Calibrating a Monitor with Color Bars

http://spareroommedia.com/video/monitor_setup.html

Here's a thorough rundown of color-bar calibration written by Mark Gamble at Spareroom Media:

Using Color Bars to Calibrate

an NTSC CRT Monitor

These instructions are for calibrating professional video monitors and consumer television sets. They are not for calibrating computer monitors. Make adjustments in the order they are presented.

In these instructions, monitor controls are given with their professional name followed by one or more names that they are known by on prosumer and consumer monitors. For example: Phase/Tint/Hue indicates that a broadcast monitor will have a control labelled “Phase”, while prosumer and/or consumer monitors will have a control labelled either “Tint” or “Hue”. Professional monitors should have all of the controls listed. Prosumer and consumer monitors, however, may not have all controls.

Note: The color bar graphics used in this article have been brightened so that the PLUGE pattern will display across a wide range of hardware. Therefore, none of the color bars should be used for actual reference, or for creating your own color bar graphics.

Note: In the following steps, when instructed to adjust a control you should try to “rock” it back and forth between extremes. This will usually allow you to quickly hone in on the general area where the control should be set. Then you can tweak the adjustment. This technique works best with monitors that have physical controls for the settings listed, but it can also be used quite effectively with monitors that have on-screen digital controls.

• 1. Preparation

     1. 1. Turn on the monitor and allow it to warm up at least 10 minutes, preferably 30 minutes.

        2. Turn down the room lights and make sure there are no reflections or glare on the monitor. Cover any windows. You do not want other light sources or mixed light sources (daylight and tungsten or flourescent) influencing your perception of color. Glare, reflections and overly bright lighting will prevent you from being able to properly set the monitor’s Black Levels. You don’t need pitch blackness, just a low level of ambient light. Ideally, you want to setup the monitor under the lighting conditions in which you will be working.

        3. Feed SMPTE color bars to the monitor from your edit system, signal generator, camera, deck, or use the leader of a tape you have created that has accurate SMPTE bars.

• • • • 1. SMPTE Color Bars, with Labels and IRE

        2. (Click for larger image)

• 1. Set the Black Level using the PLUGE (Picture Lineup Generation Equipment) pattern

• • • • 1. Correctly adjusted Brightness and Contrast

• • • 1. The Brighness control actually sets the black level of the picture. The Contrast/Picture control sets the white level, or high luminance portion of the picture.

         • If Brightness is set too low, detail and texture is lost in the picture. Medium grays blend into dark grays, dark grays blend into black. Images will lack “subtlty”.

         • If Brightness is set too high, then nothing will appear truely black. The darkest grays and blacks will appear the same. Lighter grays get lighter. The overall effect is to reduce the contrast ratio of the monitor. Images appear “muddy”.

         • If Contrast is set too low, the picture will be dim and the white will lack “luster.” Also, the color bars will not appear to be evenly balanced gray scale steps.

         • If Contrast is set too high the whites will “bloom” or glow and highlight detail in pictures can distort (raster distortion). Blooming appears as white spreading into adjacent detail and defocuses the image. If you are not sure what blooming is, quickly adjust the Contrast control between its minimum and maximum settings while watching the Superwhite bar. It should appear to get wider as the Contrast gets higher, and narrower as the Contrast is reduced. (In pixel terms, blooming occurs when pixels are so brightly lit that the light spreads into adjacent pixels, softening the color of those pixels. The effect is also sometimes described as defocusing the picture.)

         • You may also notice Contrast distortion in vertical lines/edges that waver, wiggle or bend.

         • Contrast should be set just below the point where blooming or edge (raster) distortion appears. Remember that Brightness may need to be adjusted after Contrast is set. Also, while you may have Contrast correctly set, the result may be that White appears too dull or low for the picture. So to get a more acceptible white level, you may have to make a compromise between an accurate Contrast setting and a picture (in terms of white level) that is pleasing to the eye.

         • You can also double check the Contrast after you have adjusted the color levels of the monitor using the steps below. If you are sure that the Chroma/Color/Saturation and Phase/Tint/Hue controls are properly set, look at the Superwhite bar and the large gray bar on the left of the screen. If Constrast is set too high, the Superwhite and gray bars may appear slightly yellow or brownish. Try reducing Contrast to see if it lessens the effect.

         • Consumer televisions often have Contrast/Picture set between 50-100% to produce an image that appears to have a lot of “punch”. In most cases the optimum setting will be far lower.

      2. If you previously switched on “Monochrome” switch it off. If you reduced the Chroma/Color/Saturation control, return it to its middle setting.

• 1. Adjust Sharpness/Peaking/Focus/Detail

     1. • Reduce the Sharpness/Peaking/Focus/Detail to its minimum setting.

        • Gradually bring the Sharpness/Peaking/Focus/Detail control back up until you begin to see artifacts, shadowing, ghosting, or fuzziness appear between the Superwhite box and adjacent boxes. Also check the edges between all bars and sub-bars.

        • Reduce the Sharpness/Peaking/Focus/Detail until the artifacts no longer appear, and the boudaries between all boxes are distinct, not fuzzy or blurry, and no ghosting is visible.

        • In general, the optimum position for this control is between 1/3 to just over 1/2 of the control’s range.

        • Higher settings introduce noise, distortion and shadowing. The effect can be much the same as having the Contrast/Picture set too high. Edge detail bleeds over into other parts of the image, acutally appearing softer. Or, the edge detail can appear as “stairsteps”, which are familiar to anyone who’s worked with computer graphics.

        • If set too low, you will lose detail or resolution.

        • If you are calibrating a TV, You will probably find that the appropriate Sharpness level for broadcast programs and VHS or S-VHS material is too high when viewing commercial DVDs or playback of DV footage. Mark (or note) the setting for broadcast/VHS material, then adjust Sharpness while watching a high quality commercial DVD or playback of high quality DV footage (footage that has been properly exposed and shows a good contrast range and color range). Most likely you will find the need for little—if any—Sharpening. Note the position of the control so you can switch between settings as needed. (You may find after a while that you don't mind having Sharpness at a low setting; while images may appear a bit soft, you will also notice a lack of stair-stepping, artifacting and high frequency noise that accompany high Sharpness settings.)

        • The Sharpness control enhances fine detail in a picture. It was originally used to put in color TVs that used a notch filter luma/chroma seperation which removed detail in the high frequency black and white portion of a picture. The Sharpness control restored some of the lost detail. Most TVs now have a comb filter which doesn't remove luminance detail, so you really don't need a Sharpness control.

        • More detailed instructions will be posted at a later date, along with a resolution chart for better sharpness adjustment.

  2. Do ONE of the following:

     1. • Press the “Blue Only” button on your monitor. This turns off the red and green scan guns, giving you a picture made only from the blue scan gun. Depending on your monitor, the color bars will appear either as alternating light gray and black bars, or blue and black bars.

        • Hold a Kodak Wratten #47B gel, Lee 47B filter, Rosco #80 primary blue gel, or Full CTB (Color Temperture Blue) gel between your eyes and the monitor. Double up the gel if needed to ensure no yellow, green or red colors are visible through the gel.

• • • • • “Blue Only” mode of other (usually Sony)

        • monitors

• 1. If your monitor has controls for adjusting “flesh tone” or “color correctionn” turn them off.

  2. Set the Color Levels using the Color Bars (also referred to as adjusting Saturation and Hue or Color Tone)

     1. 1. Adjust the Chroma/Color/Saturation control until the outer sets of gray and blue bars and sub-bars match (have the same brightness/level). (This control sets Color Intensity.)

• • • • 1. Correctly adjusted Chroma/Color/Saturation and Phase/Tint/Hue

• 1. Press the “Blue Only” button, or remove the blue gel, to view the monitor and ensure that the colors look correct to your eye.

     1. Close your eyes and/or look away for a minute or two, then look back at the monitor to evaluate. Do this several times. Out eyes rapidly adjust to changing color/lighting conditions, which can cause an improperly calibrated monitor to appear correct. Of course, the opposite is also true. The monitor may be correctly calibrated, but because you are not used to seeing it this way it may appear to be incorrect.

• • • • • Correctly adjusted SMPTE Color Bars

• • • • Things to check for:

        • The yellow bar should be a pure lemon yellow, without any orange or green tint.

          • • If the Phase/Tint/Hue is too red, the yellow bar will appear orangeish and the cyan bar will appear greenish.

            • If the Phase/Tint/Hue is too green, the yellow bar will appear greenish and the cyan bar will appear bluish.

        • The magenta bar should be a pure magenta; it should not look red or purple.

        • If the Chroma/Color/Saturation is too high, yellow and red will bloom and/or bleed into adjacent colors.

        • If you are calibrating a TV set (as opposed to a broadcast monitor) you may have trouble matching the large bars with their corresponding sub-bars. Manufacterers will often tweak sets in such a way that this type of calibration will not work. Get the bars to match as much as possible, then check your calibration by viewing good reference material (paying particular attention to flesh tones).

If the colors on your monitor do not look correct after you have calibrated it and given your eyes time to adjust, this could be a sign of one or more issues:

• • Take a break for 10 minutes then try again. Make your adjustments quickly without trying to be too picky. The longer you take with each adjustment, the more time your eyes and brain have to adjust to what they see, which can trick you into thinking something is correct when it’s not and vice versa. When you are adjusting a control and trying to decide between two settings, look away after you change the setting and let your eyes rest for a minute. Then look back and evaluate the image. Most of the time a monitor can be just as accurately calibrated quickly as when taking a lot of time with each step.

  • You’re working with a cheap monitor that cannot accurately display colors. (I don’t want to be snarky and say “Get a better monitor,” but.... In this regard, the situation is no different than someone attempting to mix audio on a good system but with cheap speakers (OK, technically they should also be called monitors). It may work for you, but what happens when someone looks at your video—or plays your audio—on their system? Which may or may not be high quality and calibrated?)

  • There is a problem with one or more components of your monitor. If feasible, have it serviced. Otherwise you may need to replace it. And remember that monitors will wear out over time. The older the monitor the more difficult it will be to calibrate.

  • Consumer oriented monitors can be tricky to adjust because they are often tweaked by the manufacturer to give a good viewing image, not necessarily a perfectly accurate one. Red is often over-saturated, which makes adjusting both Chroma/Color/Saturation and Phase/Tint/Hue difficult. In this case, the best you can hope for is to get the majority of the color bars to look correct. Double-check by playing a tape or DVD with well recorded images of people and check the skin tones. Adjust Chroma/Color/Saturation and Phase/Tint/Hue so that the skin tones appear correct.

  • In the same vein, the trend for both print and film/video in North America is for a slightly to significantly over-saturated look, especially in advertising. Compare several different American publications, television shows and films to European ones and you'll notice that, side-by-side, the European versions feel undersaturated, although they may more accurately reflect good saturation. But once again, the subjective creeps into the picture. When not viewing comparitively, you may never realize the difference because your eye will quickly adjust to the quality and color of what you are viewing (see below). It should be obvious that for evaluative purposes you want to view imagery that has not been post-processed to achieve certain special effects, such as bleach-bypass and various desaturation techniques.

  • You’re male. One in eight men (12.5%) have some form of color deficiency while only one in two hundred women (0.5%) have a color deficiency. Have several of your peers look at the monitor and get their (subjective) opinion. Better yet, ask a woman for her opinion as women suffer from far fewer color problems with their eyesight than men. It’s all in the genetics; two x chromosomes are better than one! (In case you’re curious, the way we perceive color is inhereted and does not change over the course of our life. Age and illness can and do affect perception, and experience can teach you to work with any color deficiency you may have, but—unlike eyesight—color vision does not change; you have to work with what you’re born with.)

All other things being equal, a professional broadcast monitor is easier to calibrate than a consumer television. But cost and manufacturer reputation alone doesn't guarantee quality. Inexpensive to mid-range monitors—professional or consumer—can provide an accurate color reference; it often just takes a lot of testing and trial and error.

When all is said and done, you may feel that it’s impossible to get an accurately balanced monitor, especially if you are using non-professional equipment. However, if you are able to route your video signal through a waveform monitor and vectorscope, you can use both to determine if the luminence and color is accurate in the video signal itself. So even if you can’t be sure of the accuracy of your monitor, you can be reasonably sure of the accuracy of your video signal.

I'll post information about using waveform monitors and vectorscopes at a future date.

It’s also worth noting that if you are working in a production environment it is advantageous that the same person be responsible for calibration. This generally insures a reasonable level of consistency working with different devices. Of course, this could also result in things being consistently off... ;)

Background

The human eye is wonderfully adept at “auto-whitebalancing”. The eye (and brain) are able to compensate for different color temperatures almost automatically. This can make calibration (among other things) difficult.

The eye (and brain) interpret a white piece of paper viewed under “normal” daylight (5600 K) as white, generally with no hint of blue even though you are viewing the paper under bluish light. The eye views much as a camera, but our brain “knows” the paper should be white and compensates (and better than a camera’s auto-white balance).

Likewise, that same piece of paper appears white when viewed under indoors under orangish tungsten light (3200 K) or greenish flourescent light (technically, flourescents don't have a color temperature the way other lights do, but they are sometimes listed around 3300 K).

Unfortunately, this trait—which is otherwise so beneficial—really gets in the way of evaluating the color values or qualities of still and moving images, whether captured on film or digitally, in print, on a television or projected.

This is why digital cameras have to be white balanced, and why all types of monitors should be calibrated. (When shooting film you either shoot with a film that matches the color temperature of your lighting or you use gels or filters to get the film and/or lights to match.) Photographers often have the the color temperature of their computer monitors set at either 5500 K or 6500 K to closely match daylight, and they view transparencies and/or prints under tungsten or flourescent lights that match that color temperature. Otherwise, it becomes nearly impossible to evaluate images with any accuracy.

Incidentally, this should give you some idea as to why artists like to paint in certain kinds of natural light.

(Given all of this—and the statistics on color deficiency—is it any wonder that people have trouble identifying colors, let alone agreeing if they match?)

It’s also worth noting that while most computer monitors allow you to set a reference color temperature (usually either 5500 K (D50), 6500 K (D65) or 9300 K (D93)) not all video monitors give you this option (or they may limit the choices to D65 or D93). 55 K generally appears too yellow or “dirty” to most people, and 93 K often looks too blue (although subjectively I think it’s easier for our eyes to adjust to this temperature rather than 55 K). 65 K, though a little bluish, usually appears the best to most people. This is often the default setting for monitors and televisions. No matter what reference color temperature the monitor is set to, it must still be calibrated.

No wonder, then, that NTSC is referred to as “Never Twice the Same Color”, “Never The Same Color”, “Not The Sharpest Colors”, “No Two Setups Compare”, or—my favorite—“Not Tested Since Christ”.

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Copyright © 2006 Mark Gamble. All rights reserved.