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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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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.



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