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Comment by ncmncm | Hacker News Reader

Dark mode, for the first time ever, has legitimate, objective reasons to want, now that we have pure LED displays: a black LED pixel consumes no energy.

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That was also the case with CRT displays. If a pixel is to stay black, no electron must be emitted to light up its phosphor.

This Google search frontend has been around long enough to claim it helped save considerable amount of energy on that principle:

http://www.blackle.com/about/

What they report about LEDs and CCFL isn't entirely correct, however. It may become in the future.

Technicalities aside, what once could be regarded as an environmental measure, today is a much more practical battery saving strategy.

CRTs consume so much power that the beam current is negligible. The comparison is with backlit LCD screens, which have to have the backlight on regardless of how many black pixels you have.

Not the case for LCDs with variable brightness backlight regions.

Actually traditional tft displays need to apply a current to turn a pixel black, so if you don't use adaptive brightness that would dim the backlight if the display is mostly black, you actually consume more power with a black theme. I have a low power Pentium notebook with 15" led backlit screen and the difference between a terminal with black or white background is significant.

> I have a low power Pentium notebook with 15" led backlit screen and the difference between a terminal with black or white background is significant.

How significant? Turn the screen black, and show the remaining battery time estimate. Turn it white, do the same. I'm curious.

Surely it isn't extrapolating from current instantaneous power consumption?

I'm pretty sure that's how it works. The drivers report total battery capacity, current battery capacity, and current power draw, so the meters just do the math.

Battery at 74%. Sitting at black screen for about a minute, the highest estimate I got was 4:50, lowest 4:05. It was mostly showing numbers close to either of these, almost never something in between.

Switching to white I got 5:40 and 4:50 showing pretty much the same behavior.

It's an HP pavilion 15-p005ng with a Pentium N3530. Terrible machine btw. Linux freezes in irregular intervals, almost immediately with OpenGL stuff...

https://duckduckgo.com/tty/

https://duckduckgo.com/settings#theme

I remember my high school electrical engineering teacher challenging me to make a project out of comparing ways to save power with CRTs and LCDs. All black vs. all white screen had no measurable difference on my multimeter. I couldn't answer it at the time. I was convinced that it should save power.

I regularly get migraines, where "bright" lights are searingly painful to look at. When the symptoms aren't too bad, dark themes on a text editor are okay, but light themes are not.

Migraines aren't an uncommon problem.

I think that is a fairly objective reason driving the preference.

Conceded.

I dislike sites with dark background. Maybe it is a problem with my poor quality LCD screen, but I switch to reading mode on such sites (so that the background becomes white) because they are hard to read.

I'm pretty much the exact opposite. I typically use my computer/phone in a low light environment (monitor and phone are almost always on lowest brightness settings), and a white background hurts my eyes. I use a dark background wherever possible, especially when reading ebooks/longer articles on my phone.

Any good software designed for reading should have good support for both.

Don't most dark themes actually use dark grey rather than black? The material design dark theme guidelines [0] were posted recently and recommend dark grey as the primary surface colour.

[0] https://material.io/design/color/dark-theme.html#

You must mean OLED. And no, there are almost no desktop OLED monitors in the wild.

"LED display" term is commonly used for LCD displays where backlight is also actively controlled to increase dynamic range.

Backlight control for dynamic range is common, but not necessary for being an “LED display.” It just means the backlight uses LEDs instead of CCFLs like they used to.

Thus the distinction "pure LED" in my previous post.

That there are plenty of deceptively labeled LCDs does not mean there are no actual LED displays.

There is nothing deceptive about LED backlit LCDs, they are an important upgrade over CFL backlit LCDs.

I've seen a fair few TVs with LED backlit screens, advertised as LED displays. Which what I think was meant by "deceptively labeled"

It's a display and it "has LED". Given a clean slate language reboot for today's technology we would probably reserve the term "LED display" to "LED per subpixel" models and call LED LCDs "LED backlit", but the term got into widespread use when the backlit kind was the only one available so that's where we are now.

LCD displays where backlight is actively controlled probably still draw more power when displaying black than white

They are not uncommon on laptops, and quite common on phones. And, I have a couple on my desk at the office.

> a black LED pixel consumes no energy

At last years Android Dev Summit, Google provided some interesting stats on how much power dark mode saves.

[1] https://www.theverge.com/2018/11/8/18076502/google-dark-mode...

[2] https://youtu.be/N_6sPd0Jd3g

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