Not All Lumens Are Created Equal: What Your Eyes Really See in LED Lighting

Benjamin Latson

Changing a light bulb – it seems so simple. Just swap out your incandescent bulb with a new energy-efficient LED, right? That’s easy enough at home, but in commercial applications, things tend to get a little more complicated. For example, if you try to replace a 400W metal halide lamp with an LED version, you won’t find a lamp with the same lumen output. You will find a slew of LED lamps with half as much lumen output as you need, claiming to be direct replacements for the lamps you already have. So why the discrepancy? The explanation lies in a decades-old series of experiments designed to explore how the human eye processes light.

The human eye perceives light with cells called photoreceptors that come in two types: rods and cones. Cones are responsible for sensing color but only work when lots of light is present, resulting in what we refer to as photopic vision. Rod cells, on the other hand, cannot sense color but allow us to see even in very low-light conditions through scotopic vision. Because of their sensitivity, rod cells have long been considered to only be active in low-light conditions. As a result most light meters are not designed or calibrated with scotopic vision in mind and most lamp outputs are measured in raw photopic lumens. After all, most people don’t expect to encounter a low-light situation in their daily work environments. But here’s the crux of the problem: scotopic vision actually plays a much greater role than we ever thought.

In fact, experiments conducted by Dr. Sam Berman and Dr. Don Jewett more than a decade ago revealed our scotopic vision is actually quite active at nearly all levels of lighting. Not only that, it turns out that scotopic vision is responsible for controlling pupil dilation, a key factor in the sharpness of vision, perception of glare, and overall perceived brightness. But what does this have to do with LEDs? Does LED lighting cause our rod cells to impact our ability to see? The short answer is yes—but only if the manufacturer chooses to make it so.

The rod cells in our eyes have evolved to respond most strongly to certain frequencies of light and, as a result, increase our ability to see clearly in conditions where that kind of light is present. LEDs, by their very nature, emit very specific frequencies of light as determined by the manufacturer. Generally these frequencies are spread across the spectrum most commonly associated with sunlight. For the vast majority of cases this is a perfectly acceptable method of LED design. But occasionally an LED designed in this manner is simply incapable of putting out the necessary amount of light. Because of this, manufacturers design some LEDs to emit more light in frequency ranges that activate our rod cells and thus, our scotopic vision. As our rod cells activate they “instruct” our eyes to adjust the size of our pupils and increase the sensitivity and fidelity of our vision. The result is a quality of light that seems brighter than it actually is! In the case of the 400W metal halide replacements mentioned before, using scotopically tuned LEDs more than doubles the Visually Effective Lumens (VEL) of the light source. Even with fewer lumens, the light appears just as bright as the bulb you are replacing.

Luminous Efficacy Graph
Image Source:

Now for the all-important question: Why haven’t we heard anything about this before? Why don’t manufacturers include the scotopic effectiveness of a light on all products they sell? After all, this seems like this information is important to the consumer and any testing agency responsible for qualifying these products.

The answer may lie in the limited use of scotopically-enhanced LEDs for extremely high output applications. It may also be that the concept was deemed too complicated for common usage and left out, even when it was being used to create a better product. Or it may simply be that the “if it’s not broken, don’t fix it” mindset has prevailed among the regulators, policy-makers, and engineers responsible for the lighting standards and practices we use today.

What we do know is that advances in lighting science and LED technology have the power to create a world where lighting isn’t just about what can be done to make more lumens, but rather what those lumens can be made to do.


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