The colour mix of a grow light changes how tall cannabis grows, how much THC or CBD it makes and how strongly it smells. This part compares the colour mix of LED, high-pressure sodium (HPS) and ceramic metal halide (CMH) lights. It summarises studies from 2018 to 2025, gives settings for UV and far-red add-on lights and ends with a checklist for choosing a fixture. It is for growers in Thailand and Southeast Asia, from licensed farms to small indoor rooms.
Two terms appear throughout. PAR is the light between 400 and 700 nm that plants use for photosynthesis. PPFD is the amount of that light landing on one square metre of canopy each second, measured in µmol/m²/s. Part 1 covers light per watt, heat, lifespan and five-year cost.
This is Part 2. Part 1: Indoor Grow Lights: LED vs HPS vs CMH — Comprehensive Comparison 2026
1. The colour mix of each light
The colour of light is described by its wavelength in nanometres (nm). Blue is around 450 nm and red around 620–660 nm. UV-A (315–400 nm) and UV-B (280–315 nm) have shorter wavelengths than visible light. Far-red (around 730 nm) lies just beyond red, outside the 400–700 nm PAR band. For the basics, see light spectrum and DLI.
- LED (white, full spectrum): covers 400–700 nm, with peaks near 450 nm (blue) and 620–660 nm (red). Modern fixtures add separate far-red (730 nm) and UV-A (380–400 nm) channels. You can adjust the mix.
- HPS: most of the light falls in 565–600 nm (yellow-orange). Blue is under 5% of the output, and there is no UV. Because there is little blue compared with red, plants stretch: they grow longer stems with more space between leaf sets.
- CMH: a broad, unbroken mix from 360 to 780 nm, close to sunlight. It includes UV-A and UV-B, a strong share of blue and balanced red. Its colour rendering index (CRI, a 0–100 score for how true colours look) is around 90.
What to do: if you move from HPS to LED, expect less stretch and adjust your plant training to match. Record plant height at the switch to flowering and at harvest. This shows how much the new light changed the height of your cultivar.
2. What the colour mix does to THC and aroma
THC and the colour mix
Studies do not agree that a blue-rich mix raises THC. Westmoreland, Kusuma and Bugbee (2021, PLoS ONE) grew medical hemp under HPS and several LED mixes at the same light level, with blue making up 4% to 20% of the light. Spectrum had no effect on THC or CBD concentration. Yield fell by about 12% as the blue share rose from 4% to 20%. The authors concluded that light per watt matters more than spectrum for cost-effective production.
Section 3 shows that UV on its own also does not reliably raise cannabinoids (THC, CBD and related compounds) in modern high-THC cultivars.
What to do: if you switch from HPS to LED, keep the same cultivar, feeding and harvest timing for one cycle. Send a flower sample to a lab and compare the THC figure with a certificate from your last HPS cycle (how to read a certificate of analysis). This shows whether the result holds for your genetics, not just in the study. To find testing labs in Thailand, ask the community in B2B Services.
Aroma
Huebner and colleagues published a 2024 study in Frontiers in Plant Science. They added several UV treatments to the main light and measured how terpenes, the compounds behind cannabis aroma, responded. One treatment (1.81 W/m² of UV, almost all UV-A with about 1% UV-B) raised the three terpenes below, compared with plants without UV. Yield and cannabinoid levels did not change. No UV treatment in the study changed the cannabinoid profile.
| Terpene | Rise under the best UV treatment, compared with no UV |
|---|---|
| Linalool | +29% |
| Limonene | +25% |
| Myrcene | +22% |
The original concludes that LED fixtures with added UV are a good way to raise terpene levels. That rests on one treatment in one study, and other UV studies found no terpene gain (Section 3). Terpenes shape the aroma that buyers judge flower by.
What to do: test UV on a small batch first, using the settings in Section 4. Judge the result by lab terpene figures or blind smell tests, not by impression. This keeps you from paying for UV that makes no difference to your cultivar.
3. Research findings
How much light is enough
- Hemp and CBD (Hahm and colleagues, 2025): a study in South Korea grew the hemp cultivar ‘Queen Dream’ at 200, 400 and 600 µmol/m²/s. Total CBD concentration was 36.88% higher at 600 than at 200 µmol/m²/s. Higher levels were not tested.
- More light without added CO2 (Rodriguez-Morrison and colleagues, 2021): at normal room CO2, dry flower yield rose in a straight line as light increased, up to 1,800 µmol/m²/s. Light level did not change cannabinoid concentration. A second study, also without added CO2 (Llewellyn and colleagues, 2022), found about 1.6 times the dry flower weight at 1,000 µmol/m²/s as at 600.
In these studies, more light gave more flower, but not stronger flower. Each extra step of light also adds electricity and heat, so the right level for your room depends on your cooling and your costs. CO2 enrichment is a separate decision with its own costs.
Translator’s note: CO2 enrichment needs a closed room, because ventilation blows the CO2 out. In Thailand, a closed room relies fully on its air conditioner to remove heat, so a power cut overheats it quickly. If you do not add CO2, aim for 600–800 µmol/m²/s and measure at canopy height.
Light bars inside the canopy
Inter-canopy lighting means LED bars hung among the plants, between the top buds and the lower buds. Plants grown this way produced:
- 29.95% more dry flower weight
- 24.4% more total THC per plant
These figures come from Garrido and colleagues (2025, Plants), who compared top lighting alone with top lighting plus bars inside the canopy in medicinal cannabis. Lower buds normally get little light and stay small and airy, which growers call “larf”. Side light reaches them, so they develop into usable flower.
- What you get: more flower weight, with the gain clearest in the middle and lower parts of each plant, and more total THC per plant.
- In Thailand: each bar adds watts and heat, so recheck your cooling with the method in Part 1. A dense, well-lit canopy also needs strong airflow in the rainy season to prevent bud rot (botrytis).
- Where to start: put one bar in one row and leave a second row without one. At harvest, weigh the lower buds of each row separately. This tells you whether the gain pays for the bar and the extra cooling.
UV: not reliably more THC, and mixed results for aroma
Many growers believe UV raises THC. Two Canadian studies found that UV did not raise cannabinoid content in modern cultivars (Rodriguez-Morrison and colleagues, 2021; Llewellyn and colleagues, 2022). Older studies found a link between UV and cannabinoids. That link may reflect older genetics, because breeding may have pushed modern cultivars past the point where UV makes a difference.
The results for aroma and resin are mixed. Huebner and colleagues (2024) found that one low-dose UV treatment raised some terpenes (Section 2). Other studies found no change in total terpenes (Llewellyn and colleagues, 2022) or a fall in total terpenes as UV-B rose (Rodriguez-Morrison and colleagues, 2021). Trichomes are the tiny resin glands on the flowers where cannabinoids and terpenes are made. One study saw denser trichomes on sugar leaves under UV-A plus UV-B, but did not measure them (Llewellyn and colleagues, 2022).
What to do: if you try UV, do not expect more THC. Any gain in aroma or resin depends on the dose and the cultivar, so measure both and you will know what it actually did for you.
Far-red
Peterswald, Purdy and colleagues published a 2025 study in Scientific Reports. They tested far-red light on three medicinal cultivars. In one cultivar, Northern Lights, 10 hours of normal light followed by 2 hours of far-red at the start of the dark period gave nearly 70% more total cannabinoids per plant than a normal 12-hour day. The other two cultivars responded differently.
The paper’s introduction describes the Emerson enhancement effect: photosynthesis runs faster when far-red is added to red light than the two would achieve separately. But far-red has downsides:
- It triggers shade-avoidance stretch. The plant reads far-red as shade from its neighbours and grows taller. In this study, far-red made plants taller in all three cultivars.
- The study did not measure terpenes, so it tells you nothing about aroma.
- Its effects depend strongly on the strain.
What to do: treat far-red as a trade-off between yield, height and aroma. Test it on one cultivar before using it across the room (Section 4).
4. Settings for UV and far-red add-on lights
UV strength is given in watts per square metre (W/m²), not in photons. A normal PAR meter does not read UV. You need a UV meter, or the maker’s chart of UV intensity at your hanging height.
UV
| Setting | Recommendation |
|---|---|
| When to start | Week 2–3 of flowering |
| Starting strength | 25% of maximum output |
| Hours per day | 2–4 hours, in the middle of the light period |
| Best strength | 1.81 W/m² of UV-A and UV-B combined (almost all UV-A; the treatment that raised terpenes in Huebner and colleagues, 2024) |
| Increase | Add 25% per week until full power |
| Safety | UV-protective glasses are a must; keep UV off your skin |
Risks: too much UV bleaches leaves, slows photosynthesis and stresses the plants.
On a 12-hour flowering day, “the middle of the light period” means, for example, hours 5 to 8 after lights-on.
Far-red
| Setting | Recommendation |
|---|---|
| Safest method | End-of-day (EOD) treatment |
| End-of-day duration | A short period (minutes, not hours) after the main lights switch off |
| All-day use | Risky; watch stretch closely |
| Upper limit | Keep far-red a small share of total PPF; raise it only after testing |
The end-of-day method is safest because the far-red runs for minutes, not hours. That gives the plants much less exposure than all-day use.
Risks: too much far-red causes extreme stretching between leaf sets, a thinner canopy and possibly lower terpene levels. Test it on each strain before using it across the room.
Translator’s note: anyone who works in a room with UV is exposed, not only the owner. Give every worker UV-protective glasses, and switch the UV off before people work among the plants. Start with one cultivar in one tent or one row, and keep a matching batch without the add-on. Record height at the switch to flowering and at harvest, dry weight, and lab THC and terpene results. These records tell you whether the add-on pays off for your genetics in your room.
5. Choosing a fixture: a checklist
Use these points to compare fixtures, not a list of brands:
- Efficiency: compare fixture-level PPE (µmol/J) from the spec sheet or the DLC Qualified Products List, not chip figures. The DLC list is published by the DesignLights Consortium, an organisation that lists the tested values of qualifying fixtures. PPE decides your electricity use and your heat load.
- Output and coverage: match the PPF and the PPFD map to your canopy size and target PPFD. This makes sure the plants actually receive the light you are paying for.
- Spectrum: full-spectrum white with added red is the current standard. UV and far-red are optional, and you need to test them (Sections 3 and 4).
- Heat and running cost: estimate electricity and cooling for your own room, using the methods in Part 1.
- Warranty and support: check how long the warranty runs and whether drivers can be serviced locally.
Translator’s note: in Thailand, add three checks. First, check the IP rating (the code for sealing against dust and water), because rainy-season humidity damages electronics. Second, plan for a surge protector to guard the driver against voltage spikes. Third, ask the official distributor in Thailand who honours the warranty and whether spare drivers are in stock. To compare real experience with a fixture, ask the community in Equipment and Supplies.
6. Conclusions
- LED is the best overall choice for new rooms in 2025–2026. The best fixtures pass 3.0 µmol/J, need no bulbs, send less heat onto the canopy and let you adjust the spectrum. That gives the best mix of performance, running cost and flexibility.
- CMH still has a place. It suits growers who value natural UV-B, very true colours (CRI 90+) and a low starting price. A pair of 315 W CMH fixtures is a real option for small, aroma-focused grows on a budget. Over five years it costs about the same as LED in the original’s table, but it gives much less light (Part 1, Section 5).
- HPS is being phased out. Double-ended HPS still gives high PPF. But at 1.95–2.0 µmol/J, with heavy heat output and recurring bulb costs, it is no longer a sound choice for a new installation.
- Use UV and far-red with care. The research is promising, but the effects vary. Adding them without testing each strain and following the settings can lower quality instead of raising it. Start low, measure the results and adjust.
What to do: before you replace your lights, write down your current input watts, PPFD readings, electricity cost per cycle and dry yield per cycle. Compare the same four figures after one cycle under the new lights. This shows what the change actually gained you.
Do you have results from your own room? Post them below this topic: fixture, input watts, PPFD, cultivar and any lab results. Questions about the studies or the settings are welcome too.
References
Scientific papers
- Westmoreland, F. M., Kusuma, P. & Bugbee, B. (2021). Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids. PLoS ONE, 16(3), e0248988.
- Huebner, D. et al. (2024). Influence of different UV spectra and intensities on yield and quality of cannabis inflorescences. Frontiers in Plant Science, 15, 1480876.
- Hahm, S. et al. (2025). High light intensity enhances cannabinoid biosynthesis through concerted gene expression in hemp (Cannabis sativa) flowers. Frontiers in Plant Science, 16, 1687794.
- Llewellyn, D. et al. (2022). Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content. Frontiers in Plant Science, 13, 974018.
- Peterswald, T. J., Mieog, J. C., Kretzschmar, T. & Purdy, S. J. (2025). The effects of far-red light on medicinal Cannabis. Scientific Reports, 15, 17435.
- Rodriguez-Morrison, V., Llewellyn, D. & Zheng, Y. (2021). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science, 12, 646020.
- Rodriguez-Morrison, V., Llewellyn, D. & Zheng, Y. (2021). Cannabis inflorescence yield and cannabinoid concentration are not increased with exposure to short-wavelength ultraviolet-B radiation. Frontiers in Plant Science, 12, 725078.
- Eichhorn Bilodeau, S. et al. (2019). An update on plant photobiology and implications for cannabis production. Frontiers in Plant Science, 10, 296.
- Garrido, J. et al. (2025). Subcanopy and inter-canopy supplemental light enhances and standardizes yields in medicinal cannabis (Cannabis sativa L.). Plants, 14(10), 1469.
- Magagnini, G., Grassi, G. & Kotiranta, S. (2018). The effect of light spectrum on the morphology and cannabinoid content of Cannabis sativa L. Medical Cannabis and Cannabinoids, 1(1), 19-27.
Technical sources
- DLC Horticultural Qualified Products List (V4.0, 2025).
- Samsung LED Horticulture Solutions Datasheet (2025).
- ams-OSRAM OSCONIQ P 3737 Gen 2 Datasheet (2024).
Manufacturers named in the original
- Spider Farmer, HLG, Gavita (Signify), Lumatek, Fluence (Signify), Growers Choice, Sun System, Phlizon, Mammoth Lighting.
Community
- Asiannabis Community Forum — asiannabis.com
Legal note
Cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. The notification was issued under the Thai Traditional Medicine Protection Act B.E. 2542 and has been in force since 26 June 2025. Flower may be sold only with a PT 33 prescription, for at most 30 days. Advertising and online sale of flower are prohibited.
Two ministerial regulations of B.E. 2569 followed. 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 and tightens the licence criteria for cannabis flower.
Research, export, sale and processing of cannabis flower require a licence (Licensing and Compliance in Thailand). This article is for education and information within the Asiannabis community. It is not legal advice and does not encourage unlicensed cultivation. You are responsible for following the laws where you live. For current requirements, contact the Department of Thai Traditional and Alternative Medicine (for cannabis flower), the Thai FDA (for cannabis extracts) or a qualified lawyer.