There seems to be no explicit report about the safety of the iris scanner in new Windows Mobile devices. Some users have reported experiencing some eye pain, but they seem to be isolated cases.
I'm not sure how the iris scanner works, but I'm deducing it's using some form of infrared laser, and the process of iris recognition is similar to how barcode scanners in groceries work (although the bar code scanners use visible red lasers). We all have heard about the safety issues lasers have with human eyes. Lasers come in five classes: Class I, II, III, IVA, and IVB. Most laser pointers are Class I and are generally safe for the human eye, i.e. accidental somewhat prolonged exposure will not cause permanent blindness. Even some (slightly) more powerful lasers (equipped with an infrared filter and are optically de-focused) are used in law-enforcement as "photonic disruptors" by temporarily blinding the target. I have a class IVA laser "pointer" that has an output of 1.4W (compared to a typical laser pointer that has an output of LESS than 1mW, or 0.001W) - now this laser really burns, and will result in permanent blindness once shined into an eye. It's somewhat the same laser than can be found in some optical drives.
The conclusion is: check the phone specifications in the phone manual for any signs of a laser class. If it is a Class I infrared laser, then it is safe to use.
I don't own a Lumia 950, so I can't check this for you.
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