Choosing a Grow Light: PAR, PPFD, and DLI Explained
Most grow light marketing describes wattage and lumens - neither of which is useful for plant growing decisions. This guide explains PAR, PPFD, and DLI: the three measurements that actually tell you whether a light will grow your plants, and how to use them to choose between LED, T5, and other light types.
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Walk into any hydroponics store or search "grow light" online and you'll be bombarded with wattage numbers, lumen ratings, and color temperatures. Most of these numbers are not the right metric for growing plants. This guide explains the three measurements that actually matter.
Why watts and lumens are the wrong metrics
Watts measure electricity consumption, not light output. Two 100W lights can deliver radically different amounts of plant-useful light depending on their technology and design efficiency.
Lumens measure how bright light appears to the human eye, calibrated to the sensitivity of human vision (which peaks in the green range at ~555 nm). Plants photosynthesize most efficiently using red (~660 nm) and blue (~450 nm) light - wavelengths human eyes are relatively insensitive to. A light can be dim to human perception but rich in plant-useful wavelengths, and vice versa.
PAR: the relevant spectrum
PAR stands for Photosynthetically Active Radiation - the range of light wavelengths (400–700 nanometers) that drive plant photosynthesis. PAR is not a measurement of quantity; it defines which wavelengths count.
Within the PAR range:
- Blue light (400–500 nm): Drives compact, dense vegetative growth; regulates stomata opening
- Red light (600–700 nm): The most efficient driver of photosynthesis; also signals flowering
- Green light (500–600 nm): Less efficient but penetrates leaf canopy better than red/blue; useful for multi-layer growing
"Full spectrum" LED lights that approximate white sunlight (3000K–6500K color temperature) typically provide a good balance across the PAR range.
PPFD: how much PAR hits your plants
PPFD (Photosynthetic Photon Flux Density) measures the number of PAR photons hitting a surface per second, per square meter. Units: µmol/m²/s (micromoles per square meter per second).
PPFD is the number you need to evaluate a grow light's practical output for your plants. It varies with:
- Distance from the light: PPFD drops with the square of distance (doubling distance = one-quarter the PPFD)
- Position in the footprint: PPFD is highest directly below the light and lower toward the edges
- Light design: reflectors, lenses, and chip design affect how uniformly PPFD is distributed
Target PPFD by crop: | Crop | Minimum PPFD | Optimal PPFD | |---|---|---| | Microgreens | 100–150 µmol/m²/s | 150–250 µmol/m²/s | | Lettuce, spinach, herbs | 150–250 µmol/m²/s | 250–400 µmol/m²/s | | Basil, kale, chard | 200–350 µmol/m²/s | 350–500 µmol/m²/s | | Fruiting (tomatoes, peppers) | 300–400 µmol/m²/s | 500–800 µmol/m²/s |
DLI: the cumulative daily dose
DLI (Daily Light Integral) is the total amount of PAR light a plant receives over a full day, accounting for both intensity (PPFD) and duration (hours of light). Units: mol/m²/day.
DLI = PPFD × hours of light × 0.0036
This matters because you can achieve the same DLI by:
- Running a lower-intensity light for more hours (16h at 200 µmol/m²/s = DLI 11.5)
- Running a higher-intensity light for fewer hours (12h at 300 µmol/m²/s = DLI 13.0)
Target DLI values: | Crop | Target DLI | |---|---| | Lettuce, leafy greens | 12–17 mol/m²/day | | Herbs | 12–20 mol/m²/day | | Fruiting vegetables | 20–35 mol/m²/day |
Understanding DLI helps you compensate for a dimmer light by running it longer, or a brighter light by running it for fewer hours - though there are biological limits on photoperiod for each crop.
Comparing light types
LED (Light Emitting Diode)
The current standard for home growing. Modern full-spectrum LED boards (quantum board design) deliver excellent PPFD efficiency per watt - typically 2.0–2.8 µmol/J. They run cool, last 50,000+ hours, and can be dimmed.
What to look for: Manufacturer-provided PPFD map at a specified distance, not just "equivalent wattage" claims. Look for brands that publish photometric test results (Spider Farmer, Mars Hydro, HLG all do this).
T5 Fluorescent
The traditional choice for seedlings and herbs. Lower PPFD than LED (typically 100–200 µmol/m²/s at 4 inches) but very uniform coverage and low heat. Adequate for lettuce and herbs; insufficient for fruiting plants. Being phased out in favor of LED bars with similar profile.
HPS (High Pressure Sodium)
Still used in large-scale operations. High PPFD output but significant heat generation, high electricity use, and limited spectrum. Not practical for most apartment home grows.
Proprietary countertop systems (AeroGarden, Click & Grow)
LED panels integrated into the system. Convenient and adequate for herbs and leafy greens. AeroGarden Bounty (30W) delivers ~50–70 µmol/m²/s at pod level - enough for a 16-hour DLI in the leafy green range. Not adequate for fruiting plants.
Practical buying guide
For herbs and lettuce on a countertop: Any LED panel rated 20–50W true draw from a reputable brand will work at 4–8 inch height.
For a 2×4 grow tent, herbs and greens: A 100–150W quantum board LED (true draw) at 12–18 inches will cover the canopy adequately.
For a 2×4 grow tent, fruiting plants: 200–300W true draw LED minimum. Look for PPFD maps showing 400+ µmol/m²/s across the footprint at the manufacturer's recommended height.
Always check true wattage (power draw from the wall), not "equivalent" wattage claims.
This guide is reviewed against published horticultural research. It is not a substitute for advice from a certified horticulturalist.