How far should CMH lights be from plants?

Ceramic Metal Halide Grow Lights

CMH lights were introduced in the 1980s, but with recent discoveries in horticulture research, it makes sense why many growers are utilizing this technology.The CRI of CMH's is around 90-92, close to the spectrum you get from the sun .The claim from CMH lovers is that your plants can enjoy a nearly full spectrum white light, very similar to natural sunlight that lasts longer than your HID counterpart. A CMH's Color Rendering Index (CRI) rating is pretty close to the sun. CRI is a measurement of how true colors stay when the light is set upon them. HPS grow lights are 20 - 30 (that's why it's all orange/yellow in your grow room during flowering) while Metal Halides are 60 - 65 (which is why there's a little bit of a blue tint in your growing area when you're vegging). CMH grow lights come in with a CRI rating of 90 - 92 making them only 8 points off from our most natural source of light. Right off the bat, once you turn on a CMH bulb at either 3,000k or 4,000k, you'll notice a more natural looking light spectrum. No strong orange and yellow hues like an HPS bulb, or the bright white and subtle blues from a Metal Halide. Why is that?Standard metal halide bulbs use a quartz arc tube whereas the CMH systems use ceramic, which lets the bulb get hotter to give you that perfect spectrum of light CMH's give off. Not only that, but there is less degradation to the bulb because of the stronger arc tube material- that means they last a lot longer than other HID bulbs out there.That's why it operates at much higher pressure than standard MH bulbs. The ceramic arc tube combined with a unique mixture of salts and halides in CMH's is what creates its broad spectrum. And in turn, gives growers the enhanced photosynthetic photon flux levels.An added bonus to the quality spectrum is their lower heat output and power consumption. Lower watts means a lower toll on your electricity bill, and with lower watts, we also have lower heat. One watt from a grow light creates four British Thermal Units (BTUs) of heat that need to be cooled. So in comparison, one 600w HPS grow light puts off 2,400 BTU 's whereas one 315 W CMH grow light puts off only 1,260 BTU's, which is almost less than half the HPS light. With all that being said, with CMH lights your tent will be cooler meaning you have to run your AC less, again lower your electricity bill.

CMH Grow Light Coverage

CMH lamps are recommended for universal positioning. This means they can be positioned vertically or horizontally above the canopy. The position won't cause damage to the bulb.

The position won't affect the light distribution, and every plant will have a consistent absorption rate. The grow light must be hanged minimum 3ft above the canopy to avoid any hot spots. This also provides an efficient and even light distribution.

The light distribution is so intense that a 315W bulb can have a coverage area up to 4ft x 4ft. This is larger than what higher wattages MH and HPS bulbs provide.

An increase in the watts leads to a bigger coverage area. The higher watts contribute to greater heat produced. Make sure you hang the grow lights at least higher than 3ft above the plants to cancel out the effects of the heat.

The table below shows the recommended coverage of each CMH bulb and the distance from plants you should keep.

CMH Bulbs - Recommended Coverage and Distance From Plants
Power             Coverage         Distance
315W                3ft x 3ft           4 ft x 4ft 2 ft
630W               5 ft x 5 ft         6 ft x 6 ft 3 ft 

Spectrums of a CMH bulb

Despite depending on the type of bulb temperature you get, CMH grow lights all have very wide spectrum wavelengths. They produce high red, yellow and green color temperatures. Each color has its own role in promoting the plants through the different lifecycle stages.

• Red: This color is responsible for the flowering and fruiting of plants.
• Yellow: This colored light spectrum improves the effects of photosynthesis.
• Green: This is used to elicit specific plant responses such as stomatal control, phototropism, photomorphogenic growth and, environmental signaling.

CMH lamps don't have high outputs of the blue spectrum like other 2700K bulbs or blue LED lights on the market, which are great for veg stages. However, they do have sufficient blueish light colors to grow in the veg stages.

Commonly, you'll run into two versions of CMH bulbs on the market.

• 3,100K (full-spectrum) bulb has more colors in yellow, red and green, which can be used for full growth (from cloning to flowering)
• 4,200K (broader spectrum) bulb contains wide wavelengths but has more blueish wavelengths; So is more suitable to grow seedling and veg plants; They can be used to grow flowering phases, but the 3100K is more efficient.

The Benefits of CMH Lights

The money you save on your power bill is the No. 1 perk of switching to CMH grow lights. Although the initial cost of the system is more expensive than other HID systems on the market, CMH systems will save you money in the long run. Some jurisdictions even recognize them as energy-efficient lighting systems and offer incentives to offset the initial cost.

Some suppliers claim one 315W CMH is equivalent to one 1,000W HPS lamp in terms of yields. Others have reported that you need two 315W CMH lamps to produce the same yields as a 1,000W HPS. The varying reports probably have to do with other elements in the garden and the gardener's skill level, and the truth probably lies somewhere in the middle. In any case, the general consensus is that your yield per watt is greater with ceramic metal halides, and you will be spending much less on electricity to power your lights.

These lights also have a low heat output because of their low wattage. One watt from a grow light creates about four British Thermal Units (BTUs) of heat that need to be cooled.

Let's say you have a small room with four 1000W HPS grow lights. Your four lights alone create 16,000 BTUs of heat that need cooling. If you trade those lights in for four 630W fixtures, which will match, if not exceed, your current yields, you will only create 10,000 BTUs of heat. Your air conditioner won't have to run as often, and you'll see savings on your power bill. The low heat output of CMHs make them an ideal choice for small grow spaces, such as closets and grow tents.

Another benefit is that with the ballasts required to run them, you won't have to deal with the electromagnetic interference (EMI) issues many growers experience with digital ballasts. High levels of EMI from digital ballasts are known to interfere with cellphone service, radio, Wi-Fi and cable television, and the Federal Communications Commission hands out citations to people who violate EMI emission standards.

One final advantage ceramic metal halides bring to the table is their superior light quality compared to other HID lamps, which is due to a few factors. First, they have a wider spectrum than other HID lamps-a result of the near-perfect, unique combination of salts, halides, and gases used in the arc tube.

This mixture creates a spectrum close to that of the sun and emits a stunning, crisp, white light. In fact, the color rendering index (CRI) for ceramic lamps ranges from 90-92. To give you some perspective, the sun's CRI is 100. HPS lamps range from 20-30, and metal halides range from 60-65. The closer the CRI is to 100, the closer the spectrum is to that of the sun.

In terms of light quality, ceramics include ultraviolet and infrared rays in their spectrum, which plants love. The higher amounts of UV rays create stronger, woodier stalks. This means the growth and vigor of your plants will be unmatched, especially in the vegetative stage. Due to the incredible growth CMHs produce, growers have reported great success when using silica supplements with these lights.

Silica helps with cell division and creates stronger cell walls, which makes the leaves, stalks and branches significantly stronger and better able to withstand the weight of the fruits. As an added bonus, pests don't like woody stalks because they are too hard to chew on. Compare it to eating a tender, juicy steak as opposed to eating an overcooked, dry steak. Which do you prefer?

Finally, CMH ballasts use square-wave technology. When you look at the graph on the back of the box your bulb is packaged in, notice the high peaks and low dips. The peaks are an indicator of energy pushing out of the bulb. The low dips indicate light trying to travel to the next peak. You can see this with your own eyes when you take photos and videos of your growroom. With most HID lights, you'll see bands of light on your photos.

The bands of light are the peaks in the graph, and the spaces between them indicate the dips. With square-wave technology, there are fewer dips. A steadier beam of light is produced, which means your plants receive more light over their lives.