When considering lighting fixtures, the lumen works as one measure of the amount of light that a fixture can produce. While the lumen measures the total perceived power of a light source, consumers as well as many professionals use the term “lumen” to define brightness. However, the measurement of lumens points to the distinction between two different types of light measurements.
Radiometric measurements cover all the visible and invisible wavelengths of a light source. To take this concept a bit further, a radiometric measurement serves as an absolute measurement of the intensity of electromagnetic radiation. For years, manufacturers, retailers, and consumers recognized the use of a radiometric measurement—the watt—as a standard for defining the performance of light bulbs and lighting fixtures. A watt measures radiant flux or the total electromagnetic energy that a light source emits across all wavelengths.
In contrast to radiometric measurements, photometric measurements cover only the visible wavelengths of light or light seen by the human eye. At this point, we can find our way back to lumens through several photometric quantities. Luminous intensity measures the light intensity of an optical source as perceived by the human eye. Luminous flux represents the total energy emitted by a light source or the sensitivity of human eyes to different colors of light. Lumens measure luminous flux.
With the measurement of lumens for different colors, we have reference points that can fit on a curve. Measuring lumens shows how bright a light will appear to human eyes. Scientists use the eye sensitivity curve as a reference when given weight to different wavelengths of light and when calculating lumens. Each reference point on the eye-sensitivity curve represents different wavelengths of light. Because human eyes perceive colors as brighter than other colors, two different colors with the same amount of radiant energy will have different levels of brightness. As a result, one color produces more lumens than the other color.