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 I

Rendering Color I

“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

As Mr. Outerbridge so succinctly states, color is a property of light…or, otherwise put, a function of light.  Color and light are intrinsically entwined…part and parcel of each other… mystic twins, or co-dependents, depending on your point of view.

Sunlight, or white light,  is the combination of the entire electromagnetic spectrum ( a light source’s spectrum is a distribution giving its intensity at each wavelength, and most light sources emit light at many different wavelengths) essence, all the colors of the rainbow.  Their combination creates white light, while  lack of light, no color, is perceived by us as black.  Though artists may see black as a color, it is actually the lack of color: a state of no color.  We are discussing color, and color mixing from the point of view of light wavelengths, not color mixing as regards to paints.  This is physics…the physics of light and color.

The terms “color” and “wavelength” here may confusing.  Our eyes will perceive the color based on not only the wavelengths of light that the object we are viewing reflects or absorbs, but  also on the actual measurement of those wavelengths…measured in nanometers. We can see the reflected colors of light which lay in a very small region of the electromagnetic spectrum called, aptly enough, “visible light”.  We cannot see color wavelengths that are absorbed by an object or surface.

Wavelengths ranging from about 400-750 nanometers make up the visible spectrum of light that can be perceived by the human eye.  When light strikes an object certain  of its wavelengths are absorbed by that object, and others are not. Those that are not, those wavelengths of light which bounce  or are reflected off an object, are perceived by the human eye as color.   In essence, “An object appears a certain color because it reflects certain light wavelengths, which are then  perceived by the eye.”

Surfaces we see as black absorb all of the wavelengths of light in the visible spectrum which reach them.  All the “color”  thus is absorbed, and we see no color, and thus black. Surfaces we see as white are reflecting, or bouncing back to our eyes, (specifically, the rods and cones -photoreceptor cells- within our eyes), all of the wavelengths of visible light which reach them.  Thus, as all of these visible wavelengths are being reflected “to us”, and their combination, as discussed above, creates white light, as white light (discussed above)  is the combination of entire electromagnetic spectrum, or all the colors of the rainbow!  (Mind you, if you mix a wide spectrum of varying paint colors together, you will  not get white! We are discussing color in reference to light only in this post!).

Each light source emits different wavelengths of light, thus the way we perceive colors varies depending on how the object we are seeing is illuminated, I.E., the wavelengths of light it absorbs and reflects. we begin to get to the heart of the story…the Color Rendering Index, or CRI.  It will take a second post on this complex subject to further demystify it.

“The Color Rendering Index is an international measurement scale that describes how colors are rendered under an artificial source of light. The standard against which artificial lighting is compared is daylight, because daylight renders the widest variety of colors. Artificial lighting, by contrast, can render very many or very few colors, depending on the nature of the light source. The color rendering index has many applications, especially in art and photography.”

“The Color Rendering Index (sometimes called color rendition index), is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI are desirable in color-critical applications such as photography and cinematography.[1] It is defined by the International Commission on Illumination as follows:[2]

Color rendering: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant.

The CRI of a light source does not indicate the apparent color of the light source; that information is under the rubric of the correlated color temperature (CCT).” —

Because of the complexity and relative “thickness” of this subject, I will delve further into the Color Rendering Index, Color Rendering itself, The International Commission on Illumination, and Correlated Color Temperature, in a subsequent post.

Until then…I am wishing you much light and color in your life.  May your spirit be illuminated, and your soul rendered…in all the colors of the rainbow!