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Color

Light is one small portion of the electromagnetic spectrum.

The Visible Spectrum

As seen above, light at one end of the visible spectrum has shorter wavelengths near the 400nm range of the spectrum producing a "blue" visual sensation.

Medium wavelengths in the 500-600nm range produce a "yellow to green" sensation. Longer wavelengths produce a "reddish" sensation.

Color Temperature 

The color temperature of a light source is a numerical measurement of its color appearance. It is based on the principle that any object will emit light if it is heated to a high enough temperature and that, as the temperature increases, the color of that light will change along with the temperature. 

If you have ever seen a blacksmith at work heating a horseshoe (or maybe in an old Western), you would see that the horseshoe glows "red hot." It was heated enough so that it gave off light, red light in this case. Keep heating it and it would go from red to orange to yellow to white and, finally, to blue white. 

The Kelvin scale

A light source's color temperature, then, is the temperature at which the color of the horseshoe would match the color of the light source. We express this temperature in degrees, but not Fahrenheit degrees like we are used to, but in Kelvin.

This scale was named after Lord Kelvin, a British inventor. The correct unit is "kelvin" (lower case), not "degrees Kelvin."

So, for example, a light source may be specified as having a color temperature of 4100 kelvin or 4100K. This means that this particular light source would have a slightly bluish, somewhat cool color appearance, the same as if our horseshoe were heated to 4100 degrees on the Kelvin scale. 

The object used for correlating the color temperature (CCT) is a theoretical object called a "blackbody radiator." Keep in mind, however, that the light source really isn't the same thermal temperature as the theoretically heated object. Color temperature is just a way of correlating the source's appearance to that of a heated object.

Warm vs. Cool - the Psychology of Light

Some people find it confusing that low color temperature light sources are called "warm" while those with higher color temperatures are called "cool." In fact, these descriptions have nothing to do with the temperature of the horseshoe (blackbody radiator). They refer to the way color groups are perceived - the psychological impact of lighting. Colors and light sources from the violet/blue end of the spectrum are referred to as "cool," and those toward the red/orange/yellow side are "warm" 

How Light Affects the Colors of Objects

Have you ever noticed that the rug you bought in the store wasn't the same color when you brought it home? Well, the reason is the light sources were different. Here is a way to assess how light sources make objects appear. It's called Color Rendering Index (CRI) - a system derived from visual experiments. If you had a light source you wanted to assess, here is what you'd do. First, get out your set of eight standard color samples as seen here:

Munsell Color Chart

Illuminate them with your light source, then with a "standard" source, and compare the way they look. If none of the samples appear to be a different color, then your light source is given a rating of 100 CRI. Any change in color of the samples would evoke a lower rating. The CRI decreases as the average change in color appearance of the eight samples increases (in the real world, we use some calculations to determine the rating.)

With knowledge of both CCT and CRI, a general impression can be given of how objects or a space will appear. Selection of a light source for a particular application however, must take into account many other factors, such as the interior design, the "mood" one wishes to convey, and even the economics.

Spectral Power Distribution

Above A spectral power distribution (SPD) curve shows the precise color output of a given light source by charting the level of energy present at each wavelength across the visible spectrum. SPD diagrams can also be very useful in understanding how various lamps differ in the color composition of their light output.

A Few Lighting Metrics

COLOR RENDERING INDEX (CRI)

A number between 1 and 100 used to describe the ability of a lamp to accurately render all the colors in the lighted space. For example, a CRI of 80 or above normally indicates that the source has good color properties; it would not significantly distort or diminish the color of the object being illuminated.

COLOR TEMPERATURE

The absolute temperature of a blackbody radiator, having a chromaticity equal to that of the source.

CORRELATED COLOR TEMPERATURE (CCT)

Temperature, describes the overall color appearance of a lamp. CCT is measured in Kelvins (K). Color temperature is used to describe the overall color tone of a white light source. Common warmer light sources, similar to incandescent color, have a Kelvin temperature of 2700K or 3000K.

Somewhat cooler light sources commonly used in offices include 3500K and 4100K. Very cool color temperatures, often used to match daylight, are 5000K and 65000K.

Spotlight
Color Vision 

Color vision is an illusion created by the interactions of billions of neurons in our brain. There is no color in the external world; it is created by neural programs and projected onto the outer world we see. It is intimately linked to the perception of form where color facilitates detecting borders of objects.

Color is created by utilizing two properties of light, energy and frequency of vibration or wavelength. How our brain separates these two properties of light, energy and wavelength, and then recombines them into color perception is a mystery that has intrigued scientists through the ages. We know much about the nature of light and the subjective impressions of color, definable by physical standards (Wright, 1946) but ultimately color should be explained at the level of single cells in our brain. Examination of the responses of single neurons or arrays of such neurons provides the best insights into the physiology of color vision. Ultimately our understanding of this process will allow us to model the neural circuits that underlie the perception of color and form. Although still beyond reach, progress is being made in deciphering these clever circuits that create our perception of the external world.

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