformation that I saw at AndroidForums today.

Novox77 wrote:

…They (PenTile displays) use less energy because if you look at the surface area that the subpixels cover, it’s less than a standard square matrix. In other words, the black space between subpixels is larger. That is where your power saving comes in, and that’s also why we perceive the screendoor effect.

Which means a PenTile screen is less bright than a square matrix screen. You could compensate by increasing the max brightness of the sub-pixels, but that would nullify the power savings.

This is classic energy-balance. You can’t have brighter AND more power saving, unless you’ve achieved a fundamental efficiency somewhere else, and in the case of PenTile, there’s no difference in efficiency; only the reduction of subpixel size…

Novox77 did pretty well in the past at explaining PenTile OLED screen door effect.  However, in his recent comment  #51,  he had his information confused.  Candice Elliott, Nouvoyance’s CEO, has posted a correction comment at this site as #65, but I would like to restate some of the key points here and add a few points as well.

OLEDs get their power savings by only lighting up what you want to see, and not an entire backlight.  That is true for RGB stripe as well as PenTile.  Power savings is not the primary value proposition for PenTile when applied to OLEDs.  PenTile RGBG has it principal value in extending life by reducing current density, also allowing manufacturers to drive it harder for improved brightness without damaging the panel.

This blog is For LCDs PenTile RGBW improves power efficiency by about 2X in three different ways:

  1. By eliminating one-third of the subpixels each subpixel can be one-third larger.  Opaque portions like the FETs and black matrix border at each subpixel stay the same size for a given backplane technology, but the aperture area increases.  It is like a window screen with a coarser mesh, letting more of the light pass through from the backlight.  Note that display makers can improve the width of the black matrix grid by going to low temperature polysilicon (LTPS), continuous grain silicon (CGS) or high aperture ratio amorphous silicon (aSi) backplanes, which all help to improve light transmissivity, but even these technology have their limitations at high dpi which can be improved though the use of PenTile.  I like to say that nobody is ever too thin or too rich, and likewise display backplanes are never too transmissive.
  2. Clear/white subpixels have no color filters to block light.  With typical image information containing so much white and pastel color, this has a very significant effect of increasing the overall transmissivity of the panel.
  3. Dynamic Backlight Control (DBLC) allows the backlight to be turned down while increasing panel transmissivity for suitable images.  Unlike the conventional global dimming of RGB stripe this is also used to maintain the look of fully saturated colors like yellow to avoid simultaneous contrast error.  It leads to less clipping artifact than typical RGB stripe global dimming since white is never clipped, even in aggressive dimming modes.

The combination of these three factors allows display makers to typically double the light throughput of their LCDs.  This will continue toward even greater savings as resolution presses beyond 300 dpi, primarily due to the effect of #1 above.    You can use the benefit to light transmissivity to either double the brightness, double the battery life or a combination of both benefits.

So to summarize:

  1. The black matrix between the pixels is the same as for the equivalent backplane process used for RGB stripe, but accounts for less percentage space in PenTile RGBW LCDs.
  2. Almost every implementation of PenTile leads to a display that is brighter and has higher contrast than the equivalent RGB stripe LCD.
  3. You can achieve more brightness and more power savings because you have implemented a fundamental change in the display structure.
  4. There is about a 2X improvement in display efficiency for typical usage models with display of 250dpi or higher.

PenTile may look different than RGB stripe panels if you look very closely at fully saturated colors on black, or conversely, but when it comes to saving power it is unbeatable.  It is also faithful for rendering even small, single stroke black and white text.


PenTile Displays Now in Space

According to a post by IKing201 and an article by Eric Zeman of InformationWeek, from amongst the many things being carried into space for the last space shuttle flight, there were three Samsung Nexus One smartphones.

These smartphones will be added to the SPHERES (Synchronized Position Hold, Engage, Reorient, Experimental Satellites), which the astronauts aboard the ISS use daily to perform a wide number of tasks.  As you may recall, Nexus One smartphones use using Samsung’s PenTile OLED technology, so it would seem that now PenTile technology has made it into space.

Motorola Battery Indicator- Really a Color Error?

While I do not have the time to respond to everything that is written about PenTile technology, occasionally I will see something that is not quite right which is then picked up by other blogs and gets a life of its own. This can happen from a single photograph. Perhaps you have seen one or more recent blogs that claim that PenTile cannot render color correctly and then go on to illustrate this using the same badly focused photo.

Here is one that appeared today from

As indicated by the logo, this photo seems to have originated with XDA.CN.

Several bloggers have shown this photo of a Motorola smartphone display stating that this proves that PenTile RGBW displays in Motorola’s phones exhibit this type of color error. I would be the first to agree that photographing high resolution displays is very challenging, especially at such high magnification. It is easy to overexpose the image, as was done here. Overexposure can cause color shifts, because the elements in the camera’s CCD of the proper color will become saturated and can no longer respond. Only at the right hand edge, where there is less light due to blur, did the camera capture the correct color.

Battery indicator on Motorola Atrix

With the proper exposure, from the equivalent display in a Motorola Atrix, you can see above that the highly magnified battery icon is rendered with the correct color over the entire area. You will see that there are more red subpixels on the top of this, since it was the intention to shade the green toward yellow at the top. And, here is the same photo, but zoomed in to show just the battery indicator:This photo matches exactly with what I see with my own eyes when looking through an eye loop.

Some of you may say it is different in a Droid X2 or another Motorola phone, but it is the same technology and the same PenTile firmware.

Of course, it was never intended that viewers examine these displays at this magnification, so you will see pattern visibility in these photos that is not apparent to people with normal vision for normal use. Fully saturated green on PenTile RGBW will have a bit of a checkerboard look that can be seen at this magnification. This does not occur with less saturated colors.

Finally, I tried to reproduce the color error by overexposing my photo of this same display and was able to reproduce the effect below. Of course, this will be slightly different for different cameras with different camera CCDs.

Battery Indicator Overexposed - note the yellow center with the green edge.

As you can see for yourself, PenTile RGBW is rendering the colors in this battery icon correctly in Motorola phone displays.