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Picture This: In Search of a Consistent Gray

Dec 1, 2008 12:00 PM, By Jeff Sauer

Demystifying consistent color temperature.

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Presentation modes in projectors and flatpanels raise the color temperature of the display because we tend to perceive blue light as crisper and sharper. Movie modes, when not set to D65, tend to go in the other direction, creating a warmer viewing experience. In either case, while it doesn't make for the ideal viewing experience, our eyes can quickly adjust to whatever the reference white is in order to discern the visual information.


Yet while our eyes can adjust to different overall references for white and can accommodate different color-temperature setup configurations on a display, it's more awkward if color temperature varies within a frame or between frames of a video sequence. Indeed, that is where color temperature and grayscale collide.

When we measure grayscales on a display, we're looking for a smooth ramp from black to white, ideally all the way from 7.5 IRE to 100 IRE. If that ramp isn't smooth, then visual details in darker shadows can be lost, crushed down to black, and brightly lit areas in an image can blend into white. It's the result of the unmarshaled quest for a higher contrast number on the spec sheet.

Yet in addition to that smooth luminance ramp, a display also should maintain a consistent color temperature as luminance increases. In other words, higher luminance should not beget a higher (or lower) color temperature. Imagine a scene of a person with a white shirt on walking down a lamp-lit sidewalk. That shirt should stay white, regardless of how far the person is from the next lamp and that can only happen if the color temperature of the display remains consistent across the range of gray.

While the Casio XJ-S57 is a powerful projector in a remarkably small package that makes it quite flexible for tight installations, its strength is as a data projector more than a video projector. In an effort to maintain high brightness to combat midday ambient light, the projector fails to maintain a consistent color temperature. At about 50 IRE, color temperature was about 1000 degrees higher than it was at 90 IRE, in which case our walker's white shirt would inherit an unfortunate blue hue each time he walked away from a street lamp.

That variation probably won't matter too much if presentation slides and spreadsheets are the subject, but it is part of what separates data-oriented displays from those that cater to video. And it's also an attention to detail that can separate a value-oriented model from one calibrated for performance.

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