Grow Light Spectrum Explained Simply

Spectrum simply means the mix of light colours, or wavelengths, a light produces. Sunlight contains the full range, from blue through green to red. Plants respond most strongly to red and blue wavelengths, so early grow lights used mostly red and blue diodes, which is why they glow pink or purple. The spectrum of a light tells you which of those wavelengths it delivers and in what balance.

Wavelengths are measured in nanometres (nm). As a rough map: blue light sits at about 400 to 500 nm, green at 500 to 600 nm, red at 600 to 700 nm, and far-red just beyond that, at about 700 to 750 nm, at the edge of what our eyes can see. A grow light's spectrum chart is simply a graph of how much light it puts out at each of those points.

Because they emit mostly red and blue light, the two bands plants use most efficiently, and red plus blue looks pink or purple to our eyes. It is not a gimmick, but it is not the only option. Modern full-spectrum white LEDs deliver plenty of red and blue within a natural-looking white light, which is easier to live with in a kitchen or living room and grows plants just as well.

Yes, more than the old idea that leaves 'reflect green' suggests. Chlorophyll absorbs red and blue light most strongly in the top layer of a leaf, but green light passes deeper into the leaf and down into the lower leaves of a crop, where it still drives photosynthesis. Leaves look green because they reflect some of it, not because they ignore it.

Green light also matters for a practical reason: our eyes are most sensitive to it. That is why a light rich in green looks very bright to us, and why a pink red-and-blue light looks dim even when it is delivering plenty of usable light to the plants. It is also why white light is kinder to the grower. Under a full-spectrum white LED you can see whether leaves are yellowing, spot pests early and judge when herbs are ready to pick, which is hard under a purple glow.

PAR, or photosynthetically active radiation, is the range of light wavelengths plants can use for growth, roughly the visible spectrum. When you compare grow lights, the useful question is how much PAR reaches your plants, not what colour the light is. A light that looks bright to you can still be weak in PAR, which is why an efficient purpose-built grow light beats a normal room bulb even when both look bright. Brightness to the human eye is measured in lux or lumens, which are weighted towards green, so a lux reading or a phone light-meter app is only a rough guide when you compare two lights with different colours.

These are the two numbers that turn PAR into something you can plan with.

PPFD (photosynthetic photon flux density) is how much usable light lands on a patch of leaves each second, measured in micromoles per square metre per second (µmol/m²/s). It falls quickly as you raise a light, and it is usually highest in the centre of the footprint and weaker at the edges, which is why good manufacturers publish a PPFD map at a stated hanging height.

DLI (daily light integral) is the total usable light a plant receives over a whole day, in moles per square metre per day. It is the number that best predicts how well a crop grows, because it combines how strong the light is with how long it is on. If the light is constant, the sum is simple:

PPFD × hours on × 0.0036 = DLI

So a light giving 200 µmol/m²/s at leaf height for 14 hours a day delivers about 10 mol/m²/day. Leave it on for 16 hours and that rises to about 11.5. Doubling the hours does roughly what doubling the brightness does, which is useful when a small light cannot be made any stronger.

Less than fruiting crops, which is the good news for anyone growing indoors. As a rough guide:

In a UK winter the daylight on even a bright windowsill can fall well short of these figures, and much of it arrives from one side, which is why indoor herbs stretch towards the glass. A grow light does not need to replace the sun entirely; topping up a windowsill for the dark months is often enough. Our guide to what to grow indoors in winter picks out the crops that cope best.

Kelvin (K) is the colour temperature of a white light: how warm or cool it looks to us. Around 2,700 to 3,000 K looks warm and yellowish, like a traditional bulb; 4,000 K looks neutral; 5,000 to 6,500 K looks cool and slightly blue, like daylight.

A warmer white contains proportionally more red and a cooler white more blue, but the Kelvin figure says nothing about how much usable light the lamp puts out. Both warm and cool full-spectrum LEDs grow herbs and salads well. In practice, choose the colour you are happy to live with in a kitchen or living room, then compare lights on PPF and PPFD rather than on Kelvin.

Each band nudges how a plant grows as well as how fast. None of this needs fine-tuning for herbs, but it explains what you see on the shelf.

Full spectrum is the sensible default, but herbs are not fussy. A full-spectrum white LED gives balanced light, looks natural indoors, and grows leafy herbs and salads without you having to think about ratios. Unless you are chasing maximum yields from fruiting crops, you do not need to tune the spectrum - you just need enough usable light over the growing area, delivered efficiently.

Once spectrum is out of the way, five figures tell you most of what you need to know. An independent comparison of greenhouse light fixtures by Utah State University researchers measured efficiency in exactly these terms, as micromoles of usable light per joule of electricity, which is the fairest way to compare lights of different sizes.

For herbs and salads, 12 to 16 hours a day on a simple plug-in timer is the usual range, and most self-contained indoor gardens default to around 16. Longer days let a modest light deliver a respectable DLI, but plants still benefit from a dark period each night, so avoid leaving a light on around the clock.

Running costs are small for herb-sized lights. Our breakdown of hydroponic garden running costs works through the sum at current UK unit rates, and the same watts-times-hours method applies to any standalone grow light.