Light & Color
A simple definition of light is “visually perceived radiant energy”. This “visible” light is just a small part of the electromagnetic spectrum. Light is what energizes our visual system. Light reflected from objects and into our eyes enables us to see.
Light enters the eye through the pupil, is focused on the retina, and is transmitted to our brain via the optic nerve. A significant part of our brain is dedicated to the processing of visual information One of the vagaries of light perception is the fact that it is often highly subjective. It is impossible to prove that any two people see a particular object in exactly the same way. This makes it a real challenge to measure certain attributes of lighting such as color and quantity.
We all tend to think of an object as having fixed colors. In reality, an object’s appearance results from the way it reflects the particular light falling on it. Under pure white light, a red apple appears red because it reflects the red portion of the spectrum and absorbs the rest. Same thing for the quantity of light. While it is fairly easy to determine when there is relatively more or less light on a given object or in space, actual measurements must take into account the way the eye sees and how it responds to that spectrum thing.
Now let’s look at the link between light and color. In fact, you could say that light in color. Light at one end of the spectrum has a shorter wavelength and appears violet in color. Light at the other end has a longer wavelength and appears red. All the other colors are between those two. Those of you who like playing with prisms have probably already figured this out.
When sunlight shines through a prism, you see all the colors of the rainbow (or visible spectrum). Daylight is a mix of all colors. Actually, for electric lighting, we don’t need all the colors to make white. All we need are the primary colors of light – red, green, and blue – and our amazing visual system takes care of the rest. This method of just using particular portions of the spectrum is employed in fluorescent and high-intensity discharge lighting to produce “white” light. It’s not the same as daylight, but it’s close enough.
Color Temperature – The Appearance of Light
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. We express this temperature in degrees, but not Fahrenheit degrees like we are used to, but in a funny scale called Kelvin.
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, and as it happens, the lighting folks have come up with a way to assess how light sources make objects appear. It’s called 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:
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 the color appearance of the eight samples increases
For example, a CRI of 80 or above is normally considered high and indicates that the source has good color properties; it would not significantly distort or diminish the color of the object being illuminated. With knowledge of both CCT and CRI, a general impression can be given of how objects or 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.
