Pupil Lumens

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:

• 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 / square foot.
• Luminance / Brightness: It is the amount of light reflected by a surface. It is measured in foot-lamberts.

The diagram below is meant to demonstrate 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 Can LED Improve Light Intensity or Luminance?

All the light output of an LED light source is directed downward resulting in better Luminous Flux. 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.

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 cells 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 homes) vision is called Mesopic vision and is due to both rods and cones. Employing ‘Photopic Lumens’ to explain the light intensity in a space unacceptably underrates the light intensity as it completely disregards the role of rod cells in a person’s vision.

The concept of scotopic lumens was developed by scientists from the Lawrence Berkley Lab. They established a factor known as a P/S ratio. A P/S ratio converts conventional Lumens into tangible lumens understood to our eyes during mesopic light situations. It provides a more precise approximation of the quantity of light.

Pupil Lumens = Photopic Lumens * [S/P] 0.78

Factoring in the S/P ratios reveals why Low-Pressure Sodium lamps, which are the most efficient with a photopic lumen output of 130 Lumens/ Watt, appear so dull. They don’t offer the spectrum of light that is needed to light objects appropriately. They also fail to stimulate the best possible response from the retina in mesopic lighting circumstances.

This improvement in lumen perception is the reason why people and municipalities are opting for full-spectrum lights. The current codes do not reflect scotopic lumens.

Therefore, if you are planning to opt for LED lights as replacements for existing lights of a generation earlier, it is prudent to avoid going by the published lumen output alone. The photopic lumen output of light may only partially reflect the reality of the situation.

Comparing absolute Photopic Lumens was acceptable as long as the same type of bulb was being compared. With differences in lighting technology, there is a marked shift in wavelength composition and CRI. It is always better to try out energy-efficient LED lights and experience their light quality first-hand.

There is another factor underlying the improved perception of light from high CRI LED fixtures, like High Bays. Below is a diagram showing the relative luminous efficiency of different wavelengths of light. Thus, cones have maximum efficiency at about 550 nm (green light) while the efficiency of rods is at its peak at around 510 nm (blue-green light).

HPS and LPS lamps that are poor in the green-yellow and blue-yellow wavelengths naturally have a lower impact on the eye’s photoreceptors. While the current method of measuring Lumens takes care of the 550 nm peak it does little to tackle issues with the lower band of 510 nm.

The Scotopic response is dependent on the blue light content of light. Thus, 6500 K fluorescent lights have better S/ P ratios than 3500 K tube lights. A look at the spectrum of light from different light sources can help clarify a lot of things.

You always suspected that the light from the HPS lamp was dull – you were right. While the absolute lumens/watt produced by an HPS lamp is high, it is the wavelength of the light that it produces that makes it appear dull.

Here is the spectrum of the HPS lamp.

(Photo credit – https://www.flickr.com/photos/79262083@N00/342148466/sizes/l/in/photostream/)

We have already seen that the eye is most sensitive to wavelengths of 510 nm and 550 nm. These very wavelengths are scarce 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 is the reason why HPS lamps do not elicit the optimum response from the human eye and far fewer lumens from a good LED source can provide the same perception of brightness.

Another way of understanding the limitations of the HPS lamp is to look at its light color spectrum. Here is a visual representation of the wavelengths of an HPS lamp. This spectrum was obtained by using a homemade spectroscope.

(Photo credit – Chris Heilman https://commons.wikimedia.org/wiki/File:Spectrum-hp-sodium.jpg)

The large gaps in the spectrum are obvious. The heaviest density is in the red-yellow region. That is not very effective in stimulating the eyes. Moreover, because the spectrum is patchy HPS lamps are not good at color rendering.

This image was also obtained by using a spectroscope. The spectroscope had a design different from the one used to obtain the spectrum of the HPS lamp, yet the results are unambiguous.  The spectrum is complete with no gaps. The wavelengths of 510 and 550 nm that are the most effective in stimulating the eyes are well represented in the spectrum. The result is better color rendering and more effective stimulation of the eye.

More research is continuing in this direction, and it is hoped that future lighting standards will reflect the biological response to light in a more meaningful manner than they currently do.

To Summarize Pupil Lumens

• When it comes to vision, it is Luminance (the amount of light reflected by a surface) that matters.
• Lumens is a unit of Illuminance that is used as a proxy for Luminance as usually, the two are directly correlated.
• The incorporation of S/P ratios in Lumen/watt calculations completely changes the efficiency rating of lamps. LEDs that already have a high Lumen/watt ratio fare even better when S/P ratios are taken into consideration while LPS and HPS lamps that are generally believed to have high Lumen/watt output fare poorly.
• Warehouses, retail stores, street lights, gymnasiums, and commercial lighting and industrial lighting areas can easily benefit from the higher S/P ratio of LED products.

(Photo credit – Jason Morrison https://www.flickr.com/photos/jason-morrison/3471835685/sizes/o/in/photostream/)