Nice post, full of understanging on the matter.The downside is ambient light getting into your sensor from the room, which can be reflected light bouncing off the walls in a darkened room. Hence the light measurement guidelines instruct use of cone frustums to eliminate ambient light noise. Which brings us back to the real world of our reality, for what ever that is(another discussion), where it isn't practical for the average person to control all artifacts and you are left with placing the sensing device at a distance that eliminates any ambient light and allow longer duration readings.
I use different lenght cone frustums for meter profiling and then I use the longer one for calibration. That way solved any problem related to ambient light, photons and small area reading.