Spirulina indoor LED growing setup: spectrum, intensity, photoperiod, and energy efficiency

Light requirements for spirulina

Photosaturation point: Spirulina photosynthesis saturates at approximately 200–400 µmol photons/m²/s PAR (photosynthetically active radiation, 400–700 nm) in shallow cultures. Above this intensity, photoinhibition begins — excess light damages Photosystem II. In practice, this means a well-lit indoor LED setup providing 3,000–5,000 lux at culture surface is optimal. More is not better beyond this range for a stationary culture.
Light penetration and mixing: In a 15–20 cm deep culture, the top 3–5 cm receives near-saturating light. Cells deeper than 5–8 cm receive less than compensation-point irradiance. Continuous aeration and mixing (air pump bubbling) creates convective circulation that cyclically exposes all cells to the light zone. This flicker effect is actually more efficient for photosynthesis than continuous high-intensity exposure at the surface.
Conversion: lux to µmol/m²/s: For white LED (5,000–6,500 K), 1 µmol/m²/s ≈ 54 lux. So 3,000 lux ≈ 56 µmol/m²/s, and 5,000 lux ≈ 93 µmol/m²/s at the surface. Accounting for mixing and convection, this is sufficient for productive indoor culture.

Red peak (660–680 nm): Chlorophyll a absorbs most strongly at 430 and 680 nm. The 680 nm red peak is the most efficient wavelength for driving the Calvin cycle via Photosystem I and II. Red LEDs at 660–680 nm are the most energy-efficient choice for maximising biomass production.
Blue peak (430–450 nm): Blue light drives chlorophyll a’s Soret band absorption and also specifically stimulates phycocyanin (absorption peak ~620 nm; blue-green light at 450–500 nm excites the phycocyanin chromophore during biosynthesis). Blue light supports phycocyanin synthesis alongside chlorophyll. A red:blue ratio of 3:1 to 4:1 is the standard recommendation for cyanobacterial indoor production.
White LED (5,000–6,500 K): The simplest and most practical choice for home growers. Cool white (6,500 K) has higher blue content relative to warm white (2,700 K). Cool white LED panels provide adequate spectrum coverage for spirulina growth without the complexity of separate red/blue channels.

Recommended LED wattage per m² of culture surface: 30–50 W/m² of actual LED power draw (not the nominal “equivalent” wattage on the packaging). For a 60×30 cm (0.18 m²) culture: a 20–30 W LED panel provides adequate intensity. For a 100×50 cm (0.5 m²) culture: 30–50 W.
Mounting distance: Position LED panel 15–25 cm above the culture surface. Closer than 15 cm: heat stress risk (LED surface can reach 40–50°C in poorly ventilated panels). Further than 35 cm: intensity falls off by the inverse square law; the culture may be under-lit. Use a lux meter (smartphone app or dedicated meter) to confirm 3,000–5,000 lux at the culture surface.
Heat management: LED panels generate some heat. In a small indoor grow space (<5 m²), LED heat contribution may reduce the supplemental heating required from a fish tank heater. Monitor culture temperature; it should remain 25–32°C. If culture temperature exceeds 35°C: raise the LED panel height or increase air circulation.

Spirulina performs dark-phase carbon fixation (CAM-like carbon concentration mechanism) and cellular repair during the dark period. A 6-hour dark period is optimal for cyanobacterial physiology. Continuous 24-hour light reduces growth efficiency and can stress the culture over time — it is not a shortcut to higher productivity.
A simple mechanical or smart plug timer controls the photoperiod. Align the light period with daytime hours for convenience in monitoring; the exact hours of the day are less important than the 18:6 ratio.

T5 fluorescent (traditional): 54 W T5 tube produces ~5,000 lux at 20 cm distance; adequate but generates more heat and has lower lifespan (~10,000 hours vs 50,000+ for LED). Higher running cost over 2–3 years.
LED panel: 20–30 W produces equivalent lux to 50–60 W fluorescent. 50,000+ hour lifespan. Lower heat output. For year-round growing, LED panels pay back the cost difference within 6–12 months of operation.
Energy cost for a 20 L home culture: 20 W LED × 18 hours/day × 365 days = 131 kWh/year. At £0.30/kWh (UK average 2026): approximately £39/year in electricity for the lighting of a single 20 L unit.

Using warm white LEDs (<3,000 K): Warm white has too much red-orange light relative to blue. While red drives photosynthesis, the very low blue content of 2,700 K warm white limits phycocyanin synthesis. Use 5,000–6,500 K cool white or dedicated grow spectrum LEDs.
Relying on nominal wattage: A “200W equivalent” LED may only draw 20–30 W actual power. Check actual power draw and use a lux meter to verify intensity at culture surface before trusting the packaging specification.
No light measurement: Do not assume any LED is producing the right intensity. A £5 lux meter app on a smartphone will tell you whether your culture is getting 3,000 lux or 300 lux — a 10× difference that determines whether you harvest in 3 weeks or 12.