Waveform monitor

Read it left to right as the picture. Horizontal position on the scope matches horizontal position in the frame, and the vertical axis is luminance, black at the bottom and white at the top. Anything flattening against the ceiling is clipped and has no detail left to recover. Grading on the picture alone is the mistake it exists to prevent, because a laptop screen at half brightness will happily talk you into an image two stops under. It reads brightness only, and colour balance is the vectorscope's job.