This article brings together research on how light colour, strength and timing affect cannabis growth, cannabinoid content, terpenes and yield. It is for growers in Thailand and Southeast Asia, from licensed indoor farms to home tents. Each section ends with one thing you can measure or change.
Translator’s notes add the Thai context: heat, power cuts, light from nearby buildings, and what equipment is available here.
1. Which light plants can use
Light reaches the plant as photons, tiny packets of light energy. Light colour is measured in nanometres (nm). Since 1972, light that drives photosynthesis has been defined as 400–700 nm, called PAR (photosynthetically active radiation). This comes from plant scientist K. J. McCree’s work and covers the colours chlorophyll a and b absorb most strongly.
In 2020–2022, Zhen and Bugbee, plant scientists at Utah State University (USA), proposed a wider range, ePAR (extended PAR): 400–750 nm in their papers. The original article’s 380–760 nm does not match. Their key finding concerns far-red light at 701–750 nm, long treated as useless for photosynthesis. It does help when combined with 400–700 nm light, and the combined effect exceeds the two added separately.
How well each colour drives photosynthesis (the McCree curve)
McCree measured how much photosynthesis each photon drives at each wavelength. As a graph, this is the McCree curve:
- Red, around 630 nm: highest efficiency.
- Blue, around 440 nm: second peak.
- Green, 500–600 nm: about 20% below red, but still drives photosynthesis.
- Ultraviolet below 400 nm and far-red above 700 nm: left out of the old definition, now reconsidered under ePAR.
A standard PAR meter counts only 400–700 nm. If your lamp gives off a lot of far-red, the meter under-reads the light plants use. An ePAR-calibrated sensor reads more accurately.
What to do: Check the spectrum chart on your lamp’s spec sheet. A clear far-red peak means your PAR readings will run low, so do not add power just to chase a meter number.
Translator’s note: ePAR sensors and add-on far-red or UV-A fixtures may be hard to find in Thailand; ask the brand’s official distributor before planning around them. Without one, a standard 400–700 nm quantum sensor still does every job in sections 2 and 3, though it under-reads far-red-heavy lamps.
2. Measuring light intensity at the canopy
PPFD (photosynthetic photon flux density) is the number of 400–700 nm photons landing on one square metre each second, in µmol/m²/s. The canopy is the top layer of leaves and buds. The original article calls poor measurement the biggest single cause of indoor lighting mistakes.
| What to check | How to do it |
|---|---|
| Sensor position | At canopy level, held horizontal |
| Measuring grid | At least 9 points per lamp: 4 corners, 4 edge midpoints, 1 centre |
| Unit | µmol/m²/s (PPFD), not lux (brightness as the human eye sees it) |
| Instrument | Quantum sensor, which counts photons (the original names Apogee and LI-COR) |
How evenly the light is spread
Uniformity ratio = lowest PPFD ÷ average PPFD (both from your grid)
Example: average 800, lowest 640, ratio 0.80.
| Rating | Ratio |
|---|---|
| Excellent | 0.85–0.90 |
| Good | 0.75–0.85 |
| Acceptable | 0.65–0.75 |
| Poor | below 0.65 |
Commercial rooms should aim for 0.75–0.90. A low ratio means some plants get much less light and, since yield follows light (section 3), produce less.
Five common measuring mistakes
- Measuring only the centre. It is always the brightest spot, so it overstates the average by 15–30%.
- Measuring at the wrong height. Measure at canopy level, not at the lamp.
- Using a lux meter. The eye is most sensitive to green, so lux does not show the light a plant can use.
- Ignoring reflections. White walls and Mylar (reflective film) noticeably raise PPFD at the edges. Measure with walls set up as for the grow.
- Ignoring LED ageing. LEDs lose 5–15% of output over 25,000–30,000 hours, roughly 4–7 years at 12–18 hours a day. Re-measure yearly.
What to do: Mark 9 points under each lamp and measure at canopy height. Record the readings and work out the average and uniformity ratio. Repeat yearly and whenever you move a lamp. This shows the real average and weak corners, so you can move lamps or plants to fix them.
Translator’s note: For a quantum sensor, ask the brand’s official distributor in Thailand, or ask members about their experience in the Equipment and Supplies section.
Light bleaching
Translator’s note: Readers asked about light bleaching, which the original article does not cover, so here is a short explanation. The tops nearest the lamp turn pale yellow or white while lower leaves stay green. Those tops get far more light than the plant can use, and lamp heat often builds up there too.
The main clue is location: damage sits on the parts nearest the lamp. If pale leaves also appear lower down, check feeding first (reading leaf symptoms).
What to do: Measure PPFD at the tallest tops as well as across the canopy. If they read well above the section 3 target for that stage, raise or dim the lamp. You can also bring tall tops down with plant training to level the canopy. This keeps every top in range instead of a few burning.
3. Daily light total for each growth stage
DLI (daily light integral) is the total photons one square metre receives in a day, in mol/m²/day. PPFD is the light at one moment; DLI adds up the whole day. The same PPFD over different light hours gives a different DLI.
DLI (mol/m²/day) = PPFD (µmol/m²/s) × hours of light × 3,600 ÷ 1,000,000
Example: 800 µmol/m²/s × 12 h × 3,600 ÷ 1,000,000 = 34.56 mol/m²/day
The last row below assumes room air enriched with carbon dioxide (CO₂).
| Stage | DLI (mol/m²/day) | PPFD (µmol/m²/s) | Light hours |
|---|---|---|---|
| Seedling / clone | 6–19 | 100–300 | 18 h |
| Early veg | 13–26 | 200–400 | 18 h |
| Late veg | 26–39 | 400–600 | 18 h |
| Early flower (weeks 1–3) | 26–35 | 600–800 | 12 h |
| Full flower (weeks 4–8+) | 35–43 | 800–1,000 | 12 h |
| Flower with added CO₂ | 43–65 | 1,000–1,500 | 12 h |
In flower, most commercial growers aim for 38–43 mol/m²/day.
Rodriguez-Morrison, Llewellyn and Zheng tested this in 2021 (Frontiers in Plant Science 12:646020), growing cannabis under 12 hours of light a day at 120 to 1,800 µmol/m²/s. Flower yield rose in a straight line, from 116 to 519 g/m², one of the clearest links between light dose and yield ever measured in cannabis.
Translator’s note: At 30–35 °C outside, the 1,000–1,500 row only works in a sealed, air-conditioned room: venting hot air outside also vents the added CO₂. Strong lamps also add heat the air conditioner must remove (sizing air conditioning for a Thai flowering room). If your room uses outside air, plan around the “full flower” row without CO₂. In greenhouses, rainy-season cloud lowers daily light; logging DLI shows which days natural light falls short.
What to do: Calculate your current DLI and compare it with your stage’s row. This tells you whether to raise light, lower it or change the hours before changing anything else.
4. Blue light (400–500 nm)
Blue light shapes the plant through two light sensors in the leaf, cryptochrome and phototropin. The original article lists percentage gains from blue light in stem weight, leaf number, stem thickness, root length and chlorophyll. It names no study and the figures could not be traced to a published paper, so they are left out. Blue light is generally linked with shorter, bushier plants, which makes a level canopy easier.
Morello and colleagues (2022, Plants 11:2982) compared cannabis under several LED colours and a high-pressure sodium lamp. Their results for blue LEDs (430 nm):
| Measured | Blue LEDs (430 nm) |
|---|---|
| THC concentration | 10.17% (highest of the treatments) |
| Total THC per plant | 1.44 g (lowest, because the plants grew less) |
| Linalool (a terpene) | 0.847 mg/g |
| Alpha-pinene (a terpene) | 5.73 mg/g |
So blue raised THC percentage but lowered total THC per plant.
A Utah State University team led by Westmoreland (2021, PLOS ONE) tested blue shares of 4–20% in flowering. The blue share is the percentage of a lamp’s photons in the 400–500 nm range. Yield fell in a straight line as blue rose: about 0.77% less flower per 1% more blue, or about 12% across the range. Blue share had no effect on THC or CBD concentration. The lowest-blue lamps had 4% (sodium lamp) to about 10% (LEDs).
What to do: Find the blue percentage on your lamp’s spec sheet. For flowering, choose a lamp or setting with 4–10% blue to avoid losing harvest weight to a blue-heavy spectrum.
5. Red light and the plant’s night-length sensor
Red light (600–700 nm) drives photosynthesis most efficiently, about 25% more per photon than blue.
A 2024 Wageningen University study (Holweg and colleagues, Frontiers in Plant Science 15:1393803) compared white light with red peaks at 640 and 660 nm against white light with 660 nm alone. The two-peak light gave more flower weight, mainly through more total dry matter and a more open structure. Cannabinoid concentrations did not change with spectrum; the study used one cultivar.
Phytochrome: the night-length sensor
Phytochrome is a plant pigment that works like a switch between red and far-red light. It has two forms:
- Pr (switched off): absorbs red (660 nm) and becomes Pfr.
- Pfr (switched on): absorbs far-red (730 nm) and reverts to Pr.
- In darkness: Pfr slowly reverts to Pr by itself (dark reversion).
Cannabis starts flowering when Pfr’s share of total phytochrome (Pfr:Ptotal) drops below a critical level during an unbroken dark period of 9–10 hours or more.
What to do: Check your spec sheet for peaks near 640 nm and 660 nm. Set your flowering timer for at least 9–10 unbroken hours of darkness: on 12/12, 12 hours of complete darkness.
6. Far-red light (about 700–780 nm)
The original article uses 700–780 nm for far-red in general; the ePAR studies in section 1 covered only 701–750 nm.
Red and far-red together (the Emerson enhancement effect)
Leaves have two light-harvesting units working in a chain: Photosystem I uses far-red and Photosystem II uses red. When both colours arrive together, both units run and photosynthesis is faster than the two colours give separately. This is the Emerson enhancement effect.
A 2025 trial: two hours of far-red at lights-off
Peterswald and colleagues (2025, Scientific Reports 15:17435) tested far-red timing on three cultivars: Cannatonic, Hindu Kush and Northern Lights. The table shows Northern Lights; the other two showed no significant change in total cannabinoid yield.
| Treatment code | What the plants received | Result |
|---|---|---|
| 12L (control) | 12 h normal light | Cannabinoids: 0.25 g per plant |
| 10L_2D | 10 h normal light, then 2 h far-red at the start of the dark period | Cannabinoids: 0.43 g per plant (about +70%) |
| 10L_2 | 10 h normal light, with far-red added in its last 2 h | Small increase |
| 12L_2_2D | 12 h normal light, 2 h far-red at its end + 2 h more in the dark (4 h total) | Flowering delayed, less flower weight |
10L_2D raised cannabinoid yield by about 70% and THCA by 25%, using 5.5% less electricity than the 12-hour control. THCA is the acid form of THC in the raw plant; heat turns it into THC. Too much far-red (12L_2_2D) delayed flowering and cut flower yield sharply.
The original summary calls this an “end-of-day” treatment; this rewrite uses the study’s exact timing (the first 2 hours of the dark period).
Far-red makes stems stretch (shade avoidance)
Far-red light tells the plant that neighbours are shading it, so it moves auxin, a growth hormone, and stems grow longer. Uncontrolled, this gives tall, weak plants. Used carefully, it can widen the spacing between nodes (where branches and leaves grow from the stem).
Translator’s note: The possible gain is about 70% more cannabinoids per plant on slightly less electricity, from one trial and one of three cultivars tested. In Thailand you need far-red LED bars on their own timer (availability: see section 1). Check that timer after every power cut: far-red running into the night moves you toward the 12L_2_2D result, delayed flowering.
What to do: Start with one tent or bench of a single cultivar. Run half the plants on 12 hours as the control and half on 10L_2D, with far-red low at first. Weigh the dried flower from both groups, lab-test a sample of each and compare the certificates of analysis. This shows whether your cultivar responds before you equip the whole farm.
7. Ultraviolet light: what the studies actually show
UV effects on cannabis are often overstated. UV-A is the longer-wave, milder ultraviolet; UV-B the shorter-wave, harsher one.
UV-B
- Lydon (1987): THC rose from 2.5% to 3.1%, about 24% in relative terms: a small change, often quoted as dramatic.
- Rodriguez-Morrison, Llewellyn and Zheng (2021, Frontiers in Plant Science 12:725078): a separate paper from the section 3 light-intensity study. In one of two cultivars, THC and CBD concentrations fell as UV-B exposure rose. Neither showed higher total THC or CBD. Total terpene content fell in both, and UV-B damaged leaves. This contradicts popular belief.
UV-A
- Zheng and colleagues (2025, Scientific Reports 15:44735): in a high-CBD hemp strain, UV-A in flowering raised CBG (cannabigerol) content by 52.7% and CBD content by 12.1%. Red light in veg plus UV-A in flower did best: CBD yield per plant rose 44.1% and CBG yield 91.8%. Its CBD yield did not differ significantly from red or UV-A alone.
The UV schedule the original recommends
| Setting | Recommended |
|---|---|
| UV-A to UV-B ratio | 99:1 |
| Intensity | about 1.8 W/m² |
| Duration | 3–4 hours a day |
| Stage | Flowering, weeks 4–8 |
| Stated purpose | More terpenes |
The original article gives no source for this schedule, so treat it as a trial starting point, not a tested setting. The studies above do not show UV raising terpenes; in the UV-B study, terpenes fell.
Overall, UV-A looks more useful for cannabis; UV-B stresses the plant and gives inconsistent results.
Translator’s note: UV harms eyes and skin: switch UV lamps off before working in the room, or wear UV-blocking glasses. The possible gain is more CBD or CBG; treat it as a trial, not a standard setting.
What to do: Test UV-A on one small batch in flowering weeks 4–8, beside a UV-free control batch, and compare lab results. This shows whether UV-A pays for its electricity on your cultivar.
8. Green light is not wasted
Many believe plants look green because they reflect all green light, so green LEDs waste power. Research says otherwise:
- Whole leaves absorb more than 70% of green photons (McCree 1972).
- The often-quoted 5–10% absorption comes from chlorophyll extracted into solution, not whole leaves.
- At high PPFD, extra green photons work better than extra red or blue (Terashima and colleagues, 2009).
- Green light reaches deeper into the canopy, driving photosynthesis in lower and inner leaves that red and blue do not reach.
Broad-spectrum lamps with green (white-looking) give healthier, more productive canopies than narrow red-and-blue lamps (purple-looking).
What to do: When comparing lamps, do not mark one down for including green; compare spectrum and efficiency (section 10). This avoids buying a narrow-spectrum lamp that leaves the lower canopy short of light.
9. Night darkness and light schedules
Cannabis is a facultative short-day plant: long nights trigger flowering and keep it going. It needs 9–10 hours of unbroken darkness to start and keep flowering, its photoperiod response.
How much stray light is too much
| Light source | PAR intensity | Safe? |
|---|---|---|
| Moonlight | about 0.002 µmol/m²/s | Safe |
| Strict limit | below 0.01 µmol/m²/s | Safe |
| Known to delay flowering | 0.1 µmol/m²/s (about 5–7 lux) | Harmful |
| Sensitive cultivars | above 0.01 µmol/m²/s (about 0.5 lux) | Harmful |
Night light leaks can cause:
- Late flowering: a delay of 1–3 weeks.
- Hermaphrodite plants: stress makes female plants grow male flowers.
- Lower yield: flowers do not develop fully.
Translator’s note: In Thailand, greenhouses and net houses are often near street lights, security lights or neighbours’ windows. Just 5–7 lux is enough to delay flowering. Blackout covers used for light deprivation solve this (greenhouse light deprivation).
Power cuts cause a second problem. A mechanical timer stops during a cut and then runs late; a digital timer may reset and switch lamps on at night. Check the timer clock after every cut, and make sure no backup lighting comes on in the grow room during the dark period.
What to do: Once a week, stand in the grow room for 10–15 minutes after lights-off. Look for glow around doors, vents and covers, and from equipment indicator lights. Tape over or block every source. This protects you from 1–3 week delays and hermaphrodite plants.
The Gas Lantern Routine for the vegetative stage
The Gas Lantern Routine is a light schedule for the vegetative stage:
12 h light / 5.5 h dark / 1 h light / 5.5 h dark
- The 1-hour light break splits the night into two short nights, so the plant does not flower.
- With 13 hours of light instead of 18, it uses about 28% less electricity than 18/6.
- It does not work reliably for every cultivar.
For example, a 600 W lamp uses 10.8 kWh a day on 18/6 and 7.8 kWh on this routine; multiply the 3 kWh difference by your electricity rate to see the saving.
What to do: Try the routine on a few plants of each cultivar for 2–3 weeks, checking for signs of flowers before switching the whole veg room. This shows which cultivars stay vegetative on it.
10. Lamp efficiency: how to judge an LED
Efficiency is measured in µmol/J: photons a lamp produces per joule of electricity. Higher means more light for the same electricity bill. PPF is a lamp’s total light output in µmol/s.
The original article lists one fixture as an example of current top efficiency, shown here as a reference point, not a recommendation:
| Lamp | Efficiency (µmol/J) | PPF (µmol/s) | Power | Note |
|---|---|---|---|---|
| Fluence VYPR 4i · BROAD R8 · clear glass | 3.3 | 2,730 | about 834 W | Spec sheet 04/2025 |
| Fluence VYPR 4i · BROAD R8 · wide beam | 3.2 | 2,660 | about 834 W | Spec sheet 04/2025 |
Efficiency levels in 2026
| Level | Efficiency (µmol/J) |
|---|---|
| Minimum acceptable | 2.5 |
| Good commercial | 2.8 |
| Premium | 3.0+ |
| Best currently available | 3.3–3.6 |
Below 2.5 µmol/J, a replacement will probably pay for itself within 12–18 months from electricity savings alone.
Translator’s note: Payback time depends on your power rate and light hours. Per 1,000 µmol/s of light, a 2.5 µmol/J lamp draws 400 W and a 3.0 µmol/J lamp about 333 W. At 12 hours a day, that 67 W difference adds up to about 290 kWh a year; multiply by your rate per kWh.
What to do: Ask the official distributor in Thailand for the spec sheet showing PPF and wattage, then divide PPF by watts for the real µmol/J. This lets you compare lamps on the same basis and work out your payback.
11. Summary of settings
| Setting | Recommendation |
|---|---|
| Lamp | Broad-spectrum LED, at least 2.8 µmol/J |
| PPFD in veg | 400–600 µmol/m²/s (18 h) |
| PPFD in flower | 800–1,000 µmol/m²/s (12 h); 1,000–1,500 with CO₂ |
| DLI in flower | 38–43 mol/m²/day |
| Blue share | 4–10% |
| Far-red | Test 2 h at the start of the dark period (10L_2D), at a low dose first |
| UV-A | 1.8 W/m², UV-A:UV-B ratio 99:1, 3–4 h a day in flowering weeks 4–8 |
| Darkness | Below 0.01 µmol/m²/s |
| CO₂ | 1,000–1,500 ppm when PPFD is above 800 |
| Evenness | Uniformity ratio above 0.75, measured at 9+ points |
Start with the two cheapest checks: map PPFD at 9 points per lamp, and check for light leaks at night. Post your PPFD map or questions below this topic so other members can compare it with their rooms.
Legal note
This article is for education and research only and does not promote unlicensed growing or recreational use.
In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568, in force since 26 June 2025. Flower may be sold only with a PT 33 (ภ.ท.33) prescription, for at most 30 days. Advertising and online sale of flower are prohibited.
Two ministerial regulations of B.E. 2569 followed (what changed in April 2026):
- The Ministerial Regulation on Cannabis or Hemp Extracts came into force on 26 April 2026.
- The Ministerial Regulation on Controlled Herbs (No. 2) came into force on 30 April 2026. It tightens licence criteria for cannabis flower.
Hold the licences your activity requires and follow the rules in force (which licence covers what).
References
- Zhen S, Bugbee B (2020). Frontiers in Plant Science 11:581156
- Rodriguez-Morrison V, Llewellyn D, Zheng Y (2021). Frontiers in Plant Science 12:646020
- Westmoreland FM, Kusuma P, Bugbee B (2021). PLOS ONE 16(3):e0248988
- Danziger N, Bernstein N (2021). Industrial Crops and Products 164:113351
- Peterswald TJ et al. (2025). Scientific Reports 15:17435
- Chandra S et al. (2008). Physiology and Molecular Biology of Plants
- Zheng Q et al. (2025). Scientific Reports 15:44735
- McCree KJ (1972). Agricultural Meteorology 9:191-216
- Terashima I et al. (2009). Plant and Cell Physiology 50(4):684-697
- Lydon J, Teramura AH, Coffman CB (1987). Photochemistry and Photobiology 46:201-206
- Rodriguez-Morrison V, Llewellyn D, Zheng Y (2021). Frontiers in Plant Science 12:725078
- Morello V et al. (2022). Plants 11:2982
- Holweg MMSF et al. (2024). Frontiers in Plant Science 15:1393803