Epson has three sensors of interest. Call them special features. Two 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! The auto sensor isn't a feature I normally think of as really useful on home projectors, but as it turns out, it can be.
For example, as mentioned, I have a similarly bright Epson in my living room - a G6550U. Bright day viewing is very different from night viewing with the lights on. So I'm used to watching at full power on the lamp in the daytime, and on very bright days at the brightest time in the afternoon, I might be in Dynamic mode - basically just shy of 5000 lumens. But let's assume I'm using a mode doing roughly 3500 lumens at full power. In the evening with lights on, I normally run Theater mode in Eco - almost 40% or so drop in brightness. With the sensor engaged, come evening, the sensor would lower overall brightness enough that I typically wouldn't have to pick up the remote to manually adjust.