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Ask HN: Do OLED displays only save energy with absolute black?
22 points by fortnum on Dec 20, 2022 | hide | past | favorite | 26 comments
It was always my understanding that OLED displays use less energy the darker the screen is. However I now came across some information that this is actually only true for absolute black (#000000).

Is that correct? Does #010101 use the same energy as #FFFFFF or do darker shades still use less energy?




That’s not true—just a very persistent myth. Your intuition is correct. Anandtech display reviews have actual measurements if you’re interested.

See this chart: https://images.anandtech.com/doci/9394/luminance-curve.png

From this article: https://www.anandtech.com/show/9394/analysing-amoled-power-e...

What’s actually true that most people don’t expect is that OLED displays often use more power than LCD displays even with very dark content.


Thanks, from that I understand the brighter the color the more power it will use. So #010101 will use more than proper black but far less than proper white.

That was my question, thanks a lot


> That’s not true;

Which question are you referring to?


The title.


Interesting article. I was surprised by how linear the luminance-power curves actually are.


Those charts are very old. Modern OLED are much better.


Complicating this is that OLED displays also strongly limit brightness in order to reduce wear, via an automatic brightness limiter. How they do it varies by manufacturer and you'll see different results if only part of the screen is bright versus if all of it is bright.

The only way to get good numbers would be to take several OLED TVs, a few LCD TVs, and measure their power usage at the outlet after calibrating them to be as close as possible, then running through several different test playbacks.


In broad strokes, an OLED turns current into visible light. More current = more light, and more current = more power. So the brighter an individual pixel needs to be, the more current is consumed by the OLED cell. There is a generally linear relationship between the brightness of a display and its power consumption.

In discrete LEDs the datasheets will typically have a table or plot of luminosity (candelas) vs current through the device, and you can figure how much current is needed (and therefore how much power is drawn) to reach a target luminosity. Brightness (nits) is a measure of the luminosity over an area. The tables in the datasheet of an OLED display unit are a bit different but the principle holds.

Now going from RGB color space in the digital world to the actual current draws of individual LED cells is not a 1:1 or even linear relationship, so while you can say in broad strokes that #ffffff will draw more power than #000000 it's not as straightforward for the rest of the color gamut. The reason to keep that in mind is that the "brightness" (or saturation, if you're in HSV instead of HSL) is not equivalent to the physical brightness of the display.


(Warning: I am not a display technology expert)

I'm guessing it's a combination of yes and no. OLED screens save power on black colors because it can completely shut off the pixels responsible for illuminating that portion of the screen. If you have a slightly brighter color than pure black, the diode will turn on but not at full-brightness.

Technically speaking, you probably stand to save the most power on a purely black background. You're still saving power on other dark colors, but it's probably more noticeable with pure black.


I found the following research paper that does seem to support your educated guess:

Energy Aware Color Sets

https://eprints.cs.univie.ac.at/4187/1/eg09_color.pdf


Could you point me in the right direction please? It does talk about OLED, but I couldn't find where it specifically referred to the question here.


My pleasure, hopefully this answers your questions, but if not, I’ll check back to see if you ask a follow up question.

From the “Applications and Results” section towards the end of the paper:

“Figure 9 documents the energy consumption of the three images of Figure 8. Energy consumption is measured according to our energy model from Equation 1, using various grid sizes. The most prominent observation is the substantial energy savings achieved by colors chosen according to our continuous optimization approach. Across the different grid resolutions, we save on average 41 percentage of energy compared to using ColorBrewer colors.”


Thank you very much


The problem is that I find it really hard to read. The contrast is too large for the eyes. Some background color and lighting makes reading more comfortable. Perfect black is good for video though


I'm reading this on an X1 Carbon with the OLED screen. The pure black is sometimes unsetting. Sometimes I'm adding brightness but the black stays black, on the IPS screen the black would've become gray which gives you visual feedback, but on OLED there is no such feedback and you think it doesn't work.

I switched everything to the dark mode, not because I like it, or want to save the battery, but because I afraid that the pixels will burn out.

When the laptop is booting it uses the full brightness, also some bootable Linux distros use the full brightness, as they lacking OLED-aware display drivers.

I also enabled auto hide for Ubuntu 22.04's taskbar, not sure for to do it for the topbar.

I use a very low brightness levels ~ 15%, otherwise the colors are too vivid for my taste.

I think dark mode on OLED somewhat saves the batery, but I'm not sure how much. I didn't measured it.


Thanks, that's exactly what I am wondering

Is #010101 effectively #FFFFFF or does it actually use less energy the darker it is? With LCDs we know it is the former, but what about OLED displays?


The brighter the pixels are, the more power they are using. A pixel uses 0 power at 0 brightness, and it goes up from there.


Well, with LCD it does not matter. It always uses the same power. Hence the question.

So you are confirming that the statement is actually wrong. The darker a pixel on OLED, the less power it uses. Right?


I'm not sure. Considering what I know about OLED, I'd assume that #010101 consumes considerably more energy than #000000 since it needs to emit light for that individual diode. The energy efficiency from there will mostly depend on the brightness/size of the display.


But does it use less power than #FFFFFF?


Yes, but it's efficiency scales with the overall brightness of the display. True OLED black will always consume the same amount of power, regardless of brightness.


The active matrix still has to be powered up with 100% black content.


The engineering answer is to test and measure the actual hardware that is being considered because implementations often vary from theory in significant ways due to the quality of components, the competence of the design team, the design priority of energy saving, the design budget, the generation of technology, and other factors that abide in meat-space.

I mean the display panel A might be 20% more efficient than display panel B, but connected to a power supply that is half as efficient as B's.

Good luck.


This obviously depends on the content you're displaying.

Mind you most "dark themes" in computer programs aren't using 100% black, but some shade of grey or green.


No. They save energy with respect to the brightness. It's like how you use a laptop, and low battery mode dims the screen.


TL;DR: No, OLED displays can still save energy even when they are not displaying absolute black because the individual pixels can be powered individually, instead of employing an always on backlight.




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