CMH/LEC Lighting

Growing, buying or possessing cannabis and seeds is legal only in some places. Check the law where you live before acting on this information.

What CMH/LEC actually is

CMH stands for Ceramic Metal Halide. LEC (Light Emitting Ceramic) is a Sun System trademark for the same underlying technology. Both refer to a metal halide arc tube made of polycrystalline alumina ceramic rather than quartz. The ceramic envelope tolerates higher internal pressures and temperatures, which produces a more stable arc and a broader, more continuous spectrum than traditional quartz metal halide or high-pressure sodium (HPS) ^[1].

The most common cannabis sizes are 315W (single-ended) and 630W (a fixture with two 315W bulbs). Bulbs come in roughly 3100K (warmer, flower-leaning) and 4200K (cooler, veg-leaning) color temperatures. CMH bulbs also emit measurable UV-A and small amounts of UV-B, which standard HPS does not.

CMH requires a square-wave electronic ballast specifically designed for ceramic arc tubes. Standard HPS magnetic ballasts will not run them correctly.

Why growers use it

Real, measurable advantages over HPS:

• Broader spectrum. CMH includes more blue, green, and far-red wavelengths plus some UV, versus HPS's yellow-orange dominant output. This makes plants look natural under the light and makes it easier to spot deficiencies, pests, or mold.
• Better lumen maintenance. CMH bulbs typically hold ~90% of initial output at 8,000 hours, compared to HPS dropping below 80% well before that.
• Natural color rendering. The continuous spectrum makes visual inspection of plant health more reliable than under HPS Anecdote.

Marketing claims to be skeptical of:

• 'CMH grows produce more terpenes.' There is no controlled study comparing terpene content between CMH and HPS or modern LED at equivalent PPFD. No data
• 'CMH yields more than HPS per watt.' On a per-watt basis, 630W CMH and 600W HPS yield roughly equivalent dry weight in published side-by-side reports Weak / limited. Modern LED fixtures now exceed 2.7 μmol/J efficacy, while CMH typically falls in the 1.5–1.8 μmol/J range ^[1]. Strong evidence
• 'UV-B from CMH will boost your THC significantly.' One controlled study (1987) showed UV-B exposure increased some secondary metabolites in a small sample, but effect sizes were modest and have not been reliably replicated in commercial settings ^[2]. Weak / limited

When to start

CMH can run an entire grow cycle from seedling to harvest, but it is most useful in two roles:

1. Small-tent all-rounder. A single 315W CMH covers roughly a 3x3 ft footprint in flower and a 4x4 ft in vegetative growth. It is a one-bulb solution for hobby growers who don't want separate veg and flower lights.
2. Supplemental finishing light. Some growers run HPS or LED as the main light and add 315W CMH bulbs for the last 2–3 weeks of flower to provide broader spectrum and UV. This is common in large operations but adds complexity and cost for small setups. Anecdote

Bulb replacement: Replace CMH bulbs every 12–18 months of regular use (roughly 8,000–10,000 hours). The spectrum shifts as the bulb ages even before total output drops noticeably.

How to set it up: step by step

1. Pick the right size. 315W for spaces up to 3x3 ft. 630W (dual bulb) for 4x4 ft to 5x5 ft. Going larger means multiple fixtures.

1. Buy a matched ballast and bulb. The ballast must be a square-wave electronic ballast rated for ceramic arc tubes. Mismatched ballasts will shorten bulb life or damage the arc tube. Stick with reputable brands: Philips Mastercolor CDM-T Elite, Iwasaki, or Ushio for bulbs; Sun System, Nanolux, or Phantom for fixtures.

1. Choose your color temperature. 4200K (sometimes labeled 'Agro' or 'Cool') for vegetative growth or full-cycle use. 3100K (sometimes 'HortiLux' or 'Warm') for flower-only or as a finishing bulb. Mixing both in a 630W fixture is a reasonable compromise.

1. Hang at the right distance. Start with the bulb 24–36 inches above the canopy. CMH runs hot and emits UV—too close burns leaves and bleaches buds. Use a PAR meter if possible; target 600–900 PPFD in flower and 400–600 PPFD in veg Anecdote.

1. Ventilate. A 315W fixture dumps roughly 1,075 BTU/hr into the room. Run an inline fan and ducted or open reflector. Air-cooled hoods help in hot climates.

1. Wear UV-blocking glasses. CMH emits enough UV that prolonged exposure damages your eyes. Get inexpensive UV-rated grow glasses; they also help you see trichomes and color accurately Anecdote.

1. Log replacement dates. Mark the install date on the bulb with a permanent marker. Replace before output drops noticeably.

Common mistakes

• Running CMH bulbs on an HPS ballast. The waveform is wrong. Bulb life crashes and the spectrum drifts. Always use a matched ballast.
• Hanging too close. CMH puts out real UV. 12 inches over the canopy will bleach top colas and burn leaves. Start high, lower gradually based on plant response.
• No eye protection. UV damage to the eyes is cumulative. Wear glasses every time you enter the room with the light on.
• Ignoring heat. CMH is more efficient than older MH but still HID—it generates substantial radiant heat. Without exhaust, canopy temps climb fast. Target 75–85°F (24–29°C) in flower.
• Expecting LED-level efficiency. If electricity cost matters and your space is large, a modern LED is a better long-term buy. CMH is competitive on upfront cost, not on running cost ^[1].
• Buying off-brand or very old bulbs. Generic 315W bulbs vary wildly in spectrum and lifespan. Check the manufacture date; old stock bulbs may have degraded phosphors.

Related techniques

For growers deciding between lighting types:

• LED Grow Lights are now more efficient than CMH and generate less heat, though upfront cost is higher. Most new commercial installations use LED.
• HPS Lighting remains the cheapest per-watt option and is well-understood, but has a narrower spectrum and shorter bulb life.
• CO2 Supplementation can improve photosynthesis efficiency under any light type, but requires careful environmental control.
• VPD (Vapor Pressure Deficit) Basics matter just as much under CMH as under any light—managing humidity and temperature keeps plants healthy regardless of spectrum.

Sources

How this page was made

Related

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• CO2 Supplementation — Raising CO2 to boost photosynthesis in sealed rooms with strong lighting.
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