Short answer

Cannabis grow lights supply the photons that drive photosynthesis. Two numbers decide most purchases. PPFD measures the photon flux that reaches the canopy in micromoles per square meter per second (µmol/m²/s). DLI measures the total delivered in one day in moles per square meter (mol/m²/day). Flowering plants need more light than vegetative plants. A 4 ft x 4 ft tent needs a fixture that holds 600 to 900 µmol/m²/s across the canopy for 12 hours per day.

PPFD and DLI targets

DLI equals PPFD multiplied by hours multiplied by 0.0036. A canopy at 700 µmol/m²/s for 12 hours receives 30.2 mol/m²/day.

  • Clones and seedlings: DLI 10 to 15 mol/m²/day.
  • Vegetative stage: DLI 20 to 30 mol/m²/day at 18 hours of light.
  • Flowering stage: DLI 30 to 45 mol/m²/day at 12 hours of light.

Above 900 µmol/m²/s at ambient CO2 near 400 ppm, extra light returns less yield unless the grower adds CO2 and controls temperature and humidity. Some commercial rooms run 1,000 to 1,500 µmol/m²/s with CO2 at 1,200 ppm.

Lamp types

LED

White and red LED fixtures convert electricity to light at 2.2 to 3.0 µmol per joule. A 320 W bar fixture covers about 3 ft x 3 ft in flower at 800 µmol/m²/s. Diode count, driver quality, and thermal design set the real output.

HPS

A 1000 W high pressure sodium lamp produces near 1.5 µmol/J and covers 4 ft x 4 ft in flower. It emits infrared heat, so canopy distance runs 18 to 24 in and the ventilation load is higher. At 120 V the ballast draws about 9.5 A.

CMH and fluorescent

Ceramic metal halide lamps come in 315 W and 630 W. T5 fluorescent tubes suit clones and seedlings. Both cost more per photon than current LED fixtures.

Coverage and distance

Light intensity falls with the square of the distance. Move a lamp from 12 in to 24 in and intensity drops to one quarter before wall reflection. Fix the height, then measure with a quantum sensor at 9 to 12 points across the canopy, not only at the center.

Spectrum

Chlorophyll absorbs blue near 450 nm and red near 660 nm. Broadband white LED at 3000 K to 4000 K covers both bands. Added 660 nm and 730 nm diodes raise red output. Blue-heavy 5000 K light keeps vegetative plants compact. PPFD affects yield more than small spectrum shifts do.

Safety and power

  • Choose fixtures with UL or ETL listing and an IP rating for wet rooms.
  • Use a GFCI circuit. Keep load at or under 80 percent of the breaker rating. A 15 A circuit carries 12 A continuous.
  • Dimming and a timer cut heat and electricity cost.

Check state rules before buying. Some states limit home grow wattage or plant counts. Grow light specs do not make a product legal to use.