Does a black LCD background really conserve battery life?

General Questions
#1

I’ve seen this bandied about for the longest time and never really thought much of it until it came up again today in the description for an iPhone app, so now I’ve just gotta ask.

Does setting a black background on an LCD screen really conserve battery life?

I’m tempted to call shenanigans here because of two rather important factors:

  1. Sending a signal for “black” to the screen still requires that the pixels be turned on just as they would be for any other colour. They’re just sent whatever the amount of current is needed for black.

  2. Black doesn’t turn off parts of the backlighting – it just blocks the light from getting through. Since the backlight is usually the biggest drain on power, setting a black screen would have zero effect here.

The only reason I can think of that setting a black background could conserve power is that maybe the amount current required to twist the crystals to black is less than that required for any other colour – but in such a case, surely the reduced power consumption must be so minor as to be almost meaningless, no?

Anyone know the straight dope?

#2

(1) I know that maintaining an LCD state requires very little power … it’s mostly changing an LCD display that drains power.

(2) I would have to imagine you’re right here. But, not having an iPhone, I’m not aware that this is a common practice. IME, the backlighting is generally turned down on displays (say, laptops) during power-save modes rather than making the screen black

#3

True enough – but whether you’re talking black text on a white background or white on black, you’re changing the same number of pixels, so the power required to change text here remains identical. (Let’s just assume here that we’re talking about reading an eBook for the sake of simplicity.)

This is only during power save mode though – and yes, the iPhone does do this in the same way other portable LCD devices do, which is to dim after X minutes of inactivity. But I’m talking about an active situation where the device being used and is therefore given no reason to enter power save mode; your backlight remains at whatever brightness level you have set for when it’s not being fed AC power. Backlights are are just a set of bright white LEDs (these days, anyway) shining through the edges of a diffusion plate, so you can’t selectively turn off portions of it, therefore all a black background does is block the light from shining through.

#4

I don’t see anything wrong with your reasoning… I guess the bigger question I have is: Is this a claim that a lot of people are making? I’ve never even heard of anyone (before you) suggesting that filling a screen with more black pixels somehow uses less power.

#5

Oh-- I see it now in your OP.

I thought you were talking about setting the ENTIRE SCREEN to black, but not changing the backlighting. I didn’t realize you meant that, say, we make the SDMB web site background black instead of white, but still keep using the site.

Yes, I would also call BS.

#6

Well, yeah, that would work too – the basic assertion being that using more black on an LCD screen reduces power consumption, in effect claiming that, as you mention, inverting the SDMB’s foreground and background colours would somehow consume less power on LCD screens.

However, your first assumption is basically the same premise, too; an all-black screen (but with active backlight as per normal operation) would consume less power than one with only 50% black coverage, which would consume less power than no black at all.

I’ve heard this assertion made frequently with regards to laptops and notebooks, that using a black wallpaper (or a white-on-black colour scheme in general) on your desktop would extend battery life as compared with a bright and/or colourful one.

Just a few examples here, here’s one (an old one) that asserts that drawing more colours takes more power, and [URL=http://forums.macrumors.com/showthread.php?t=16290]here is one that asserts the reverse – that drawing black pixels requires more power.

I think the whole lot of it is bunk. While there may be difference in power consumption in certain cases, they would be so minute as to be meaningless at any practical level.

#7

Well, here’s a guy who says that if Google alone switched to an all-black page (instead of all-white) it would save 750 megawatts a year. Maybe he’s talking about CRT style screens though.

This site pretty much confirms that it is CRTs that they are talking about, not LCDs.

Sounds like someone heard this, didn’t realize that the comment applied to CRTs, not LCDs, and drew the wrong conclusion.

#8

It also applies to plasma displays (at least, it seems to from my experience). My TV uses about 450 Watts with full color/brightness, and only about 300 when displaying a very dark or black scene.

Of course, you won’t find that on an iPhone, but I’m sure there are a few people out there with plasma desktop displays.

#9

The glow of an LCD monitor comes from flourescent tubes backlighting the LCD surface. Therefore, I was under the impression that a fully black screen requires all LCD pixels to be on to block the backlight as completely as possible (some monitors do this better than others.)

#10

This applies to CRTs, plasma displays, and OLEDs. That means it’s practically useless right now.

LCDs (liquid crystal displays) work by modifying the light being passed through them (hence the backlight), so that light has to always be on and always be the same brightness (unless the computer shuts down or partially shuts down, as in powersaving mode).

Here’s how plasma displays work:

So, yes, a black background would save power if you have a plasma display. The bad thing is that practically nobody does.

Finally, CRTs work by sending a beam of electrons to excite phosphors at the front of the screen. No electrons are sent to display black, so a black background saves power with CRTs. Again, relatively few people are still using CRTs and disposing of them is rather tricky because they’re heavy, fragile, and toxic (think Britney Spears on a five-day bender).

LEDs (Light-Emitting Diodes) including OLEDs (Organic LEDs) are really tiny and really efficient light bulbs. A black background would help with them, obviously, but I don’t know how much. I also doubt there are any OLED displays in the wild yet.

#11

Those I can understand, and it is as I suspected: This has nothing to do with LCD screens.

There are quite a few. A drop in the bucket compared to Plasma and LCD, but they’re in the wild. Sony has the XEL-1, their first OLED-based TV. There is also an increasing number of OLED and AMOLED portable devices (mostly PMPs and DAPs) popping up as well – mostly out of Japan as you might expect. Perhaps the biggest-name PMP to sport a sizeable and well-publicized AMOLED screen is the new iRiver Spinn (technically not released 'til Friday, but close enough.)

From what I understand, advances in OLED and its variants (both in finding better crystals to grow them out of and better substrates and sealing systems) have allowed OLEDs that now last long enough to be practical in consumer devices. I don’t know if they’ve solved the fading problems, but I guess these recent spate of consumer OLED-based devices will tell us something eventually.

#12

Your BS suspicions are likely correct, but there are a few possible reasons why black screens could consume less power on tiny devices.

One, manufacturers who were eager to save power could use a scheme where they set the brightest pixels to be completely transparent in the LCD, and reduce the backlighting, so that the brightest pixels were whatever brightness they were supposed to be. That is, do the first pass of throttling on the backlight power. This seems to me a klunky scheme that would be hard to make work right, especially if they used fluorescent lamps as the backlighting source. But on smaller devices they may be using LEDs for the backlight (somebody else may know more on this point).

Two, the smaller the device, the liklier it is that the display is actually an LED display (as others have discussed). A few years ago when I was interested and went looking for them, I only found LED screens on the backs of some digicams. Maybe they’re more common now?

Three, though I doubt it, some designers may turn LCDs on and off rapidly rather than turning them part way on constantly. Perhaps LCDs that work poorly turned part way on are cheaper, for instance. While any of the states of an LCD consume little power, rapidly switching them ought to consume more. If it’s black, it would probably be switching between two identical off settings in this case. Rapidly switching displays, called “strobed”, are very common in some situations, especially LED numerical readouts like old calculators and clocks have. They do this so they can address the different segments of the numbers in a special way that saves wires. If you have ten digits, and they have seven segments each, this method requires 17 wires rather than 70 wires. We can’t see the strobing because it is too fast, but if you stare straight ahead and wave the calculator in front of your eyes you can detect it. I doubt this is done on LCDs because the pixel addressing is so different, but am not sure.