Lights and switches using radar have been around for a long time. Human presence radar detector switches are common in car seats. Light bulbs that turn on using microwave radar to detection human presence are cheap on eBay:<p><a href="https://www.ebay.com/itm/255302776447" rel="nofollow">https://www.ebay.com/itm/255302776447</a><p>I'm not sure why they are not more commonly used. I have a few of the radar bulbs and they do turn off if motion is not present, so they are similar to PIR sensors. However they will turn on a light that is behind a door (such as in a stairwell or closet) before I even open the door. Maybe that is considered wasteful of electricity even though it is turning on right before it is most useful? Maybe they trigger too easily through walls?