Face ID may be safe, but it's not a time tested technology for now. No one can say it's absolutely safe without long term testing. I just want to point out that people did get eye discomfort from using Samsung's iris scanner, which also uses IR. Apple's technology may be different, but it still uses IR.
Exactly: IR. Infrared. Which inherently is safe.
The only way IR can harm you, just like visible light, would be through sheer power -> heat. So as long as you're not looking into a
very bright IR source...
Humans (like most other living beings) have been exposed to IR every day since literally forever. That's what I'd call time-tested.
I don't know why people are getting eyestrain from Samsungs Iris scanner, but I'd assume that it uses a pretty bright IR illuminator, which you're also required to look directly into.
Face ID however is no brighter than the display.
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Yep. 828p, to be precise.
That number alone gives a wrong impression (828 is just 15 % more than 720), because it ignores the other dimension. While 720p HD and 1080p Full HD imply a 16:9 aspect ratio, that of the iPhone X-ish models is much higher, about 16:7.4, in case of the XR that's 1792 by 828 pixels.
1792 is less than 7 % short of 1920, but 40 % more than 1280. When comparing screen resolutions of different aspect ratio, the only fair way is to multiply both dimensions for the total pixel count.
In total, the XR has
61 % more pixels than 720p HD. And 28.4 % less than 1080p Full HD.