Do you want to measure video signals from the bottom of sync pulses to the top of peaks (above white) or just the video signal without sync pulses, as shown on the attached picture ..
To measure parameters of video signal you need oscilloscope or what is better a Waveform monitor which is actually special video oscilloscope. It has built in video filters by which you can measure lumunance and crominance components separatelly. It is built for one TV standard only. Tektronix VM700 is intended for such measurements.
I suppose that it is difficult measure the synchronisms. I will try to measure voltage peak to peak of the video signal. Somebody knows a circuit to measure it?
I believe that the best solution is use op amps.