Light is radiant energy that travels in waves composed of vibrating electric and magnetic fields. Light waves have both frequency and length; the ranges of frequency and wavelength differentiate light from other forms of radiant energy such as heat and radio waves. These properties are expressed on the electromagnetic spectrum.
Certain light waves comprise a portion of the spectrum called the visible light spectrum. Visible light is capable of exciting the eye’s retina, producing a visual sensation that we call vision. The process of seeing, therefore, requires at least one functioning eye and visible light.
The eye, in fact, does not see objects; it sees only light that is emitted from a source, transmitted through a material, or reflected off of a surface, allowing us to perceive a source, a material, a surface. The object is visible because of contrasts of light. Its colors are perceived because those colors are contained in the light.
It is important to understand light because if we understand it, we can control it. The lighting industry has developed a series of metrics enabling practitioners to measure, understand and predict the behavior of light in a given application. In design, these metrics become tools, enabling the designer to control the behavior of light.
The most fundamental are light output (lumens), light level (footcandles) and luminance (candela per square meter). Many retrofits focus on light output and light level, although luminance is actually the most important of these three basic elements because this is the light that we can actually see.
Light output, also called luminous flux, is the quantity of light exiting a light source or luminaire, measured in lumens. Light level, also called illuminance, is the resulting quantity of light falling on the tasks in the space, measured in footcandles, or lumens per square foot (or lux, lumens per square meter), after all light loss factors are considered. Luminance, sometimes confused with perceived brightness, is the resulting quantity of light intensity radiated from the task in a given direction, measured in candela per square meter, based on the reflectance of the task.
