It is often useful to measure light with a photodiode. It is quite easy.
One needs a photodiode, such as:
A standard oscilloscope probe (also known as test probe or scope probe).
One can simply connect a standard oscilloscope probe across the 2 leads of the photodiode and tape the scope probe to the side of the monitor, so that the photodiode is near to the light source. If the light source is a screen, it is helpful to tape the scope probe to the edge of the monitor (not the glass!) so that the photodiode position does not vary during any recording. If you want to test light in a particular region of the screen, it is useful to put some cardboard on the screen and cut a small hole so that only light in the region of interest shines through to the photodiode. Then, just plug the BCN end of the scope probe into a data acquisition device or oscilloscope.