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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.
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.
Avalanche photo diodes have comparatively lower rise and fall time as compared to LDR, these APD's can be coupled with Si-PM (silicon photomultiplier) which work in the Geiger region to amplify the input through silicon and is the successor of vacuum tubes devices.
But I guess these are going to be costly as OP seems to be doing a DIY project, using a lens the light you can concentrate will be proportional to the aperture of the lens.
The attenuation factor of the light can be calculated by various models and the output can be scaled at the photocell, if possible you can also use a photocell just near the LCD surface and compare the results for various distance.
If you are looking for a commercial product that will do this job, look for photometers that are made to image the target from a distance. An example is a Pritchard Photometer.
