Understanding Kelvin, CRI and Mired

Today’s Post by Joe Farace

I often hear photographers tossing around color temperature numbers and talking about how many “K” aka Kelvin a particular light produces. At the same time I’m amazed at the misinformation about the Kelvin scale that’s used for measuring color temperature. For example, a power company stated on its website, “History of Kelvin temperature originally comes from the incandescent lamp.”

Long before Edison invented the incandescent light, Lord Kelvin proposed a temperature scale for measuring low temperatures, suggesting 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 when using Fahrenheit or Celsius scales. In his honor, the system is called the Kelvin scale and uses the unit “Kelvin” or “K” as a unit of measurement.

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. Household light bulbs measure about 2600 K. 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.

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 that have 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 the concept, UK based Lee Filters offers an on-line Colour Temperature (Mired Shift) Calculator.

How I made the above image: Rotolight’s LED-based RL-48 lights were used to make the above 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 a Moroccan Frost filter to soften the light. As I’ve mentioned before, some subjects find LED light to be harsh. All these filters are included with the RL-48 light kit. Camera used was my original 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 blog reader Joe Harris, an ancestor of Lord Kelvin, for his contributions to this post.

 

 

If you’re interested in learning how I shoot portraits and how I use cameras, lenses and lighting in my in-home studio and on location, please pick up a copy of Studio Lighting Anywhere which is available new from Amazon.com for $23.34 or used for the giveaway price of $2.23, as I write this. If you would like some hands-on training, check out my one-on-one workshops.