Sensor to rapidly sample light intensity of a small area at a distance?
Buy a cheap webcam with a screw-thread mount for the lens. Cover the lens with something opaque like aluminum foil, leaving a small hole in the foil to restrain the field of view to 1 square inch. Focus the lens at 1.5 feet distance and test.

When you're satisfied with the results, you can actually scrap the webcam and put the optical system you've made on a photodiode or a similar light sensor with a desired interface.