lightphotodiodephotosensorsensingsensor
I'm looking for a sensor or method of rapidly sensing light intensity of a 1-inch x 1-inch square of a LCD monitor at a distance of 1-1.5 feet away. Does anyone know if such a thing exists, or has experience with this challenge?
I've considered using a standard photocell with some sort of zoom-lense, but I don't have enough experience with senses to put one together, and haven't found anything pre-built.
People did detect a hand touching a glass surface via infrared light. They had the light shine on the edge of a glass plate and the light was then reflected internally. Where the hand touched the glass plate, the infrared light left the glass plate and could be detected via a digital camera behind the glass.
Your can read about it here.
For stability and accuracy, I would suggest a silicon PIN (P-Intrinsic-N) photodiode. Response time in the microseconds is not too difficult with good design (you'll want the response to be an order of magnitude faster than the time the sample is in front of the sensor).
Phil Hobbs has a good article here on photodiode front end design. You may just need a simple transimpedance amplifier as this one using the OPA128 (there's a mistake in the schematic on the datasheet- the resistor should go to the PD bias voltage, hey nobody's perfect).
if there is plenty of light available (so you can use a really low capacitance PD), but if you need more, that article (and his book) are a good guide.
Best Answer
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.