Using Color Bars for Video Calibration

When using analog devices, make sure they are calibrated for accurate brightness and color so there’s no distortion when you capture and color correct your video.

About Color Bars

Color bars are electronically generated video signals that meet very strict specifications. Because the luma and chroma levels are standardized, you can use color bars passing through different components of a video system to see how each device is affecting the signal. For example, if you record color bars that have a 100 percent white level in a camcorder and then play the videotape back on a VTR, and if the white level output from the VTR is only 90 percent, you know that you need to increase the luma level output of the VTR.

NTSC and PAL each have specific color bar standards, and even within NTSC and PAL there are several standards. When you evaluate color bars on a video scope, it is important to know which color bar standard you are measuring, or you may make improper adjustments. “SMPTE bars” is a commonly used standard.

When Should You Use Color Bars?

Analog devices always need to be calibrated and adjusted, even if only by minute degrees. This is because heat, age, noise, cable length, and many other factors subtly affect the voltage of an analog electronic video signal, which affects the brightness and color of the video image. Color bars provide a reference signal you can use to calibrate the output levels of an analog device.

Using Color Bars to Adjust Brightness and Color

Broadcast video professionals use color bars to calibrate the output of analog video devices such as:

  • Playback VTRs used for capturing analog video into your editing system

  • Your video editing system’s video interface analog output (for output to analog video)

  • Playback VTRs used for dubbing tapes to VHS

Typically, 30 to 60 seconds of color bars is recorded at the beginning of each new videotape shot in the field. Color bars from a professional camcorder are used to calibrate the on-location video monitor used to watch what’s being shot, so that adjustments for brightness or color temperature made to the camcorder are made accurately. Later, the color bars recorded at the head of each source tape are used to calibrate the clip settings in Final Cut Pro. If you don’t have color bars on your tape, see Adjusting Capture Settings on Source Tapes Without Color Bars.

Each source tape should be individually calibrated because tapes from various camcorders may have slightly different settings, and because even a single camcorder may have fluctuated slightly over the course of the production.

Calibrating Video Monitors with Color Bars

Videographers don’t rely on the viewfinder of a camcorder to adjust the image being recorded. Viewfinders and camcorder LCD displays are often too small and have color fidelity that’s too inaccurate to ensure proper focus, brightness, and color adjustments. Instead, a properly calibrated video monitor can be set up on location to monitor the video during shooting.

Similarly, editors and broadcast designers shouldn’t rely on an uncalibrated monitor when making crucial adjustments to the color and brightness of their movie footage. Instead, it’s important to use a calibrated broadcast monitor to ensure that any adjustments made to exposure and color quality are accurate.

Monitors are calibrated using SMPTE standard color bars. Brightness and contrast are often adjusted by eye, using the color bars onscreen. Adjusting chroma and phase involves using the “blue only” button found on professional video monitors. This calibration should be done to all monitors in use, whether they’re in the field or in the editing room.

To calibrate your monitor
  1. Connect a color bar or test pattern generator to the monitor you’re using.

    Alternatively, you can use one of the built-in color bar generators in Final Cut Pro. Avoid using still-image graphics of color bars.

  2. Turn on the monitor and wait at least 10 minutes for the monitor to reach a stable operating temperature.

  3. Select the appropriate input on the video monitor so that the color bars are visible on the screen.

    Near the bottom-right corner of the color bars are three black bars of varying intensities. Each one corresponds to a different brightness value, measured in IRE. These are the Picture Lineup Generation Equipment (PLUGE) bars, and they allow you to adjust the brightness and contrast of a video monitor by helping you establish what absolute black should be.

  4. Turn the chroma level on the monitor all the way down.

    This is a temporary adjustment that allows you to make more accurate luma adjustments. The chroma control may also be labeled “color” or “saturation.”

  5. Adjust the brightness control of your monitor to the point where you can no longer distinguish between the two PLUGE bars on the left and the adjacent black square.

    At this point, the brightest of the bars (11.5 IRE) should just barely be visible, and the two PLUGE bars on the left (5 IRE and 7.5 IRE) should appear to be the same level of black.

    Note: The brightness control adjusts the black level of your video signal by changing the offset of the red, green, and blue signals.

  6. Now, turn the contrast all the way up so that this bar becomes bright, and then turn it back down.

    The point where this bar is barely visible is the correct contrast setting for your monitor. (The example shown below is exaggerated to demonstrate.)

    Note: The contrast control adjusts how much the RGB signals are scaled.

    Figure. Color bars and PLUGE bars.

    When adjusting the contrast, also watch the white square in the lower left. If the contrast is too high, the white square appears to “spill” into the surrounding squares. Adjust the contrast until the luma of the white square no longer spills into surrounding squares.

  7. Once you have finished adjusting luma settings, turn up the chroma control to the middle (detent) position.

    Note: Some knobs stop subtly at a default position. This is known as the detent position of the knob.

  8. Press the “blue only” button on the front of your monitor.

    Note: This button is usually available only on professional monitors.

    Notice how all of the different-colored bars turn into alternating light and dark ones.

    • If your monitor is correctly calibrated: All the gray bars will be evenly gray, and all the black bars evenly black.
    • If the two outer gray bars don’t match: Adjust the chroma control of the monitor until they do. Then adjust the phase control to even out the inner bars.
      Figure. Bar tone showing alternating grays and blacks.

    Note: NTSC and PAL video have brightness and contrast levels that are very different from those of RGB computer displays. Broadcast video viewed on a computer display often looks dull and dark, whereas the same video viewed on a broadcast monitor will be bright and clear. For this reason, the color bars above are shown for reference only and not to reflect what you should be seeing.