Color Temperature

Color Temperature

What is “Color Temperature“, and how does it relate to CRI, or the Color Rendering Index?

We know CRI to be an international measurement scale or rating of how accurately an artificial  light source renders, or shows the color of an object (often called its “color appearance” ) as compared to daylight, which is capable of depicting, rendering or showing the widest range of colors. Thus, CRI rates, describes or measures how the  colors of objects  appear (compared to daylight) under a specific light source…IE, in a given light.

Color temperature however refers to the actual color and type of  light emitted by a particular light source.  High color temperatures, those over 5,000K (K = Kelvin) are termed cool colors  and are bluish white, while lower color temperatures (2,700–3,000 K) are called warm colors  and are yellowish white through red.

Yes, counter-intuitively, the higher color temperature describes a cooler color, while a lower color temperature describes a warmer hue. When considering interior lighting,  color temperature can play a significant role in how we feel and function in a space.  A warmer light, which has a  lower color temperature, can  promote our relaxation, while a cooler  light with a higher color temperature may enhance or help to increase our concentration.

Thus, color temperature refers to the color of light, and CRI refers to the ability of a light source to render the color of objects in a manner comparable to the way daylight does/would.

Whew…what a mouthful!

The rub?  The color rendering index can be used as a basis of comparison between light sources only if they have the same color temperature. To compare light sources, or to compare an artificial light source to daylight, said artificial light source must have the same color temperature as the daylight to which it is being compared.  Remember, daylight renders, or makes visible, the widest range of colors…and there are yet many more colors in the light spectrum that we cannot see with our naked eyes.  We are not able to see the entire light (thus color) spectrum. Light values beneath the visible part of the spectrum are referred to as infrared, and above the spectrum as ultraviolet.

CCT, or Correlated Color Temperature  is expressed in Kelvin, and describes whether a white light source appears more yellow/gold, or more blue.  Thus, it  is  a numerical description of a  light’s color appearance.  It can be used as a means of correlating the color of an artificial light source with the color of daylight. “The correlated color temperature (CCT) is a specification of the color appearance of the light emitted by a lamp, relating its color to the color of light from a reference source when heated to a particular temperature, measured in degrees Kelvin (K).

Here, we bring our discussion of Color Temperature, and its relationship to CRI to a close, at least for the moment.  It is a complex subject, but one that can come into play when dealing with any matters of color and light in the realms of photography, film, video, interior design, theater, the visual arts, and many other areas.  Good for us to have some idea of what the terms mean!


I hope that this series of posts on the relationship between Color and Light has helped to demystify it for you…at least to some extent. As much as we can learn, observe and discover, we will never know it all…and maybe that is as it should be.  Some things, such as the majesty and magnificence of the natural world should retain some mystery…no matter how much physics we attempt to wrap our minds around.




Rendering Color II

Rendering Color II

“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

In the post, “Color Rendering I”  I delved into the nature of color and light…as Paul Outerbridge  says above,  color is a property of light…the color that we see an object as “being”, is in essence, light…the wavelengths of light it reflects, as opposed to absorbs.

In this post, I am seeking to clarify CRI…what does that mean?

CRI, or the Color Rendering Index, is a scale that measures not color, not light, but ” the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source”.

The ideal or natural light source being daylight, because “it (daylight) displays (1) a great variety of colours, (2) makes it easy to distinguish slight shades of colour, and (3) the colours of objects around us obviously look natural.”P.J. Bouma

The International Commission on Illumination  (which is usually abbreviated C.I.E.  for its French name Commission Internationale de L’Eclairage), the international authority on color, color spaces, light, and illumination, has defined CRI as the  “Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant”.

Is the concept and definition of CRI becoming any more clear?

Trying to explain CRI reminds me of trying to translate from one language to another, in a manner that makes the meaning of a phrase in one language, comprehensible in another.  It isn’t enough just to translate the words…the whole meaning, context, and  sense of the phrase must be understood.

How’s this:

  The closer the red of your child’s red beach ball inside, under the light of say, your dining room chandelier, looks to the red the same ball appears to be outside, on the beach, under the sunlight, the higher the CRI is of that dining room chandelier illuminant.  CRI measures the ability of a light source to reveal, render, depict or show color the way daylight would.

Put another way, the color rendering index describes the  effect of a light source on how the color of an object appears to us.  It is the measurement of how much an object’s color appearance shifts when illuminated by an artificial (other than daylight)  light source compared to the color appearance of the same object when  illuminated by a “reference” light source (daylight), of comparable color temperature.

Whoops!  Color Temperature!?!  Suffice it to say, right here, right now, the CRI of a light source can only be determined when it is being compared to a reference illuminant, (natural light/daylight), with the same, or comparable color temperature.  The role of “Correlated Color Temperature”  in CRI will be discussed in a future post.

So…until then chew on the above…and I hope the light bulb goes on for you about what Color Rendering Index is.

May both your days, and nights be illuminated with light sources of the highest CRI!