PAR

PAR (Photosynthetically Active Radiation) is the foundational concept of horticultural lighting, defining the specific range of light wavelengths that plants can use for photosynthesis. Definition: PAR includes all light between 400nm (violet) and 700nm (deep red), which approximately matches the visible spectrum but excludes UV (under 400nm) and infrared (above 700nm). Plants cannot use light outside this range for photosynthesis — UV light damages plant tissue (with rare exceptions), and infrared is just heat energy. Why PAR matters more than lumens: lumens measure light intensity weighted to human eye sensitivity, which peaks in the green-yellow range that plants use poorly; a yellow-spectrum HPS bulb produces high lumens per watt but only modest PAR output; conversely, a deep red LED produces low lumens (since red light isn't bright to human eyes) but extremely high PAR output for flowering plants. PAR measurements: PAR is measured in micromoles of photons per square meter per second (µmol/m²/s), called PPFD (Photosynthetic Photon Flux Density) — see separate PPFD entry for details. PAR ranges by plant requirement: low-light plants (lettuces, herbs) — 100-300 µmol/m²/s; medium-light plants (peppers, tomatoes) — 300-700 µmol/m²/s; high-light plants including cannabis vegetative — 400-700 µmol/m²/s; cannabis flowering without CO2 — 700-1000 µmol/m²/s; cannabis flowering with CO2 supplementation — 1000-1500 µmol/m²/s. PAR efficiency of light sources (µmol/J = micromoles per joule of electricity): incandescent — 0.3 µmol/J (terrible); CFL/T5 — 0.7-0.9 µmol/J (mediocre); HPS 600W — 1.7-1.9 µmol/J (decent); CMH 315W — 1.7-2.0 µmol/J (good); modern full-spectrum LED — 2.5-3.0 µmol/J (excellent); cutting-edge LED 2024 — up to 3.5 µmol/J (best available). Quality grow light marketing should always specify PAR output in µmol/J and PPFD distribution maps showing coverage area uniformity. Avoid grow lights advertised only in "watts" or "lumens" — these are misleading metrics for plant growth. Apogee MQ-500 quantum sensor is the industry-standard tool for measuring actual PPFD at canopy level, which serious growers use to verify manufacturer claims and optimize fixture height.