Working with Kelvin, CRI and Mired

I’m always amazed at the misinformation about the Kelvin scale of measuring color temperature. On the Internet, a power company stated that the “History of Kelvin temperature originally comes from the incandescent lamp.” During the nineteenth century and long before Edison invented the incandescent light, Lord Kelvin (William Thompson)  proposed a new temperature scale that was suitable for measuring low temperatures. He suggested that absolute zero should be the basis for a new scale. His idea was to eliminate the use of negative values that occurred when measuring low temperatures using Fahrenheit or Celsius scales. In honor of his contributions the system is called the Kelvin scale and uses the unit “Kelvin” or sometimes “K.” On a clear day at noon, the sun typically measures approximately 5500 degrees Kelvin. On an overcast day, that temperature rises to 6700 degrees Kelvin, while you’ll experience 9000 degrees Kelvin in open shade on a clear day. Traditional “hot” lights have a temperature of 3200 degrees Kelvin. Household light bulbs measure about 2600 degrees.

Color Rendering Index (CRI) is a unit of measure defining 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.

Another unit of color you might occasionally see  is the mired (micro reciprocal degrees,) a unit of measurement equal to 1,000,000 one million divided by the color temperature and indicates color correcting filter densities. Higher Kelvin color temperatures are at the cool (blue) end of the spectrum while warmer (red) temperatures are at the lower end of the spectrum.

Rotolight’s RL-48’s produces a color temperature that’s close to daylight with 5600K with 6300K, 4300K and 3200K (Tungsten) options available by using the included color correction filters. For the portrait of Olivia, I placed one Rotolight RL48 at camera right with a Bright Pink filter in place. The other RL48 was at camera left with the Moroccan Frost filter. Camera used as Panasonic Lumix GH4 with Olympus M.45mm f/1.8 lens and an exposure of 1/125 sec at f/1.8 and ISO 1600.

Thanks and a tip of the old Farace chapeau to Joe Harris, an ancestor of Lord Kelvin, for his contributions to this post.

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If you’re interested in shooting portraits, make Mrs. Farace’s oldest son happy by picking up a copy of “Studio Lighting Anywhere” which is available from your favorite book or camera stores as well as Amazon.com, where your purchase helps this blog. And it makes a great holiday gift too. Prine members can get it in time for Christmas giving.

Author: Joe Farace

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