Apple Color 1.5 - Manual (Page 197)

The I Bar
The –I bar (negative I bar) shows the proper angle at which the hue of the dark blue box
in the color bars test pattern should appear. This dark blue box, which is located to the
left of the 100-percent white reference square, is referred to as the Inphase signal, or I for
short.
The I bar (positive I bar) overlay in the Vectorscope is also identical to the skin tone line
in Final Cut Pro. It’s helpful for identifying and correcting the skin tones of actors in a shot.
When recorded to videotape and measured on a Vectorscope, the hues of human skin
tones, regardless of complexion, fall along a fairly narrow range (although the saturation
and brightness vary). When there’s an actor in a shot, you’ll know whether or not the skin
tones are reproduced accurately by checking to see if there’s an area of color that falls
loosely around the I bar.
If the skin tones of your actors are noticeably off, the offset between the most likely nearby
area of color in the Vectorscope graph and the skin tone target will give you an idea of
the type of correction you should make.
197 Chapter 8 Analyzing Signals Using the Video Scopes