Color Temperature & Color Rendering Index DeMystified
"What is color? No object of itself alone has color.

We know that even the most brightly colored object, if taken into total darkness, loses its color. Therefore, if an object is dependent upon light for color, color must be a property of light.

And so it is."

Paul Outerbridge, Photographer 1896 - 1958
color temperatures
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Color Temperature Defined
Color temperature has been described most simply as a method of describing the color characteristics of light, usually either warm (yellowish) or cool (bluish), and measuring it in degrees of Kelvin (°K).

That's a little too simple to be of more than introductory value.

A more technical definition assigns a numerical value to the color emitted by a light source, measured in degrees of Kelvin. The Kelvin Color Temperature scale imagines a black body object--- (such as a lamp filament) being heated. At some point the object will get hot enough to begin to glow. As it gets hotter its glowing color will shift, moving from deep reds, such as a low burning fire would give, to oranges & yellows, all the way up to white hot. Light sources that glow this way are called "incandescent radiators", and the advantage to them is that they have a continuous spectrum. This means that they radiate light energy at all wavelengths of their spectrum, therefore rendering all the colors of a scene being lit by them, equally. Only light from sources functioning this way can meet the truest definition of color temperature.

Note - the light spectrum is wider than our ability to see it. Light values falling beneath the visible part of the spectrum are referred to as infrared, and above the spectrum as ultraviolet. Each can adversely affect an image, and you may need to add some filtration to remove them.

Light sources that are not incandescent radiators have what is referred to as a "Correlated Color Temperature" (CCT). It's connotations to any part of the color temperature chart are strictly visually based. Lights with a correlated color temperature do not have an equal radiation at all wavelengths in their spectrum. As a result, they can have disproportionate levels (both high & low) when rendering certain colors. These light sources are measured in their ability to accurately render all colors of their spectrum, in a scale is called the Color Rendering Index (CRI). Incandescant radiators have a CRI of 100 (the max.) More on this below.

Color Temperature in Imaging
color temperatures
The above is not a true Color Temperature chart. Instead it is a hybrid, showing the color temperatures of light sources most commonly encountered in professional imaging. In our scale, tungsten-halogen has a color temperature of 3200°K. Household fluorescents are accepted to be around 4500°K, depending on the lamp. (They are shown for reference, but would not be part of a true Color Temperature chart, for reasons described below). Sunlight is 5600°K, with shade & skylight hitting higher temperatures. These are basically averages which became standards when they were selected, back at the beginning of color film manufacturing, as the choices for various emulsions to be made sensitive to (daylight film, tungsten film, etc). There are variations on these standards, but this is a good start to understanding the relationship between different colored light sources. It's important to keep in mind that even tho' reddish light has a technically lower color temperature, its frequently described as warm. Bluish light, which has a higher color temperature, is described as cool. In this instance, warmer & cooler describe color, not temperature.

4500 k
The greenish color of 4500°K fluorescent would not appear in a true Color Temperature chart because a fluorescent lamp does not get its color by heating a black body object to the point of glowing, it uses gases & phosphors instead.

Household quality fluorescent lamps can have either too much green or magenta rendered in their color. The degree to which this occurs will affect the lamps CRI rating.

See Color Rendering Index (CRI) below for more information.

  4500 k
Tungsten incandescent, most common in household lamps, has a slightly lower color temperature at 2900°K than tungsten-halogen (aka quartz) at 3200°K, so its output will be slightly warmer. Incandescant lamps also shift their color, growing warmer as they age, something tungsten-halogen lamps don't suffer from.

Cinematographer Tom Robotham has a theory that we prefer the warm color of tungsten in our living environments because of our long pre-historic practice of sitting around campfires, and our pre-electric history of lighting early dwellings with fireplaces & candles. How many people have a wall dimmer in the dining room to set an intimate mood? Notice that when you dim tungsten lights, they get warmer in color, closer to flames & candles. Their color temperature is shifting lower when this happens.

4500 k
Daylight is not the same as sunlight. Sunlight is the light of the sun only, where daylight is a combination of both sunlight & skylight.

Outdoors, shadows are lit by skylight, since sunlight is being blocked to create them. This is why shadows in exterior day-lit shots are bluish in color.

4500 kSunlight changes its color as it crosses the sky (or more accurately, as the Earth rotates in relation to it).

At dawn & sunset the sun appears more reddish, due to the filtering nature of the denser atmospheric layer it's rays are passing thru at that angle. It has a correlated color temperature of approximately 2000°K at sunrise / sunset, and 5600°K when directly overhead.

Sunset Photo by Marrike Van Irsel

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Color Rendering Index
Color Rendering Index (CRI) Defined
A simple definition of Color Rendering Index (CRI) would measure the ability of a light source to accurately render all frequencies of its color spectrum when compared to a perfect reference light of a similar type (color temperature). It is rated on a scale from 1-100. The lower the CRI rating, the less accurately colors will be reproduced. Light sources that are incandescent radiators have a CRI of 100 since all colors in their spectrum are rendered equally. As stated earlier, light sources that are not incandescent radiators will have Correlated Color Temperatures. Examples of light sources with Correlated Color Temperatures, having CRI levels that are less than 100 would include: HMIs, and also most photo quality fluorescent lamps, as well as LEDs. With lower CRI ratings these sources may also have too much green or magenta in their spectrums. An acceptable Color Rendering Index level for professional imaging is considered to be 90 or above.

Tungsten-halogen 3200K
The lamp formerly known as Quartz has a more stable color temperature throughout the life of the lamp, than tungsten-incandescant. These lamps get hot and have shorter lifespans than some others. CRI is 100.
Fluorescent (Photo Quality)
3000-3200K, or 5000-5600K

Fluorescent lamps made for photo use are available in tungsten-halogen or daylight colors, with CRI's over 90. High frequency ballasts are flicker free even when shooting slo-motion.
6000  k
HMI 6000K
HMI stands for Hydrargyrum Medium-arc Iodide lamp. This discharge lamp has a very high output of daylight color (usually 6000K), normally with a CRI of 95+.
LED (for imaging) 3000-6000K
LED stands for Light Emitting Diode. A semi-conductor based light source, that is energy efficient, with a long life. CRI is normally 70 - 90+, but subject to controversy.

Comparison of High & Low CRI Fluorescent Lamps
These 2 images are worth really studying. Both were shot with daylight colored fluorescent lamps and a camera with daylight preset white balance. The image on the left was shot with Lowel 27w day-flo lamps (CRI 92+), and the image on the right side was shot with a household day-flo lamps (CRI not listed, but assumed to be aprox. 80). Compare the details of each image, noting where the colors are pretty similar, such as the red & orange peppers, and the radishes. Then look at the items with colors rendered differently, such as the floor, cutting board, carrots, cabbage, and lettuce leaf, for example. A low Color Rendering Index does not mean all colors will shift, and no 2 lamps with the same low CRI rating will necessarily have the same errors in rendering.

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