Ask HN: Do OLED displays only save energy with absolute black?

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.

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.

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.