Lux Meter vs PAR Meter

What each meter actually measures

A lux meter measures illuminance — visible light weighted by the human eye's sensitivity curve (the photopic response), which peaks around 555 nm (green-yellow) and falls off sharply in blue and red ^[1]. Units are lux (lumens per square meter).

A PAR meter (more precisely, a quantum sensor) measures photosynthetically active radiation: the number of photons between 400-700 nm hitting a surface per second, regardless of color. Output is PPFD (photosynthetic photon flux density) in µmol/m²/s ^[2][3]. This is the unit plant scientists use because, to a first approximation, a photon at 450 nm and a photon at 660 nm both drive photosynthesis Strong evidence^[2].

The two instruments are measuring different physical quantities. There is no universal conversion between them — the ratio depends entirely on the light source's spectrum.

Why growers use them

Cannabis yield and cannabinoid production scale with light intensity up to roughly 1500 µmol/m²/s PPFD under enriched CO₂, with diminishing returns above ~900 µmol/m²/s in ambient CO₂ ^[4] Strong evidence. Too little light = stretchy, low-yielding plants. Too much = light burn, bleaching, and wasted electricity.

Without a meter, growers rely on manufacturer charts (often optimistic), wattage rules of thumb (unreliable across LED generations), or visual guessing. A meter — any meter — turns light intensity from a guess into a number you can repeat and tune.

The practical question isn't 'lux or PAR' — it's 'which gives me actionable numbers at my budget.'

When to start measuring

Measure before your first grow under any new fixture, and re-measure whenever you:

• Change lights, bulbs, or drivers
• Raise or lower the fixture
• Move from veg to flower (targets change)
• Add or remove plants (canopy reflectance changes intensity)
• Notice symptoms of too much or too little light

Target PPFD ranges most cultivators use ^[4][5] Strong evidence:

• Seedlings/clones: 100-300 µmol/m²/s
• Veg: 300-600 µmol/m²/s
• Flower (ambient CO₂): 600-900 µmol/m²/s
• Flower (supplemented CO₂, 1000-1500 ppm): 900-1500 µmol/m²/s

How to do it: step by step

Using a quantum PAR meter (easy mode):

1. Turn on the grow light and let it warm up for 15-30 minutes (especially HPS).
2. Hold the sensor flat at canopy height, sensor facing straight up.
3. Keep your body and arm out of the light path — your shadow tanks the reading.
4. Take readings in a grid: directly under the fixture, then at quarter points, then corners.
5. Record values. The center-to-edge spread tells you about uniformity.
6. Adjust fixture height or dimmer to hit your target PPFD for the current growth stage.

Using a lux meter (budget mode):

Follow steps 1-5 above, then convert lux to approximate PPFD using a spectrum-specific factor ^[3]:

• HPS: PPFD ≈ lux ÷ 71
• CMH/LEC (3100K): PPFD ≈ lux ÷ 67
• White LED (3000-3500K, typical 'full spectrum'): PPFD ≈ lux ÷ 54
• White LED (5000K): PPFD ≈ lux ÷ 72
• Pure sunlight: PPFD ≈ lux ÷ 54

These conversions are approximate — published values vary by ±10-15% ^[3] Weak / limited. They fail completely for narrow-spectrum LEDs (pure red, pure blue, UV, or heavily blurple fixtures), because lux meters underweight or miss those wavelengths.

Smartphone lux apps: Generally unreliable. Phone ambient light sensors are uncalibrated, vary by model, and often clip at high intensities Weak / limited. Use as a relative tool only, not for absolute targets.

Common mistakes

• Using lux meters under blurple or narrow-spectrum LEDs. The photopic curve barely sees deep red and blue. Your reading will dramatically under-report actual PPFD. Get a quantum sensor for these fixtures.
• Forgetting to convert. Reading 60,000 lux under an LED and calling it 'high PAR' means nothing without the conversion step.
• Single-point measurement. A reading directly under the fixture tells you the peak, not what the plants in the corners are getting. Always grid.
• Measuring at the wrong height. PPFD targets apply at the top of the canopy, not the floor or the pot rim.
• Trusting manufacturer PPFD maps. Marketing maps are often measured at suspiciously close distances or in reflective rooms. Verify in your space.
• Confusing DLI with PPFD. DLI (daily light integral, mol/m²/day) = PPFD × photoperiod hours × 0.0036. PPFD is intensity; DLI is total daily dose ^[2].
• Buying an Apogee-clone with no calibration data. Cheap 'PAR meters' under $80 are often just relabeled lux meters with a fixed conversion baked in — i.e., they lie under different spectra Disputed.

Related techniques

• Daily Light Integral: the metric that actually correlates with biomass; PPFD is the input.
• Defoliation: changes canopy light penetration; re-measure after.
• CO2 supplementation: raises the ceiling on usable PPFD.
• Light burn vs nutrient burn: how to recognize when your PPFD is too high.
• LED vs HPS: spectrum differences that determine which meter type you need.

Sources

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