Understanding Pupil Lumens: Why LED Lights Appear Brighter Than Their Lumen Rating Suggests

Pupil Lumens is a concept that is not well-known outside of the lighting industry, but it is an important aspect to consider for lighting upgrades. Here is a helpful example: Recently, a warehouse building modernized its lighting system. Before its lighting upgrade project, it was lit by low High-Pressure Sodium (HPS) lamps, and they were replaced with higher CRI LED High Bays. The luminous intensity before the retrofit was 35-foot candles, which fell to only 25-foot candles after the retrofit project! This was a reduction of 28% in light intensity. Yet most users of the facility rated the new lights as being significantly brighter than the older lamps.

This defies common sense and begs that a few questions should be answered:

1. Why is it called a lighting upgrade project when the light intensity is falling?
2. Why would a company pay to reduce the light intensity in its facilities where security and safety are very important?
3. How can a setting that has lower light intensity appear brighter?

The answers stem from the term ‘Pupil Lumens’. Before delving further, it will help to clarify a few concepts:

Key Lighting Terminology

• Luminous flux / Light output: It is the amount of light produced by a lamp. It does not take into consideration the direction in which the light is sent. Most lighting devices produce light in a 360-degree sphere. This light must then be redirected by a set of optics to the area where it is needed. Light output is measured in lumens.
• Illuminance / Light level: It is the amount of light incident on a surface. It is measured in foot candles (FC) or lumens per square foot.
• Luminance / Brightness: It is the amount of light reflected by a surface. It is measured in foot-lamberts.

The diagram below demonstrates each one of these terms:

The human eye sees Luminance. It is NOT concerned with Luminous flux or illuminance. Thus, a dark-colored surface absorbs most of the light and has a low luminance, while a lighter-colored surface reflects more of the light rays incident on it and has better luminance.

How LED Technology Improves Perceived Light Quality

All the light output of an LED light source is directed downward, resulting in better Luminous Flux distribution. This is only half the story in light sources.

Compared to High-Pressure Sodium (HPS) and Low-Pressure Sodium (LPS), LED lights have a far higher Color Rendering Index (CRI). A higher CRI is achieved because the light produced by LED bulbs is not monochromatic but has a richer palette of wavelengths of light.

An incandescent bulb has a CRI of 100. However, CRI values between 75-100 are considered to be excellent for general-purpose lighting, 65-75 is considered good, while anything less than 55 is considered to be poor and dim light. When illuminated by a high CRI source, the surface colors of an object appear brighter, thus resulting in a perception of higher Luminance.

The Science Behind Human Vision and Light Perception

From physics to biology – understanding the eyes’ response to Light

• Cones and Photopic vision: The central part of the eye is the fovea, which is rich in a type of cell called cones. Cones are responsible for color vision and are involved in vision during bright light. This is called Photopic vision.
• Rods and Scotopic vision: Rods are sensitive to dim light and are active during low light intensity conditions. Rods cannot perceive color. Vision due to rods is ‘black and white.’
• Light measuring instruments measure Light intensity in Lumens. This only considers the response of the cones. Thus, the Lumens measured by a standard light meter are also called Photopic Lumens.

In conditions of low light intensity, entire vision is due to rod cells (scotopic vision). In medium light intensity conditions, often found under street lights and in commercial facilities is called Mesopic vision and is due to both rods and cones.

Employing ‘Photopic Lumens’ alone to explain the light intensity in a space may underestimate the perceived brightness, as it does not account for the role of rod cells in a person’s vision. The concept of scotopic lumens was developed by scientists from the Lawrence Berkeley Lab. They established a factor known as the S/P ratio (Scotopic to Photopic ratio). An S/P ratio can be used to convert conventional Lumens into a measurement that may better represent how our eyes perceive light during mesopic light situations.

Understanding the Pupil Lumens Calculation

Pupil Lumens = Photopic Lumens × [S/P]^0.78

This formula represents one approach to understanding perceived brightness, though it should be considered alongside other factors when making lighting decisions.

Factoring in the S/P ratios helps explain why Low-Pressure Sodium lamps, which are highly efficient with a photopic lumen output of 130 Lumens/Watt, may appear dull. They don’t offer the spectrum of light that stimulates the optimal response from the retina in mesopic lighting circumstances.

Therefore, if you are planning to opt for LED lights as replacements for existing lights, it may be helpful to consider more than just the published lumen output. The photopic lumen output may only partially reflect the perceived brightness. With differences in lighting technology, there is a marked shift in wavelength composition and CRI.

Comparative Performance Table

Why LED Spectral Quality Makes a Difference

There is another factor underlying the improved perception of light from high CRI LED fixtures. The diagram below shows the relative luminous efficiency of different wavelengths of light. Cones have maximum efficiency at about 550 nm (green light) while the efficiency of rods peaks at around 510 nm (blue-green light).

HPS and LPS lamps that are poor in the green-yellow and blue-green wavelengths naturally have a lower impact on the eye’s photoreceptors.

While the current method of measuring Lumens accounts for the 550 nm peak, it gives less weight to the 510 nm range. The Scotopic response is influenced by the blue light content of the light source. Thus, 6500K fluorescent lights have better S/P ratios than 3500K tube lights.

HPS Lamp Spectral Analysis

Here is the spectrum of the HPS lamp:

Photo credit: flickr.com/photos/79262083@N00/342148466

We have already seen that the eye is most sensitive to wavelengths of 510 nm and 550 nm. These very wavelengths are limited in the spectrum of an HPS lamp. The most prominent wavelengths in the spectrum of an HPS lamp are between 575 and 650 nm. The sensitivity of the eye to these wavelengths is 20-80% of the peak response.

This helps explain why HPS lamps may not elicit the optimal response from the human eye, and why fewer lumens from a quality LED source can provide a similar perception of brightness.

Here is a visual representation of the wavelengths of an HPS lamp obtained using spectroscopic analysis:

Photo credit: Chris Heilman, Wikimedia Commons

The gaps in the spectrum are notable. The heaviest density is in the red-yellow region. Moreover, because the spectrum has gaps, HPS lamps typically have poor color rendering capabilities.

LED lights provide a more complete spectrum with no significant gaps. The wavelengths of 510 and 550 nm, which are most effective in stimulating the eyes, are well represented in the LED spectrum. The result is better color rendering and potentially more effective stimulation of the eye.

More research continues in this direction, and it is anticipated that future lighting standards may better reflect the biological response to light.

Summary: Key Points About Pupil Lumens

• When it comes to vision, it is Luminance (the amount of light reflected by a surface) that matters most for perception.
• Lumens is a unit of Illuminance that is used as a proxy for Luminance, as typically the two are directly correlated.
• The incorporation of S/P ratios in Lumen/watt calculations can change the efficiency comparison of lamps. LEDs that already have a high Lumen/watt ratio may perform even better when S/P ratios are considered, while LPS and HPS lamps may show different relative performance.
• Warehouses, retail stores, street lighting, gymnasiums, and commercial and industrial facilities may benefit from considering the higher S/P ratio characteristics of LED products when planning lighting upgrades.

Photo credit: Jason Morrison, Flickr

Professional Consultation Recommended

When planning significant lighting upgrades, it’s recommended to consult with qualified lighting professionals and licensed electricians to ensure optimal results for your specific application. Contact our lighting experts to discuss your project requirements and explore LED solutions that may provide both energy savings and improved light quality for your facility.