How does reducing brightness of screen increase battery life?

Question

For all I have heard, reducing the screen brightness of your phone can increase your battery life. Just about every expert in the field agrees to this statement. But today I came across this article which kind of raised quite a few questions in my mind. The excerpt from the article in question here is:

All these screen dimming/color cast applications function in essentially the same way: by overlaying a graphic on the screen to reduce the brightness and/or change the color cast of the screen. Think of it like adding a partially opaque layer to an image in Photoshop. When you tell the Lux application, for example, that you want the screen 50 percent dimmer than the actual hardware in the phone can provide via LED adjustments, the application essentially cheats by layering a gray mask over the screen that decreases the brightness because the screen elements are darker. Other apps like Screen Adjuster, Darker, Easy Eye, Twilight, and even the brightness adjustment function in popular battery-saving app JuiceDefender all work the same way.

1. So if the screen brightness applications (like Lux) only add a partially opaque layer they are essentially not dimming the backlight (correct me if I am wrong here) so in turn it would not have any affect in battery life. Right?

2. The article continues saying – Anything that layers something over the screen in anyway disables the “Install” button as the button is rendered unclickable in order to prevent malicious software from creating a false overlay that leads the user to think an application has a different set of permissions or that the application is an entirely different app altogether. So does reducing the screen brightness via Settings uses some other technique (probably it actually reduces the backlight)?

Answer 1

I think you've got to differentiate between "software-layers" and "hardware-layers" in the first place. If you put a "display foil" on top of the screen, that will certainly not reduce battery consumption (though it "dims" your display). But if it's a software layer reducing brightness, that's something completely different: How should that be achieved other than reducing the "amount of light created"? It's not a "physical layer on top", though it kind-of works like that when it comes to LCDs.¹

I have no means of proving it "physically", but here are two ways I can imagine²:

• reducing light intensity of the "light generators" (LEDs for OLED, or backlight for LCD) by feeding less power to them – which would work for both types of displays, and is the way the actual brightness control works
• altering the color values (e.g. convering "#FFFFFF" to "#DDDDDD", making "white" a "light gray") – primarily working for OLED to "safe power", but in theory also for displays using differently colored LEDs (or LED arrays) to compose the color, if such exist³

As Dan put it¹: Using a filter/overlay like this will make either type of screen look darker, but it'll only reduce power use on OLED screens. On LCD screens, only decreasing the backlight brightness saves power.

Apart from that, your quotes nowhere state the filters would not affect power consumption in either way.

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¹ Following up a discussion in chat between Dan and me, on LCDs it in fact is similar; quoting Dan: Using a software overlay to darken pixels on an LCD screen won't reduce the power consumption [… which works] by putting a transparent grey full-screen window on top of other windows; that's more likely to increase overall power consumption, because you're giving the window compositor more work to do. Doesn't contradict whith what I wrote, but gives more insight in "applying the right filter to the wrong display type": The same amount of "backlight" is generated, just the crystals (LCD = Liquid Crystal Display) are darker.

² There might be other possibilities which escaped me, so I don't claim the list to be complete. It should give some insight nevertheless.

³ There are differently colored LEDs at least for Red, Green, and Blue (read: RGB, so such a combination would cover the full color spectrum), making displays like this potentially possible (note I didn't say they already exist!); different colors feature different efficiency – so "red-ifying" the screen would safe power (red-orange LEDs feature the most lumen-per-Watt according to Wikipedia).

Answer 2

Even if that is true, that would not apply to all screens such as an (AM)OLED screen that doesn't use a backlight.

From Wikipedia article on amoled:

The amount of power the display consumes varies significantly depending on the colour and brightness shown. As an example, one commercial QVGA OLED display consumes 0.3 watts while showing white text on a black background, but more than 0.7 watts showing black text on a white background, while an LCD may consume only a constant 0.35 watts regardless of what is being shown on screen."

Also read the claims on the apps themselves, most don't even claim to effect battery life on other screens

from "Screen Filter" app:

Applies a shade that acts as a dimmer to ensure your eyes don't hurt. Far more powerful than Android's built-in brightness setting. Great for low-light gaming, web browsing, and eBook reading. It even saves battery life for AMOLED displays!

From "Night Mode" app:

• If you have an AMOLED display, you can also save battery!

Answer 3

This is an old question, but aside from Dan Hulme's comments it doesn't really have a clear and correct answer.

To the questions in the OP: Yes to both, if you're using a backlit display.

There are two very different types of screens in phones: backlit and non-backlit.

LCD displays are backlit. The backlight brightness is the main factor in screen power consumption, and the image shown on screen doesn't matter. The phone's brightness setting controls the backlight.

OLED displays are non-backlit, as the OLEDs themselves emit light and don't need a separate backlight. An OLED's color output determines its power consumption. The phone's brightness setting is a multiplier for each OLED's brightness.

A screen dimming application puts a transparent black image on top of the user interface, which darkens the color of the display output. If there's a backlight, it won't be dimmed and the power consumption won't change. With an OLED display, the change in color reduces power consumption.