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Picture This: In Search of a Consistent Gray

Demystifying consistent color temperature. 12/01/2008 7:00 AM Eastern

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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When SVC reviews display products, such as the Casio Super Bright XJ-S57 (see p. 62 of this issue), there is often a discussion of color temperature and how the display maintains the color temperature across a range of grayscales. Yet it's admittedly a rather strange concept: the “temperature” of color. How can color have a temperature? And, if it does, how does it have anything to do with how a product displays visual images?

A simple web search can find a lot of different explanations of color temperature, such as low color temperatures being represented by the color of a single candle and high color temperatures being the color of a bright blue sky. You can find an interesting visual analogy of how color temperatures are measured on the Kelvin scale using the changing color of a black iron pot heated by an increasingly hot fire. As the pot gets hotter, it turns from black to red and then orange, much like an iron rod being forged by a blacksmith, and then white and blue as it reaches beyond 7000K.

It's the shades of white in the middle between roughly 5000K and 7500K that are arguably most important because white is the reference by which we perceive other colors. Anyone with experience using the manual settings on a still camera understands the importance of a proper white balance when it comes to capturing accurate colors, and that's really the root of proper color temperature. In more automated point-and-shoot cameras, white balance is often simplified with preset modes such as “daylight,” “cloudy,” “indoors,” and “fluorescent,” but the function of establishing a correct white remains critical to good picture-taking, as well as displaying good images. Use the wrong setting that adds just a hint too much blue to a person's face, and that person can quickly go from looking happy and healthy to appearing sick.

CIE Standard Illuminant D65, or D6500, is recognized by the International Commission on Illumination (CIE) and the display industry as the proper balance of red, green, and blue light to create the purest white. On the Web, you'll see 6500K described as the color of white at midday in the sunlight or something akin to that. And 6500K is often the setup target for displays (at least in standard mode). Indeed, being able to create a pure white is one of the reasons why several companies (Samsung, Sharp, and Sony) have introduced LED-backlit LCD panels. LEDs are adjustable and can be calibrated to achieve that 6500K color temperature (or any other desired) more easily than a Cold Cathode Fluorescent Lamp (CCFL)-backlit LCD panel. And that, in turn, affords a fuller range of color when red, green, and blue are filtered out of the white light by the LCD filter layers.

On the other hand, the color of the light around us has a great effect on how we see white and other colors. Traditional tungsten light bulbs and fluorescent office lighting both produce a white light, but tungsten bulbs are typically described as having a warmer look or feel, while office lighting appears colder.

Many people tend to find that the warmer white light of tungsten creates a more pleasant environment than the colder light of an office environment. Although in either case, our brains do a good job re-establishing our own internal white balance, thereby creating a new reference by which we judge a healthy face or edible fruit. Ironically, while those descriptions of cool and warm are well understood, the colder fluorescent lamps actually have the higher color temperature than the tungsten bulbs as measured in degrees on the Kelvin scale.


Picture This: In Search of a Consistent Gray

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

Demystifying consistent color temperature.




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.

COLOR TEMPERATURE AND GRAYSCALE

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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