Ask HN: Can we make a computer screen light in color when turned off? | Hacker News Reader

Ask HN: Can we make a computer screen light in color when turned off?

Or is keeping displays on all the time the only alternative to black rectangles?

There's one problem and that's how our eyes work. They need photons to enter them to be able to see. The photons are being "reemitted" by the body we look at. The body absorbs the light shining on it and just reemitting the photons that aren't absorbed by the body's surface. So, actually, what we perceive as color are photons that haven't been absorbed by the body.

Black screen means it absorbs everything. Because it's a translucent body consisting of a few layers, the light simply gets trapped within the layers and is not reemitted into our eyes. We perceive that as black.

If you want to make it whatever color, you need a not translucent screen, so the photons are partly absorbed and partly reemitted. Then we have some photons in our eyes and sensations that can be interpreted by our brain as information/color/whatever.

The screens of e-ink utilize that. But they're slow.

So to answer your question, no it's not possible to make ordinary screens to have a color, except there's a solid body in front of the layers to absorb and reemitt the photons.

To build a screen like that, one would need a backlight. No matter where it comes from. Or, inverted LCD on a white plane that alters the refraction when there's a current flowing. That would consume more energy because all the pixels need to be constantly powered (and after powering off, they would automatically alter their refraction..)

So, no..

What about constantly low powered screens like Samsung's the frame?

If you just want a different aesthetic when your system is powered down, you don't need to change the screen itself - just drape a piece of cloth over it.

Your typical e-ink screen will be white when turned off. I think transflective LCDs too, or alternative display techs like https://daylightcomputer.com/

You can get monitors using those technologies if you don't want it to be black when off.

Didn't search for it, but there was a young man (backed by his parents) who regularly got eye irritation while staring at a screen. So that guy just pulled the backlight and started to sell it on the webz as eye friendly screen. Guess, didn't start off like a rocket. It's been one or two years ago here on hackernews.

Thanks for the detailed reply.

I imagine a black back on the screen improves contrast when it is on, as well.

Guess I’ll throw a nice colored cloth over the screen when it is off for now :)

I remember those silver-gray displays in old calculators fondly, and love the word “transflective”!

Thanks for the Daylight Computer link!

I didn't come up with the word transflective BTW. It's just what the industry calls them, and you can actually buy them from niche vendors.

I mean, the absence of light is darkness, and light requires energy. That's why screens are black when they're off -- there's no energy source. The alternative would be constant energy drain for no reason, or something like e-ink (but that's expensive and afaik incapable of producing color).

Not all displays default to blackness when there's no power... just think of a pocket calculator or old-school black and white LCD watch, for example.

(And also, there are color e-ink devices in the wild now, like https://us.kobobooks.com/products/kobo-libra-colour or https://remarkable.com/store/remarkable-paper/pro)

Thanks for the links!

I’d love a display that let me pick the color or picture showing on the screen when it is off, or maybe it’s the wrong question and power-efficient things with solar panels directing sun energy to batteries that keep them going on all the time is the way.

My black and white Kobo displays a book cover when it's asleep (with eink, the display doesn't use any power unless it's redrawing, so once a picture is up there, it stays there even if the battery dies). But I don't know what the color ones do, like if they let you choose a background color or if there are even color ebooks yet. Presumably you could make your own color ebook with Calibre and set that as your background cover, but I haven't tried it.

To make them go white, they actually have to use power to clear the image. If you just let it sit there and let the battery die, the image will be there until you recharge it and turn it back on.

E ink is really cool tech (just patented and expensive). But the daylight computer from the other comment might be more what you're looking for.

And yeah you can totally run a solar setup. Anker makes a bunch, for example, of portable li ion batteries that can be directly connected to their solar panels.

Edit: If what you want is a digital billboard or picture frame, there might be some of those out there too, or I think I saw some DIY projects building those out of eink panels and an Arduino or such. Some of them get commercialized on Kickstarter.

Thanks for the info and ideas! I’ll look these up.

I’m excited for the future with solar, battery, and really most electronic technology becoming cheaper and simpler.

With the right set of engineering requirements, yes. But typical engineering requirements preclude it because typically opposed features are more important for a display.

And for some use cases, placing a sheet of white paper over the display might be good enough. Good luck.

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