Why PenTile RGBW Improves LCD Efficiency

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

http://androidforums.com/motorola-photon-4g/374557-photon-lcd-screen-pentile-not-2.html

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 http://pentileblog.com/?p=586 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.http://pentileblog.com/?p=619

 

One comment on “Why PenTile RGBW Improves LCD Efficiency

  1. It seems that your LCD is running, but the back light is not on. Check your brightness control, but it is likely that the LED backlight drivers are not functioning. BTW I do not thing this is a PenTile display as I have yet to see PenTile in a video display.

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