Eliminate Dark Picture Color Shifts with the CP5000 ColorPro Would you like to improve the quality of your white balance calibrations? Even if you nail the tracking on your calibrations from 30 IRE to 100 IRE, there may still be room for improvement. You can make your customers’ calibrated displays look much better if you pay a little more attention to the color of the very darkest parts of the picture.
	The human eye is most critical of color shifts in very dark gray, and not nearly so critical of color shifts in medium and bright white. You’ve probably noticed this yourself. You’ve likely watched a movie on a display that looked pretty good until you got to a night scene. Then you could see that the dark shadows were tinted slightly green or slightly red. That suspends the realism of the movie so quickly, as your mind gets drawn away from the action by the colors that don’t look right.
You can easily adjust  incorrect coloration in the darkest parts  of the picture by   slightly modifying  your white balance  calibration procedure.						Figure 1: You can use your CP5000 to eliminate dark picture color shifts (these clouds shouldn’t be green).
	Have you ever had this incorrect coloration in the darkest parts of the picture, even though you calibrated the display for dead-on white balance accuracy at 30 and 80 IRE? Why is that? Did you do something wrong in the calibration? Likely not. But if the display is a CRT type direct-view or rear projection display, you can easily correct the situation by slightly modifying your white balance calibration procedure (see the section below for information on bulb-type projectors).
	CP5000 Most Accurate For Low Luminance White Balance Calibration The CP5000 Color Analyzer is ideal for accurate low luminance calibrations, as it holds its color accuracy at extremely low light levels, as witnessed by recent industry tests. The CP5000 Dark Calibrate function helps to insure that the three color light sensors produce a balanced, accurate output, even at critical low light levels. For the best measurement stability at these low light levels, set the Update Rate/Averaging slider way to the left.
	CRT Type Display Dark Offset Tint On CRT type displays, a dark offset problem is usually caused by poor CRT gun linearity at the low end of their conduction range. So, if you can’t have both 5 IRE and 30 IRE tracking perfectly at D65, which do you suppose is more critical? When you are watching program material, your eye will tell you that 5 IRE is more critical, every time. Your job then, is to satisfy the human eye as best you can. You can produce a great calibration at low light levels by first calibrating a display in your normal manner. Use the CP5000 to balance the drive controls at 70 to 80 IRE, but balance the cutoff controls at 15 to 20 IRE, to get as close to the critical low end as possible. When you have completed a normal calibration, look at the Pluge bar on the VP300 or S802B Multimedia Video Generator White Window pattern, set to your low IRE cutoff calibration level. If the low IRE Pluge bar is still tinted slightly green or slightly red, simply decrease the green or red cutoff control slightly to visually balance the very darkest parts of the picture, compared to the calibrated white window.
Is the low IRE Pluge  bar still tinted slightly  green or slightly red?				Figure 2: Visually match the 4 IRE Pluge bar to the color of the calibrated white window.
	If you re-check the white window with the CP5000 at your low IRE cutoff calibration level, you’ll likely see that it has changed less than 500 K from 6500 K. Your eye probably won’t be able to see the difference. But you’ll certainly see that dark shadow detail now looks exactly the right color, with no annoying coloration. The picture will be much more realistic and enjoyable to watch, with no more suspension of realism in dark shadow scenes.
	Bulb-Type Projector Dark Offset Tint Bulb-type projectors also occasionally have a slight coloration in the darkest parts of the picture after proper white balance calibration, although for different reasons than for CRT type displays. Most projector bulbs, especially the UHP type, produce about twice as much green and blue light as they need to balance with the red light output. In very dark parts of the picture, the primary determinant of the light output balance is the bulb itself, unless the manufacturer has included color filtering to improve the balance. Also, LCD and LCoS/DILA projectors use electronically variable polarizers to control the individual red, green, and blue light output (DLP uses moving mirrors). These polarizer disks are rotated to null their output at a signal level of 0 IRE. Since their adjustment is very touchy, an individual polarizer may be misadjusted to null at 1-5 IRE, rather than 0 IRE. This would cause the associated color output to go to zero whenever the signal level is at the level to which the polarizer is nulled. If, for example, the red polarizer disk is incorrectly nulled at 2 IRE, whenever a very dark picture level of 2 IRE is produced, the dark gray will have a blue-green tint (minus red). You can check for this incorrect polarizer null with the VP300 or S802B video generators by gating on only one color at a time on the White Window pattern. Then, adjust the window IRE level from 10 IRE down to 0 IRE. If a polarizer null is adjusted incorrectly, you will see the color output go to black at some level above 0 IRE, then reappear at a lower IRE level. Or, the color will not turn off entirely at 0 IRE. This will cause dark parts of the picture to have an incorrect coloration. The only solution for this is to readjust the polarizer nulls (not real easy).
Use the VP300 or  S802B video  generators to check for incorrect polarizer null.					Figure 3: Check each color for incorrect nulls above 0 IRE.
	If none of the colors null at a signal level above 0 IRE, yet the projector has a blue-green tint at very dark picture levels (probably due to an unfiltered bulb), consider applying a color correcting filter in front of the projector lens (available from photography supply sources). Either a CC20R or CC40R filter will usually do a good job of color balancing the projector output at very dark picture levels. These filters reduce all colors except red by either 20% or 40%.You will need to devise a way to mount the filter in front of the projector lens. Since the filter reduces the bulb’s excessive green and blue light output at low light levels, the color correcting filter gives you the added bonus of deeper blacks and improved contrast ratio.
	As the final step, use the CP5000 Color Analyzer to calibrate the projector’s white balance at 70 to 80 IRE and at 10 to 15 IRE. The trick to calibrating at this very low level is to point the CP5000 ColorPro III pod directly at the projector (in an absolutely dark room), rather than reading the light reflected off the screen. You can color balance the projector directly, without concern for the screen’s effect, because the currently available high accuracy screens seldom shift the color more than 0.005 in either x or y with projected D65 light.
The trick is to point the CP5000  ColorPro III pod directly at the projector in an absolutely dark room.	Figure 4: Point the CP5000 directly at the projector for very low IRE cutoff calibrations, watching the CP5000 tripod shadow to keep it square to the projector.
	To calibrate the projector at even lower IRE levels, just move the ColorPro pod closer to the projector. Be sure to keep the pod squared up with the projector light output by minimizing the shadow of the tripod holder accessory, and don’t readjust the projector focus; keep it focused on the projection screen. With this method, you can accurately calibrate a projector’s cutoff white balance at signal levels below 10 IRE with the CP5000 Color Analyzer, for absolutely no color shift in dark picture areas.
			Figure 5: To white balance a projector at even lower IRE levels, move the CP5000 closer to the projector.
Give your customers that super accurate white balance calibration you promised them.	Ultra-Accurate Low Luminance White Balance Give your customers that super accurate white balance calibration you promised them. With just a little more attention to the very darkest parts of the picture, you can calibrate your displays with the CP5000 to look even better than they’ve looked before. No other color analyzer on the market comes close to the low luminance color accuracy of the CP5000 ColorPro Color Analyzer. Use your CP5000 ColorPro to best advantage when calibrating low luminance white balance for maximum customer satisfaction.