It's colored black because the beam isn't illuminating the phosphors on the tube so they remain dark, though as you turn up the brightness the scanned area above and below will bleed into that area and make the dark area smaller.
It's colored black because the beam isn't illuminating the phosphors on the tube so they remain dark, though as you turn up the brightness the scanned area above and below will bleed into that area and make the dark area smaller.