I don't no if this would work for you but I used a CCD camera It has 10 IR leds so it can see at night.
This is what these things can do It will output a voltage change when you move, but it's read able you could tell what is in front of it.
It could do what you want I hooked the power to 5 volts and used ADC to read the video and you get movement readings
that change with size and heat.
And there repeatable