Epson has three sensors of interest. Call them special features. both affect brightness, so are discussed here.
The first is more of image processing - rather than a physical sensor. That is the Auto mode on the Eco menu. I just mentioned that as a way of maintaining maximum brightness but able to save energy when the image content itself isn't a very bright one.
The other two though are sensors. The first located on the front of the projector enables the projectors Auto Fit (so the image does not overshoot the screen) and focus assist functions.
The other sensor is located on the top of the projector behind the lens focus and zoom controls. This sensor faces up, and measures the brightness of the room. When engaged, it will adjust the brightness of the projector accordingly. If the room is relatively dark, it will lower the brightness of the projector. If the room measures very bright, it will not.
Sensors used for adjusting brightness have been around for probably close to a decade. They do work, but I usually wonder if they are really needed, or more of a marketing device. You know: "sounds great - maybe I should get a projector with this capability, just in case I decide I will need it in the future?" Well, this Epson has it, and it provides another tool in the Epson's energy management and control capabilities. My point though, is that if it didn't have this feature, I can't really imagine anyone deciding to choose a different projector simply because this feature wasn't available!