We Don’t Use Lumens for Plants

If you spend any amount of time reading the side of lighting boxes, you will quickly notice that lumens appear everywhere. The numbers are usually very large, the font is usually enthusiastic, and the general implication is that a higher lumen rating must obviously mean a better light.

For most forms of lighting, that assumption works reasonably well. If you are trying to illuminate a warehouse, a garage, a workshop, or a parking lot, lumens are actually a very useful measurement. They tell you how bright the light will appear to the human eye, which is exactly what those spaces need.

Plants, however, are not human.

They are also not warehouses, garages, workshops, or parking lots. They have their own priorities. They are quiet about it, but they do.

And those priorities do not include caring how bright something looks to us.

What Lumens Actually Measure

Lumens measure perceived brightness to human vision.

More specifically, lumens are calculated using something called the photopic luminosity function, which describes how sensitive the average human eye is to different wavelengths of light.

Human vision is most sensitive to light around 555 nanometers, which sits right in the green portion of the visible spectrum. Because of this, the lumen system heavily weights green wavelengths when calculating brightness.

In practical terms, that means a light producing lots of green light will score extremely high in lumens, because our eyes interpret that wavelength very efficiently.

If two lights consume the same electrical power but one produces more green light, the green-heavy light will appear brighter to us and therefore receive a higher lumen rating.

This system works wonderfully for lighting designed for people, because people are the ones using the space.

Plants, on the other hand, have a completely different set of biological machinery.

Plants Are Running a Different Operating System

Plants do not “see” light in the same way humans do.

Instead of visual perception, plants rely on photoreceptors that respond to specific wavelengths of light. The most important process for growth, of course, is photosynthesis, which uses photons to drive the chemical reactions that create plant sugars.

The wavelengths that drive photosynthesis most effectively fall within a range known as Photosynthetically Active Radiation, usually shortened to PAR. This range spans approximately 400 to 700 nanometers, covering blue, green, and red light.

Within this range, plants tend to use red and blue wavelengths very efficiently, while green light is somewhat less active in driving photosynthesis.

This is where things become slightly awkward for the lumen system.

Because lumens heavily emphasize green wavelengths — the ones our eyes love — a light can produce a very impressive lumen rating while still delivering a relatively modest number of the photons plants actually use most effectively.

In other words, a light can look incredibly bright to a human standing underneath it and still be somewhat mediocre from the perspective of a tomato plant.

The tomato plant will not complain about this. Tomatoes are polite like that. But it is still happening.

A Brief Detour Into Footcandles

Sooner or later, when discussing lighting, another unit tends to appear: footcandles.

A footcandle is simply a measure of how much light lands on a surface. Specifically, one footcandle equals one lumen per square foot.

If lumens measure the total brightness of a light source, footcandles measure how bright that light appears once it spreads out over an area.

This can be extremely useful in architecture, photography, and workplace lighting design. Building codes, for example, often specify minimum footcandle levels for offices, corridors, and industrial workspaces.

Unfortunately for growers, footcandles inherit the exact same limitation as lumens: they are still based on human visual response.

So while a lighting chart might proudly state that a canopy is receiving “2,000 footcandles,” that number still reflects how bright the light appears to a human observer rather than how many photosynthetic photons are actually reaching the plant leaves.

Footcandles are essentially lumens with geography.

Helpful for people.

Still not the language plants speak.

What Growers Actually Use

Because plants respond to photons rather than perceived brightness, horticultural lighting uses a different set of measurements.

The most important ones are:

PPF (Photosynthetic Photon Flux)
This measures the total number of photosynthetic photons a fixture emits each second.

PPFD (Photosynthetic Photon Flux Density)
This measures how many of those photons actually land on a given area of plant canopy.

PPE (Photosynthetic Photon Efficacy)
This measures how efficiently a fixture converts electricity into photosynthetic photons.

These values are expressed in micromoles, which sounds a bit scientific at first, but really just means we are counting the photons plants can use.

Together, these numbers provide a much clearer picture of how a grow light will perform in an actual growing environment.

Plants, it turns out, are extremely interested in photons.

They are considerably less interested in how impressed we are by the brightness of the room.

Why Lumens Still Appear on Grow Lights

If lumens are not very useful for plant lighting, you might reasonably ask why they still appear on so many grow light boxes.

The answer is fairly simple: lumens are part of the general lighting industry standard, and manufacturers are often required or encouraged to list them. They are also familiar to consumers, which makes them convenient for packaging.

This does not mean lumens are wrong. They simply answer a different question than growers are asking.

Lumens tell you how bright a light looks.

Growers usually want to know how well a light grows plants.

Those are related questions, but they are not the same question.

The Short Version

Lumens measure brightness as perceived by human vision.

Footcandles measure how that brightness spreads across a surface.

Plants, meanwhile, are quietly conducting photosynthesis using photons in the PAR range.

So while lumens and footcandles are excellent tools for lighting offices, warehouses, and parking lots, they are not especially helpful for evaluating plant lighting.

Parking lots and plants are different critters.

One needs to help people find their cars at night.

The other is trying to turn light into sugar and grow a tomato.

They deserve different measurement systems.