Which is the Best CDM/CMH Grow Light for Me?

How to choose a CDM

Going back a few years, Metal Halide lamps were the standard choice for vegging or supplementary lighting to give a more balanced spectrum in a flowering setup.

Ceramic Discharge Metal Halide (also known as CDM or CMH) grow lights have risen considerably in popularity in the last few years. They represent a big advancement over the old Metal Halide (MH) type grow lights which, although are still available to buy, are no match for the newer CDM type of grow light.

Both MH and CMH have a much better light output spectrum balance than traditional HPS. The reason for this is that the output spectrum contains more blue and other colours apart from orange and red. The predominantly orange/red spectrum output of an HPS makes it ideal for maximising productivity during the fruiting/flowering stage. However, the better balance and greater blue content made Metal Halide lamps the usual choice for vegging. It was not uncommon to find growers using an MH during veg and a HPS for flowering or fruiting.

The Importance of Efficiency

The downside to the old Metal Halide grow lights is one of efficiency. Typical HPS lamps emit around 1.6 – 2.1 μmols/J (1.6 for a cheap lamp, up to 2.1 for a premium 400V version). By comparison, the old type MH lamps only manage about 1.4 μmols/J.

Despite this relative inefficiency, it was a common scenario for growers to use the combination of a 400W Metal Halide alongside a 600W HPS. The HPS produced the yield, while the MH lamp provided blue light and some UV, which encouraged terpene & essential oil production.

Ceramic Metal Halide lamps are more efficient than their original counterparts. In fact, the best ones can deliver around 2.0 μmols/J which is about 20% more efficient. Also, they give the manufacturer more scope to tweak the spectrum. CMH lamps are commonly available as 4K, 3K, and 3K Agro.

So, What on Earth is this Colour Temperature thing about?

The Kelvin scale does not give a detailed description of the colour of a lamp. Although the explanation for where the scale comes from gets rather scientific, understanding what it is describing is fairly simple. The colour temperature describes the colour balance of a light source.

Natural sunlight has a colour temperature of around 5K to 6.5K. The exact colour temperature depends on the time of day, the weather, and to some extent the season. In the 5K to 6.5K colour temperature range, the balance of the colours in the light spectrum is fairly even. As the colour temperature rises above 6.5K there is a progressive tilt towards the blue end of the visible spectrum. As the colour temperature goes downwards (below 5K) the spectrum balance goes the other way – with a progressive tilt in favour of the red end of the visible spectrum.

This means that a CDM lamp with a colour temperature of 4K will appear cooler (or bluer) than a CDM with a colour temperature of 3K. CDM lamps denoted as “3K Agro” still have a colour temperature of around 3K but have slightly more of the highly productive reds than the standard 3K version.

To add more uncertainty to the choice, you will most often find that each manufacturer’s version of these lamps comes with their own signature spectrum. Each one varies according to the design, the construction and the materials used to make them.

Looking at Real-life lamps

So, let’s take a look at some of the more common, branded options:

  1. Philips Master – 942 Daylight (4K), 930 Agro (3K)
  2. Dimlux – 4K, 3K, 3K Agro
  3. LUMii Solar – Grow (4.2K), Pro (3.2K)
  4. Maxibright – Daylight (4K), Agro (3K)

It is probably fair to say that the brightness of these lamps will vary a little from manufacturer to manufacturer. However, before choosing the brand of CDM to buy, it is a good idea to decide which colour temperature you need.

The decision is actually not as difficult as it may first appear. The 3K Agro lamps are perfect for flowering/ fruiting as standalone lights. There is plenty of blue and UV in the spectrum for quality enhancement over a HPS, yet there is a bias towards the red part of the spectrum which ensures that yield is not compromised.

If CDMs are being added to a grow light setup which is based on HPS lighting, then the 4K option will probably be the best. While your HPS grow lights will provide oodles of red for yield, the 4K CDM will fill in the blue and the UV parts of the spectrum to enhance the quality and potency of your crop. Using a 3K Agro alongside HPS grow lights would be a little bit of a waste. The 3K Agro CDM will not be able to compete with a 600w HPS (or above) in terms of the red output. Also, the 3K Agro produces less of the blues and UV than a 4K version, and it is these parts of the spectrum that will make adding a CDM to your grow a seriously savvy move for improved quality produce. Reports have it that using 4K CDMs with HPS lamps in a 1:1 or 1:2 ratio gives the best of both worlds – Excellent yields and Serious quality!

Taking a look at the Spectrums

So, the colour temperature helps us to choose the right type of CDM (CMH) grow lamp will be best for the purpose that we want it for. However, taking a look at the actual spectrum of a lamp can give us a bit more information to help us to make a decision about which specific lamp is best for us. For example, the spectrums of different manufacturer’s 4K lamps are not all identical. For comparison, here are the spectrums of some of the different lamps that are available. First of all, let’s take a look at the spectrums of some different 4K lamps:

Philips 942 4.2K CDM/CMH Lamp
Dimlux 4K CDM/CMH Lamp
Maxibright 4K CDM/CMH Lamp

Here are the spectrums of some 3K Agro CDM/CMH lamps:

Philips 930 Agro 3K CDM/CMH Lamp

Choosing the Best Reflector for the Purpose

So, we have established that it is best to use 4K lamps for vegging or as supplemental lighting for a HPS grow light setup, whereas 3K Agro lamps are best for where the CDMs will be exclusively used in a flowering/fruiting setup on their own. However, the decisions don’t end there. The wise grower will also want to get the best out of their lamp by buying the right reflector for the job.

Focussed Reflectors

Where 4K lamps are going to be used for vegging then a focussed reflector will make the most of the available light and intensity. This also goes for where 3K Agro CDM lamps are to be used exclusively in the flowering setup. There are plenty of choices for this and one of the best is the Sun Systems LEC 315 RA.

An alternative to this is the Maxibright Focus. It is available in either a “Remote” version as a normal standalone reflector which is remote from the ballast, or it is available as a “Connect” version where the ballast is attached to the back of the reflector.

Wide-Angle Reflectors

Where 4K CDM lamps are going to be used as supplemental lighting alongside HPS lights, a wide-angle reflector, which will cast its light to overlap the light from the HPS lamps either side of it, is the way to go. The Maxibright Horizon reflector is purpose designed for this application. It casts a very wide footprint that gets the light where it is needed – out to the sides as well as directly below. As with the Focus reflector, it is available in either a “Remote” version, or as a “Connect” version.

Complete Full Fixtures

There are 4 brands that make full fixtures which include a lamp, reflector and ballast. This simplifies things no end, and will usually save you money over buying the parts separately. Most are available as single lamp fixtures and also as a dual lamp. If you like the idea of using a dual lamp (630w) setup to replace a single 600w HPS then this is a convenient way to do it.

Almost all the units below have focussed reflectors. The exceptions are the Maxibright Horizon kits, which have a wide-angle reflector for greater sideways spread and are therefore perfect for using in overlapping gardens alongside a couple of HPS lights.

Gavita Pro 315e SE CDM / CMH Full Fixture Grow Light Kit

Gavita Pro 630e SE CDM / CMH Full Fixture Grow Light Kit

Dimlux Expert Series – 315w CDM / CMH Full Fixture Grow Light Kit

Dimlux Expert Series – 630w CDM / CMH Full Fixture Grow Light Kit

LUMii Solar 315w CDM / CMH Full Fixture Grow Light Kit

LUMii Solar 630w CDM / CMH Full Fixture Grow Light Kit

Maxibright Horizon Connect 315w CDM / CMH Full Fixture Grow Light Kit

Maxibright Horizon Remote 315w CDM / CMH Complete Grow Light Kit

Maxibright Focus Connect 315w CDM / CMH Full Fixture Grow Light Kit

Maxibright Focus Remote 315w CDM / CMH Complete Grow Light Kit

Products mentioned in the article

630w dimlux cdm
315w dimlux cdm
315 cdm main
Choose your Gavita CDM fixture:
Gavita Pro 315e with 942 Lamp (Veg) (Add: £50.00)

£399.95 £399.95
630w cdm main
Choose your Gavita CDM fixture:
Gavita Pro 630e with 942 Lamps (Veg)

£749.95 £749.95
Choose your LUMii CDM Kit:
LUMii Solar 315w CDM grow light fixture only

£149.95 £149.95
Lumii solar 630w main
Choose your LUMii CDM Kit:
LUMii Solar 630w fixture only

£269.95 £269.95
Horizon connect solo
Horizon connet
Choose your CDM kit:
Maxibright Ballast with 930 Elite Agro lamp (flower)

£259.95 £259.95
Cdm kits focus connect
Choose your CDM kit:
Maxibright Ballast with 930 Elite Agro lamp (flower)

£259.95 £259.95
Focus remote full cdm kit
Choose your CDM kit:
Maxibright Ballast with 930 Elite Agro lamp (flower)

£259.95 £259.95
Choose your CDM kit:
Sol Digital Ballast with 942 Mastercolour lamp (veg)

£259.95 £259.95
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