The Nature of Sunlight and Color
Light energy from the sun spans a wide range of wavelengths. Much of the total energy is absorbed in our atmosphere and never reaches the Earth’s surface. The light that does get through ranges from 300-2500 nanometers (nm) in wavelength.
A portion of this sunlight is visible to the human eye, and it is these wavelengths, from 400-700 nanometers, that are responsible for color. If an object reflects across this entire wavelength range, then it is white. Black surfaces absorb these wavelengths. If some regions of this light are absorbed and others reflected, then the object is colored. For example, an object that absorbs all visible light except the region 400-450 nm appears blue to our eyes, while another that reflects only 650-700 nm light has a red color.
Other solar wavelengths are invisible to us. Ultraviolet light (<400nm) is full of energy and is responsible for sunburn. Infrared (IR) light (>700nm) is less energetic but comprises a large percentage of the solar energy that actually reaches us. Both Ultraviolet and Infrared light are invisible, and have no affect on color. However, all light, whether visible or invisible, will heat an object that absorbs it. The more solar energy the object absorbs, the greater the heat build-up. Conversely, the greater the reflectivity of an object, the less it will build up heat sitting under the sun.
Two objects can be identical in visible color, yet have very different reflectance characteristics in the Infrared spectrum. The object that reflects IR-light will remain cooler than the object that absorbs it. And because IR-light comprises fully half of sunlight, the IR-reflectivity of an object is even more important than its color when it comes to heat build-up. In other words, an object doesn’t have to be white to be cool.
