The Trouble With Lumens

The way that lumen output is traditionally measured, reported, and interpreted poses a few potential problems for evaluating and comparing LED lighting luminaires:

• Since complete and accurate definitions of lumens and related photometric terms can be technical and complex, they are often misunderstood. Without a good understanding of these terms, however, the unique properties of LED lighting sources cannot be clearly grasped.
• Lumens are an imperfect measurement of the perceived intensity of light sources, with known shortcomings. The specific spectral properties of LED light sources exaggerate these shortcomings, especially toward the blue end of the spectrum.
• Conventional lighting luminaire manufacturers often report total lamp lumens more prominently than or instead of total luminaire lumens. Because many LED lighting luminaires do away with the distinction between lamp and luminaire, only total luminaire lumens can serve as a basis for valid comparisons between LED and conventional lighting luminaires.
• LED lighting luminaires and conventional lighting luminaires are tested differently, and therefore some photometric data is reported differently. These differences must be taken into consideration to accurately compare conventional lighting luminaires and LED lighting luminaires.
• A luminaire’s total lumen output does not account for wasted light. Because LED lighting luminaires are fundamentally directional and natively create shades of white and colored light without filtering or additional lensing and shading, LED luminaires typically waste much less light than their conventional counterparts and deliver more of their total light output to a task or target area.

An LED lighting luminaire with lower rated lumens, therefore, may deliver the same or more useful light in a specific application than a comparable conventional lighting luminaire with a higher rated lumen output. Each of these issues is discussed in greater detail in the sections that follow.

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.

The spectral luminous efficiency function weights the perceived intensity of light of different wavelengths based on the varying sensitivities of the human eye. The eye is most sensitive to light around 550 nm in the green-yellow area of the spectrum and is less sensitive at either the red or blue end.

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. 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.

Lumens are calculated using the efficiency curve’s weighting function. Two lights with the same radiant flux but producing light at different wavelengths, therefore, have different measured lumens.

Various modifications of the standard eye-sensitivity curve, such as the Judd-Vos correction shown here, have been proposed to improve its accuracy in representing eye responses to different wavelengths of light.

Deficiencies of the Eye-Sensitivity Curve

It is well understood that the eye-sensitivity curve underestimates the perceived intensity of wavelengths toward the blue end of the spectrum. Although none has been widely adopted, various modifications of the eye-sensitivity curve have been suggested over the years. The Judd-Vos correction, for example, adjusts the curve to more accurately represent the normal sensitivity of human vision, especially to blue light.

Visible wavelengths excluded from standard measurements of luminous flux can significantly underestimate the perceived intensity of some LEDs. The Judd-Vos modification partially corrects for this.

The Judd-Vos correction, may not look like much, and it has relatively little effect when comparing conventional light sources with one another. But the correction can make a great deal of difference when measuring the luminous flux of LED light sources and comparing it to that of conventional light sources. Conventional light sources tend to radiate across a wide range of visible wavelengths. Incandescent light sources typically radiate throughout the visible band. Fluorescent light sources show a spiky spectrum, with intense radiation in some narrow wavelength bands and lesser levels of radiation throughout, due to emission lines of mercury, which LEDs do not contain. Solid-color LED light sources usually radiate in a single, narrow band of wavelengths, which exaggerates discrepancies in the eye-sensitivity curve. The calculated lumens for a blue LED source with a peak wavelength of around 460 nm, for example, does not account for a significant portion of the visible light that the LED produces. In practice, the deficiencies of the eye sensitivity curve can result in lumen measurements that underestimate the perceived intensity of LED light sources, especially blue light. The perceived intensity of an LED lighting luminaire, therefore, could be greater — in some cases much greater — than the luminaire’s reported lumens suggest.