I just realized that the display on my 2012 Macbook Air is transparent. I had the screen brightness of the built in display on the lowest (off) while using an external monitor, and the sun came shining in on the back of the laptop, lighting up the "apple logo" part of the display - and I realized that I could see my desktop background there!
The picture was actually quite clear and had proper colors and everything. This led me to think that if somebody wanted to, they could create a transparent display by just putting clear glass or plastic behind it instead of black plastic or aluminium. I'm sure that with some engineering, some sort of solution that simply reflects sunlight or ambient light onto the back of the display could be constructed.
Of course it should be combined with a cover of some sort so that you can use it as a traditional display as well, but when out and about and on battery, it could be useful to be able to turn off the background light on the display and use it.
Given that people want their laptops and smartphones to have limitless battery life, how come this is not a thing? Is it mostly aesthetics? Or are there actual technical problems doing this? If my 6 year old laptop can do it, it can't be that difficult, right?
To clarify, I'm not asking why manufacturers don't sell laptops with see-through displays or heads-up-displays. I'm asking why they're not inventing some clever way to utilize the ambient light when possible and desirable. It could be as simple as a fold-down back cover on the display, which could reflect sunlight into the display if folded down or just act as a normal display back cover when folded up. If I'm out in the sun with my laptop running low on battery, wouldn't it make sense to use free ambient backlight instead of wasting battery life on illuminating the display?