Plz help me to find a specific human recognition sensor

Im planning to start a new project I need a sensor which could recognise humans specifically A brief explanation: i need to count how long did every individual person spent in the lecture hall (time entered the lecture hall and time left the lecture hall)

could recognise humans specifically

This subject comes up here once in a while. As far as I know there has not been any reliable answer to the problem that can be applied to an Arduino. The usual solution is computer vision which is outside what any Arduino can do.

RFID tags in everyone's pockets, shop door RFID scanner...

Facial recognition is pretty good, if the TV shows are to be believed. They can get the computer to recognise a face in a reflection off the side of a moving car at night in a standard-definition security video.

Seriously though, it would not be unbelievable to put a camera over the doorway and recognise faces. But someone will be wearing a hat on the way in and not on the way out or maybe they walked out backwards. The reliability of the data will be low, even with big bucks spent. Plus now you have a privacy issue - can you even use cameras in this hypothetical location?

I wonder why you need to do this , can people not just sign in and out ? If you are not trusting them , then any system you install is likely to be able to bypassed by those who want to cheat .