Color Temperature

Seeing the Temperature

Think about an electric stove. The color of a cold burner is black. As the burner heats up, the color changes from black to red. If we were able to heat it hotter than "red hot," it would become white. This is where the term "white hot" comes from. Did you know that if you keep heating it, it will actually go past white and start to become blue? The tern used to measure that change is color temperature.

Rough Color Temperature Scale in Kelvin

Color temperature would not be such an important factor in color perception if people were not adaptable. Our eyes will adjust to various light sources which have different color temperatures. And even though the power spectrum density of the light is different, we still see the light as white.

Simply put, color temperature is the tendency of different "white light" sources to change our perception of a color.

The color temperature of a light source causes the colors of everything illuminated to change, but the change is often barely noticeable.

White

True White is defined as the equal energy point in the gamut chart. It has equal energy across the spectum. But the color of white itself actually changes with a change in its color temperature. This is true for both light source colors and pigment colors.

If you've ever had a flashlight battery become weak, you may have noticed that the light coming from the incandescent bulb was less white and actually had a red tinge to it.

This gamut chart shows the color temperature of white light. At the real white point where the white is at an even spectrum, the color temperature is approximately 5000 degrees Kelvin. If we are in a dark room, even if the "white light"source were quite blue or quite red, we would adjust and see it as white.

If we used instruments to measure the color temperature, this chart shows what we would find as the color source changes with temperature.

The Affects from various Color Temperature Light Sources

Again, different types of light sources have different color temperatures. The color temperature from incandescent lights, fluorescent lights and sunlight all have a different color temperature. Fluorescent bulbs are actually rated by their color temperature as an indication. They are given a CCT (Correlated Color Temperature) rating. Warmer colors have a lower CCT while cooler colors have a higher CCT.

In the table below, we'll show you how the same object can appear different when illuminated by different types of light.

You could do this experiment yourself by creating a bookshelf where each shelf had recessed lighting which illuminated a set of similar objects. If you stood back and looked at all the objects on the shelves, they would look different because your eyes are not solely exposed to the light from only one of these light sources. They would not adjust to just one of them.

If, however, you stood close to one of the shelves and looked only at the items on that shelf for a short while, your eyes would adjust to that one light and you would see it as normal white light.

Here is a chart which shows the effects which different types of lighting can have on a colored object. White light is not always the same color. What seems like white light is actually different depending on the light source. The spectral content of different light sources will also have an affect.

Sunlight and Temperature

Even the effect of sunlight on the perceived color of an object changes with temperature because of how much air the sunlight has to pass through. If the sun is directly overhead (warmer), it passes through less air than it does when it is low on the horizon (cooler). Colors in northern climates, especially in the winter, look different than they do in southern climates. If you live in a place like Seattle and travel to Mexico, this change can be striking.

The Effects of Color Temperature on Work Productivity

Full Spectrum Lighting

Research has been done which shows that the use of warmer, full spectrum fluorescent lights will improve worker productivity in office buildings. Cool white fluorescent bulbs have been shown to cause eye strain, headaches and irritability. They have also been shown to affect physical, emotional and mental well being.

White Balance

All cameras have to make an adjustment in order to compensate for the different properties of the light source. This is called white balance. I they didn't, the colors would not be correct in the picture.