Lighting: spectrum, PPFD and DLI

What this is (and why light is the engine)

Light is not just ‘on or off.’ It is the raw fuel a plant turns into sugar, and the single biggest lever on yield and quality you control indoors. This paper assumes you know nothing: it defines every term, gives concrete numbers to aim for at each growth stage, and explains the one switch that makes a plant flower.

Plants eat light. Photosynthesis converts light energy plus CO2 and water into sugar, so more usable light, up to a limit, means more growth^[8]. The three numbers that matter most are PPFD (how bright, right now), DLI (how much total light per day), and spectrum (the color mix). Every term is defined the first time it appears.

PAR, PPFD, DLI and umol/J in plain English

Get the gist of these five terms and the rest of the paper falls into place. They all describe the same thing from different angles: how much usable light a plant is getting.

Spectrum: what each color does

Blue light, roughly 400 to 500 nm, keeps plants compact with tight internode spacing and is linked to denser growth and resin in flower^[3]. Red light, 600 to 700 nm, is the most photosynthetically efficient band and drives flowering and stretch^[7].

‘Full-spectrum white’ LEDs are rated by color temperature in Kelvin. Higher-K and bluer (around 4000 to 6500K) leans veg, lower-K and redder (around 3000 to 3500K) leans flower. A good broad white spectrum works fine across both stages for beginners. The practical takeaway: do not over-optimize spectrum early. Intensity matters far more for yield than chasing a perfect color recipe.

Intensity and the daily dose: targets by stage

Young tissue cannot process intense light, so targets climb as the plant matures. Clones and seedlings want about 100-300 PPFD (DLI ~10-15 mol)^[1], early-to-late veg about 300-600 PPFD (DLI ~20-35 mol), and flower about 700-900 PPFD without added CO2 (DLI ~30-45 mol)^[2].

Pushing past about 900 PPFD only pays off if you also raise CO2 to 1000-1200 ppm and tighten temperature and humidity^[8]. Otherwise extra light just causes stress and bleaching. Because DLI bundles intensity and hours together, you can hit the same daily dose with lower PPFD over more hours (veg at 18/6) or higher PPFD over fewer hours (flower at 12/12).

The photoperiod flip that triggers flowering

Photoperiod-type cannabis stays vegetative under long days (commonly 18/6) and is forced to flower by switching to 12 hours light and 12 hours uninterrupted dark^[4]. This is ‘the flip.’

The plant does not count light hours. It measures the length of the unbroken dark period using a pigment called phytochrome, which flips between an active form (Pfr) and an inactive form (Pr). Once nights are long enough it produces a flowering signal, florigen, in the leaves^[5]. This is why light leaks matter so much: even a phone screen, an indicator LED, or a pinhole in a tent during lights-off can reset phytochrome and stall or revert flowering, cause re-vegging, or trigger hermaphrodites^[4].

LED vs HPS vs CMH, and reading efficacy

Modern LED is the efficiency leader at roughly 2.7-3.0 umol/J for good fixtures (budget units 2.0-2.3), runs cooler, and lasts longer^[6]. HPS (high-pressure sodium) sits around 1.7-1.9 umol/J and runs hot but is cheap to buy. CMH/LEC (ceramic metal halide) lands lower, around 1.3-1.9 umol/J, but has a pleasant broad spectrum.

Efficacy (umol/J) is the number to compare. A 3.0 umol/J LED makes about 60% more usable light than a ~1.85 umol/J double-ended HPS for the same power bill^[6]. For beginners, a reputable full-spectrum LED with a published PPFD map and efficacy at or above ~2.5 umol/J is the safe default. Ignore inflated ‘equivalent watt’ marketing and look at actual PPF (total umol/s) and coverage.

Hanging height, coverage, and a stage-by-stage setup

Light obeys the inverse-square law: roughly doubling the distance from the canopy cuts PPFD by about 75%. Height is your coarse intensity dial, the dimmer is the fine dial. Hang about 24 in for seedlings and clones, ~18 in for veg, and ~12-16 in for flower, then fine-tune with the dimmer and a PAR meter.

Verify coverage by taking PPFD readings at nine points: four corners, four edge-midpoints, and the center. Aim for a min-to-average ratio above 0.75 so edge plants are not starved while the center bleaches. Hanging higher trades peak intensity for more even spread, so use a manufacturer PPFD map as your starting point and confirm with real readings at canopy height.

Light stress, far-red and UV, and common mistakes

Too much light shows as bleaching (white or yellow bud tips directly under the fixture), upward-cupping or ‘taco’ leaves, and faded color even when nutrients are fine. The fix is to dim or raise the light, not to feed more.

Far-red (~730 nm) can speed the transition to dark via the phytochrome system and slightly stretch plants^[4]. UV-B in the final 1-2 weeks is a popular potency play, but the evidence that UV-B reliably raises cannabinoids or yield is mixed^[2], and it carries real eye, skin and plant-stress risks. Treat it as optional and advanced.

Realistic expectations

More light only helps up to the point where something else (CO2, water, nutrients, temperature or genetics) becomes the limiting factor. Past saturation you pay for electricity and heat with no extra yield, and eventually with stress^[1].

Once lighting is handled, the rest is climate, feed and genetics. Read the light acclimation paper for how to ramp safely, and the flowering stages paper for what happens after the flip.