Light Spectrum & DLI: The Science of Cannabis Lighting

ChillTokin · July 24, 2026, 3:05pm

Article 12 – Understanding how light quality, quantity, and timing drive cannabis growth, cannabinoid production, and yield.

1. PAR vs ePAR – Redefining Useful Light

For decades, PAR (Photosynthetically Active Radiation) was defined as 400–700 nm (McCree 1972). This range captures the wavelengths that chlorophyll a and b absorb most strongly. But recent research has expanded our understanding.

ePAR (extended PAR) covers 380–760 nm, as proposed by Zhen & Bugbee (2020–2022) at Utah State University. Their key finding: far-red photons (701–750 nm) are not merely inactive – they synergistically boost photosynthesis when delivered simultaneously with 400–700 nm light.

The McCree Curve

The McCree action spectrum describes the relative quantum yield of photosynthesis at each wavelength:

Peak 1: ~630 nm (red) – highest quantum efficiency
Peak 2: ~440 nm (blue) – second peak
Green (500–600 nm): approximately 20% lower than red, but still photosynthetically active
UV and far-red tails: traditionally excluded, now reconsidered under ePAR

Why ePAR matters for growers: If your fixture emits significant far-red, traditional PAR meters undercount the actual photosynthetic contribution. Sensors calibrated to ePAR (380–760 nm) give a more accurate picture of plant-available light.

2. Measuring PPFD Correctly

Poor measurement is the single biggest source of lighting errors in indoor cultivation. Follow these guidelines:

Factor	Guideline
Sensor position	Canopy level, held horizontal
Grid	Minimum 9 points per light: 4 corners, 4 midpoints, 1 center
Unit	umol/m2/s (PPFD) – NOT lux
Sensor	Quantum sensor (Apogee, LI-COR)

Uniformity Ratio

Uniformity ratio = PPFD_min / PPFD_avg

Rating	Ratio
Excellent	0.85–0.90
Good	0.75–0.85
Acceptable	0.65–0.75
Poor	< 0.65

Target: 0.75–0.90 for commercial grows.

Common Measurement Mistakes

Center-only measurement – the center is always the brightest; this overestimates average PPFD by 15–30%
Wrong height – measure at canopy level, not fixture level
Using a lux meter – lux is weighted for human vision (green-biased), not plant photosynthesis
Ignoring reflections – white walls and mylar significantly increase edge PPFD
Ignoring LED degradation – LEDs lose 5–15% output over 25,000–30,000 hours; remeasure annually

3. DLI by Growth Stage

DLI (Daily Light Integral) is the total photosynthetic photon delivery per day, measured in mol/m2/day.

Formula

DLI (mol/m2/day) = PPFD (umol/m2/s) x Hours x 3600 / 1,000,000

Example: 800 umol/m2/s x 12h x 3600 / 1,000,000 = 34.56 mol/m2/day

DLI Targets by Stage

Stage	DLI (mol/m2/day)	PPFD (umol/m2/s)	Light Cycle
Seedling / Clone	12–20	100–300	18h
Early Veg	18–25	200–400	18h
Late Veg	25–35	400–600	18h
Early Flower (Wk 1–3)	25–35	600–800	12h
Full Flower (Wk 4–8+)	35–50	800–1,000	12h
Flower + CO2	45–65	1,000–1,500	12h

Commercial sweet spot: 40–45 mol/m2/day in flower.

Rodriguez-Morrison et al. (2025) demonstrated that yield increased linearly from 131 to 543 g/m2 as DLI increased – one of the strongest dose-response relationships documented in cannabis.

4. Blue Light (400–500 nm)

Blue light plays a critical role in cannabis morphology through cryptochrome and phototropin photoreceptors.

Morphological Effects

Stem biomass: +15.1%
Leaf count: +13.7%
Stem diameter: +10.2%
Root length: +6.8%
Chlorophyll content: +7.4%

Blue light creates shorter, bushier plants – desirable for canopy management.

Effects on Cannabinoids and Terpenes

Parameter	Under High Blue
THC concentration	10.17% (highest)
Total THC per plant	1.44 g (lowest – due to reduced biomass)
Linalool	0.847 mg/g
Alpha-pinene	5.73 mg/g

The Blue Light Trade-off

Westmoreland (2021) found that each 1% increase in blue fraction reduces yield by 0.6% linearly. Higher blue = more compact plants with higher cannabinoid concentration, but lower total harvest weight.

Optimal blue fraction in flowering: 4–10%.

5. Red Light (600–700 nm)

Red light is the workhorse of photosynthesis, with ~25% higher quantum efficiency than blue photons.

Dual Red Peaks

Research shows that combining 640 nm + 660 nm red produces better results than 660 nm alone. The dual-peak approach better matches the absorption profiles of both chlorophyll a and chlorophyll b.

Phytochrome System

The phytochrome system is the plant’s red/far-red light sensor:

Pr (inactive form): absorbs red light at 660 nm, converts to Pfr
Pfr (active form): absorbs far-red at 730 nm, converts back to Pr
Dark reversion: Pfr slowly converts to Pr during darkness

Flowering trigger: Cannabis flowers when the Pfr:Ptotal ratio drops below a critical threshold during an uninterrupted dark period of 9–10+ hours.

6. Far-Red Light (700–780 nm)

Emerson Enhancement Effect

When red and far-red photons are delivered simultaneously, the photosynthetic rate exceeds the sum of each delivered alone. This is the Emerson enhancement effect, operating through Photosystem I (far-red) and Photosystem II (red) coordination.

Peterswald et al. (2025) – Breakthrough Results

Treatment	Description	Result (Northern Lights)
12L (control)	12h normal light	Cannabinoid: 0.25 g/plant
10L_2D	10h light + 2h FR at start of dark	Cannabinoid: 0.43 g/plant (+70%)
10L_2	10h light + 2h FR at end of light	Slight increase
12L_2_2D	12h light + 4h FR total	DELAYED flowering, reduced flower biomass

Key findings:

10L_2D protocol: +70% cannabinoid yield, THCA +25%
Energy savings: 5.5% less electricity vs standard 12L
WARNING: Too much far-red (12L + 4h FR) destroys flower yield and delays flowering

Shade Avoidance Response

Far-red triggers the shade avoidance syndrome via auxin redistribution, causing stem elongation. This is undesirable if uncontrolled, but can be strategically used for internode spacing.

7. UV Light – Separating Fact from Hype

UV effects on cannabis are frequently exaggerated. Here is what the research actually shows:

UV-B Studies

Lydon (1987): THC increased from 2.5% to 3.1% (~24% relative increase) – a modest result, often cited as dramatic
Rodriguez-Morrison (2021): THC and CBD actually DECREASED with increased UV-B exposure – contradicting the popular narrative

UV-A Studies

Zheng (2025): UV-A at 395 nm produced compelling results:
- CBD yield: +44.1%
- CBG yield: +91.8%

Recommended UV Protocol

Parameter	Recommended
UVA:UVB ratio	99:1
Intensity	~1.8 W/m2
Duration	3–4 hours/day
Stage	Flowering (weeks 4–8)
Main purpose	Increase terpenes

UV-A appears to be the more productive wavelength for cannabis. UV-B is stressful and results are inconsistent.

8. Green Light (500–600 nm) – Myth Busted

The Myth

“Plants are green because they reflect all green light – green LEDs are wasted energy.”

The Reality

Whole leaves absorb >70% of green photons (McCree 1972)
The 5–10% absorption figure comes from extracted chlorophyll in solution, not intact leaves
At high PPFD, green photons are actually MORE efficient than adding more red or blue (Terashima 2009)
Green light penetrates deep into the canopy, driving photosynthesis in lower and inner leaves that red/blue cannot reach

Green light is not wasted. Broad-spectrum fixtures that include green produce healthier, more productive canopies than narrow red+blue spectra.

9. Light Cycle Science

Photoperiod Requirements

Cannabis is a facultative short-day plant. It requires 9–10 hours of uninterrupted darkness to trigger and maintain flowering.

Light Pollution Thresholds

Light Source	PAR Intensity	Safe?
Moonlight	~0.002 umol/m2/s	Safe
Strict threshold	< 0.05 umol/m2/s	Safe
Causes delay	0.1 umol/m2/s (~5–7 lux)	DANGEROUS
Sensitive cultivars	0.01 umol/m2/s (~0.5 lux)	DANGEROUS

Consequences of Light Leaks

Delayed flowering: 1–3 weeks
Hermaphroditism: stress-induced male flowers on female plants
Reduced yield: incomplete flower development

Gas Lantern Routine (GLR)

An alternative vegetative light schedule:

12h light / 5.5h dark / 1h light / 5.5h dark

Prevents flowering (the 1h interruption resets the dark period)
Saves ~28% electricity compared to 18/6
Not universally reliable across all cultivars

10. Commercial LED Comparison 2025–2026

Top-Tier Fixtures

Light	Efficacy (umol/J)	PPF (umol/s)	Wattage	Notes
Fluence VYPR 4 R8 @400V	3.6	2,660	800W	Latest 07/2025
Fluence VYPR 4 R8	3.3	2,660	800W	Broad spectrum
HLG Scorpion Diablo	3.2	2,130	~665W	Samsung LM301H
Gavita Pro RS 2400e	3.2	~2,400	~750W	Samsung + Osram
IONFrame EVO4	3.14	~940	300W	Samsung LM301H EVO
Gavita CT 2000e	3.0	2,000	650W	Enhanced blue

2026 Efficacy Standards

Level	Efficacy (umol/J)
Minimum acceptable	2.5
Good commercial	2.8
Premium	3.0+
State of the art	3.3–3.6

If your fixture is below 2.5 umol/J, upgrading will likely pay for itself within 12–18 months through electricity savings alone.

11. Practical Recommendations Summary

Factor	Recommendation
Light	Broad-spectrum LED, >= 2.8 umol/J
PPFD veg	400–600 umol/m2/s (18h)
PPFD flower	800–1,000 umol/m2/s (12h); 1,000–1,500 with CO2
DLI flower	40–50 mol/m2/day
Blue ratio	4–10%
Far-red	Test EOD 2h protocol, start with low dose
UV-A	1.8 W/m2, UVA:B ratio 99:1, 3–4h/day in late flower
Darkness	Absolute < 0.05 umol/m2/s
CO2	1,000–1,500 ppm when PPFD > 800
Uniformity	Ratio > 0.75, measure 9+ points

Disclaimer

This article is for educational and research purposes only. Cannabis cultivation in Thailand is regulated under the Controlled Herbs Act 2025 and Ministerial Regulation B.E. 2569 (effective April 2026). Cultivators must hold a valid license and comply with all applicable regulations. This content does not promote illegal cultivation or recreational use.

References

Zhen S, Bugbee B (2020). Frontiers in Plant Science
Rodriguez-Morrison V et al. (2025). Scientific Reports (Nature)
Westmoreland FM et al. (2021). PLOS ONE
Danziger N, Bernstein N (2022). Plants 11(21):2982
Peterswald TJ et al. (2025). Scientific Reports 15:17435
Chandra S et al. (2008). Physiology and Molecular Biology of Plants
Zheng Y et al. (2025). Scientific Reports 15:44735
Westmoreland FM et al. (2024). Frontiers in Plant Science
McCree KJ (1972). Agricultural Meteorology
Terashima I et al. (2009). Plant and Cell Physiology
Llewellyn D et al. (2023). Light pollution study, University of Guelph
Viet Growers Community (2015). Viet Grower Handbook