The color bars in video editing
There are standard test signals designed for a perfect adjust and calibration of devices dedicated to process video. The most known and familiar of these tools is the color bar, mainly used to measure levels and chroma (color) relations plus offering a hand in monitors’ adjustment. In the digital age we can observe that many people don’t know how to use the color bars properly and don’t understand the meaning and use of its parts.
In video editing and dubbing processes (copying) to guarantee the perfect correspondence in quality of the finished material in future reproductions color bars are recorded at the head (beginning) of the videotape with an audio tone. This signal is used in the playback as reference for any system or device adjustment to obtain a transparent system, path or display for the video preserving the original quality. A little advice here: when adjusting video monitors or television sets always remember first to adjust brightness and contrast before touching anything related to color to avoid inducing error in differential gain or phase that will alter the perception.
100% Color bars
This test signal contains the traditional seven colors (white, yellow, cyan, green, magenta, red and blue) and can be used for monitors or television sets alignment and sent through measurements instruments (waveform monitors or vectorscopes) can help in checking color amplitude and phase in post-production systems. The white bar has 100% amplitude (corresponding to the maximum luminance signal level for standard signals but there are also versions with 75% amplitude which would represent the white bar as grey.
SMPTE Color bars
The SMPTE (Society of Motion Pictures and Television Engineers) Color Bars is the most used in video production and includes components in its implementation that help in the setup, calibration and maintenance of different systems.
The first bar on the left (white one) is generated at 77 IRE and not the 100 that are present in the full field bars. The following color bars help in the adjustment of saturation, color intensity and hue of color monitors. Reverse patches (middle) are added left to right with blue, magenta, cyan and gray with three intercalated black patches. Note that these bars contain an equal level of blue and if your monitor can be set to display “Blue Only” a perfect adjustment of Hue or Tint can be obtained when you see the reversed bars and the top color bars without any difference in their intensity.
To adjust the monitor after filtering the monitor through is “Blue only” option (that 
cancels the red and green guns) adjust the color or chroma gain (intensity of color) until the two vertical bars in the outside are matched in level. Later adjust the control of Hue or Tint until the two internal vertical bars and the patches are matched and look as one.
The strip in the right bottom that appears as solid black consists of 3 smaller strips with different levels of luminance at 3.5, 7.5 (setup level) and 11.5 IRE. The IRE is named from Institute of Radio Engineers and it’s a unit created to measure video analog signals being a value of 100 IRE the range that goes from black to white with a voltage equivalent to 0.7 volt, the whole signal of video including synchronism would be 140 IRE (for one volt peak to peak for a white signal).
The different steps in IRE units present at the bottom of the SMPTE color bars are useful to adjust the brightness levels of the monitor where you should be able to define each small strip or bar due to its difference in level and then adjust the contrast setting until the left ones merge in their brightness. The brightness-contrast adjustment is fine when visually the two left strips disappear in the background (under the 7.5 IRE setup or perceptual black level) and the right one (11.5 IRE) is barely visible.
In the bottom is provided a 100 % white flag (100 IRE) with the signals –I and +Q at each side. These signals (with a dark blue look) are just the components R-Y and B-Y (red minus luminance and blue minus luminance) used in the encoding process of NTSC video and can be used to check their phase relationship in relation to all other colors. The bottom of the SMPTE test signal is named IYQB because combines the -I signal, Y for Luminance, Q signal and Black.
There are many other test signals used in video such as the Bowtie, Ramp, SDI check, etc, but the most common for the video editor is the color bar in one of its usual forms.