"I bought a 'full spectrum' LED with a purple glow, and the plants are growing but look strange: stretched, pale, with large leaves and thin stems." The spectrum is "ideal" according to the manufacturer's description, 660 nm red and 440 nm blue. The plant uses green light too: it reaches deeper into the leaf and down to the lower leaves, although it adds less to growth than red and blue do. Red plus blue gives you the two outer points of the full spectrum and leaves out everything in between.
Quick glossary: Spectrum is the distribution of radiation across wavelengths; for plant lighting it is measured in nanometers (nm) within the PAR range of 400-700 nm. The blue range (400-500 nm) stimulates compact growth and a thick stem, regulates stomatal opening, and acts as the "day" signal for photoperiodic responses. The red range (600-700 nm) is the main "fuel" for photosynthesis and the band chlorophyll absorbs most efficiently, with a peak at 660-680 nm. The green range (500-600 nm) is partly reflected, which is why leaves look green, yet it penetrates deep into the leaf and the plant's tissues, where it is absorbed and drives photosynthesis in the lower layers.
What the Plant Actually Absorbs and Why
Chlorophyll a and chlorophyll b have two main absorption peaks: blue (430-450 nm) and red (640-680 nm). That is where the logic of "blue plus red equals ideal" came from. Photosynthetic efficiency, measured as quantum yield, is in fact fairly even across the whole PAR range from 400 to 700 nm, green included.
Green light (500-600 nm) is partly reflected from the leaf surface, but the part that gets through penetrates much deeper into the leaf tissue than blue or red does. In a dense canopy, green reaches the lower tiers after the upper leaves have already absorbed the blue and red. A plant grown under narrowband blue and red light with no green is a plant whose lower leaves receive less than they would under a broad spectrum.
Chlorophyll is only part of the picture. Phytochrome responds to the ratio of red (660 nm) to far red (730 nm) and regulates flowering, leaf orientation and stem elongation. Cryptochrome responds to blue and regulates circadian rhythms, stomatal opening and phototropism. Under narrowband red and blue lighting with these signals out of balance, the plant develops abnormally even when PPFD and DLI are sufficient.
How Different Spectra Shape the Plant
Excess blue (relative to red): a compact plant with short internodes, small thick leaves and a dark green color. For microgreens and dense herbs this is often desirable. For crops that need a large leaf mass, excess blue gives compactness you do not want.
Excess red (relative to blue): stretched plants with long internodes, a thin stem, and large but thin leaves. This is the typical picture under monochromatic red, or under purple lamps with a very small share of blue. The plant looks as if it is "searching for light".
Balanced white spectrum: broad coverage of the PAR range, green and yellow included. The plant develops more naturally, with the morphology typical of the crop and a normal ratio between height and leaf mass.
Far red (700-750 nm, FR): outside PAR, yet it acts on the phytochrome system. A short pulse of FR at the end of the light period (end-of-day FR) signals "sunset" and speeds up flower initiation in some crops. LEDs with an FR component let you steer flowering and some aspects of plant shape.
How Much Blue and Far-Red to Give Lettuce, Basil, and Fruiting Crops
Without blue light, photosynthesis malfunctions: cucumber leaves grown under pure red did not work properly, and 7% blue was already enough to prevent it. Trials vary blue between 7 and 35% of PPFD: the low end gives more leaf mass, the high end gives compactness and color.
Lettuce. Red-leaf lettuce under white LEDs with 7-9% blue had 78-98% more fresh mass than at about 35% blue, though leaves get redder as the blue share rises. Keep blue near the low end for mass and raise it for color.
Basil. At 215 µmol/m²/s and 16 hours of light, the highest yield came from a red-to-blue ratio of 3:1, meaning 25% blue. At 1:2, the content of several aroma compounds dropped.
Tomato, cucumber, and pepper are more sensitive to blue: at 500 µmol/m²/s, raising it from 11 to 28% cut their mass by 22, 26, and 14% respectively, while lettuce showed no difference.
Far-red. Replacing 15% of the 400-700 nm photons with 700-750 nm gave lettuce 29-31% more mass: leaves expanded and caught more light. The price is stretching, so FR goes where large leaves are wanted, and to crops whose flowering depends on photoperiod.
Practical Comparison: Purple LED vs White LED
A purple LED (narrowband blue plus narrowband red) turns a high share of its power draw into PAR and scores well on PPE. It is unpleasant to work under: it distorts colors and makes diseases and deficiencies harder to diagnose. The missing green and yellow can also lead to abnormal morphology over a long growing period.
A white broadband LED (phosphor-converted) includes green and yellow, is comfortable to work under and gives the plant its natural morphology. Its PAR per watt is somewhat lower than a purple fixture's, yet current top-end LEDs on Samsung or Osram chips deliver a PPE of 2.8-3.2 µmol/J together with a broad spectrum.
For most commercial operations, where people spend time in the growing area and visual diagnosis matters, white LED is the more practical choice.
Three Mistakes That Cost the Most
Choosing a spectrum by the color of the glow or by a "full spectrum" label without checking the actual spectrogram. "Full spectrum" on the box does not mean even coverage of PAR. Ask the manufacturer for the spectrogram (SPD, spectral power distribution) and look at how the power is actually distributed across wavelengths.
Ignoring the spectrum when diagnosing diseases and deficiencies. Under purple light, chlorosis, necrosis and changes in leaf color are practically invisible to the human eye. Keep a flashlight with a daylight spectrum at hand, or take the plant out for inspection under natural or white light. The deficiency diagnosis guide assumes you can see true leaf color.
Locking one spectrum in for the whole cycle on a crop whose needs change by stage. If your LED channels are adjustable, run more blue at the start and shift toward red during fruiting. If they are not, choose the spectrum for the most critical stage of that particular crop.
How to Tell the Spectrum Is Right
The plant has the morphology typical of the crop: the ratio of height to width, the thickness of the stem, and the size and texture of the leaf all match the standard. There is no excessive stretching at adequate PPFD. Leaf colors are natural and even, and you can diagnose diseases and deficiencies without carrying the plant out into natural light.
If you want to go deeper: PPE and lighting efficiency: how to choose a fixture explains how to pick a lamp with the spectrum you need without losing efficiency, and how to calculate the real cost of lighting once PPE and LED degradation are taken into account.