What Exactly Is a Lumen?
     Light measurements can either be radiometric or photometric. Radiometric measurements measure all the wavelengths of a light source, both visible and invisible. Photometric measurements measure only the visible wavelengths of light. The total electromagnetic energy that a light source emits across all wavelengths is known as radiant flux, and is measured in watts. The total energy that a light source emits across the visible wavelengths of light is known as luminous flux, and is measured in lumens. Since visibility only has meaning in relation to a human viewer, photometric data takes into consideration the varying sensitivities of the human eye to different wavelengths (colors) of visible light. The sensitivity of a human eye with normal vision can be plotted as a bell-shaped curve. This curve is known as the spectral luminous efficiency function, and is often referred to as the eye-sensitivity curve. The eye-sensitivity curve shows that the human eye is most sensitive to light in the green part of the spectrum, around a wavelength of 550 nanometers (nm), and is progressively less sensitive to light toward both the red and blue ends of the spectrum.  To calculate lumens, different wavelengths of light are given more or less weight depending on where they fall on the eye-sensitivity curve. Two light sources with the same radiant flux falling on different parts of the curve will therefore have different lumen measurements. Imagine, for instance, two light sources of 1 watt of radiant flux each. One source emits a blue light at 480 nm, and one emits a green light at 555 nm. As the eye-sensitivity curve shows, the blue light appears significantly less bright than the green light, even though the total energy of the two lights is the same (see the figure at the top of the next page). To put it another way, the green light produces more lumens than the blue light, even though both lights produce the same amount of radiant energy. In practice, there are variations in every individual’s experience of the apparent intensity of a light source. In 1924, the International Commission on Illumination (CIE), a recognized authority on light, illumination, color, and color spaces, standardized the responses of the human eye to visible light by defining a so-called standard observer. The standard observer has regular eye responses to visible light under specific conditions, which the standard defines. The eye-sensitivity curve used in lumens and other photometric measurements is the standard observer’s eye-sensitivity curve, not the eye-sensitivity curve of any actual observer. Lumens and related measurements are therefore approximations or idealizations, which are usually good enough for evaluations and comparisons of different light sources.
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source:  Evaluating Light Output, Philips Color Kinetics

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