Today’s Post by Joe Farace
In black and white you suggest, in color you state.—Paul Outerbridge
I often hear photographers tossing around color temperature numbers and talking about how many “K” or Kelvin that a particular studio light produces. But…
…at the same time I’m surprised at the amount of misinformation about the Kelvin scale that’s used for measuring the color temperature of light. For example, a power company’s website states, “History of Kelvin temperature originally comes from the incandescent lamp.” Before Edison invented the incandescent light in 1879, William Thomson, First Baron Kelvin, a British mathematical physicist, proposed a temperature scale for measuring low temperatures. In 1848, he suggested that absolute zero should be the basis for a new scale. His idea eliminated the use of negative values that occurred when measuring low temperatures using the Fahrenheit or Celsius scales. In his honor, the system is called the Kelvin scale and uses Kelvin or “K” as a unit of measurement.
How I made the above image: I photographed this porch in a home in one of my trips to Acapulco, Mexico—one of my favorite places outside the USA. It was made with a Canon PowerShot S400 point-and-shoot camera. It was, I believe, the last PowerShot camera that used CompactFlash cards, which is why I used it on trips where I also used Canon DSLR that used the same memory cards.
Higher Kelvin color temperatures lie at the cool (blue) end of the spectrum while warmer (red) temperatures are at the lower end of the spectrum. On a clear day at noon, the sun typically measures approximately 5500 degrees K. On an overcast day, that temperature rises to 6700 degrees K, while you’ll experience 9000 K in open shade on a clear day. Traditional hot studio lights have a temperature of 3200 K. Incandescent household light bulbs measure about 2600 K. The kind of LED light bulbs you find at Home Depot or Lowe’s are available in different color temperatures with Warm White LED bulbs having stated Kelvin ranges between 2700K and 3000K.
Other ways to measure color
Color Rendering Index (CRI) is a unit of measure that defines how well colors are rendered under different lighting conditions and uses a scale from one to one hundred. A low CRI number causes colors to look washed out or take on a different hue, while light sources with a high CRI makes colors look natural and vibrant. Read more about measuring color in my post, Testing LED Lights for Portraiture.
Mired (Micro Reciprocal Degrees) is a unit of measurement that’s equivalent to 1,000,000 divided by the color temperature and indicates color correcting filter densities. If you want to play around with this concept, UK based Lee Filters offers an on-line Colour Temperature (Mired Shift) Calculator.
Thanks and a tip of the old Farace chapeau to blog reader Joe Harris, an ancestor of Lord Kelvin, for his contributions to this post.
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The top photograph (not the bracket) is from my self-published book, Acapulco, Paradise of the Americas, that was co-authored with Don Bain, and is available on Blurb.com.