P. L. Light Systems Expert Articles: What is the Difference Between Metal Halide and Ceramic Metal Halide Luminaires?

Jillian Whitehead

Ceramic Metal Halide, Lighting Strategies, Metal Halide | March 2, 2020

We often get asked what the difference is between metal halide (MH) and ceramic metal halide (CMH) luminaires. While they both have similar technologies, there are some fundamental differences in how they operate.

To start, they are both high intensity discharge (HID) lamps that can be used for growing plants.  Both these lamps contain metal halides, or metal salts, and are built of two main sections – an outer bulb and the inner arc tube.  In general, MH and CMH lamps, because their more balanced spectrum and higher emittance of blue wavelengths, have been traditionally used in vegetative growth cycles. Plants that benefit most from metal halide lamps are vegetables like leafy greens and herbs, and cannabis during its vegetative and mother growth stages.  This is because the higher blue spectrum limits internode stretching, producing bushier, more compact plants, while also limiting the flowering response.  On the other hand, high pressure sodium (HPS) lamps are traditionally used as a flowering and production spectrum because of its high emittance of red wavelengths – which are most efficient for inducing flowering and promoting fruit development in plants. Another similarity between MH and CMH lamps is they both emit some UV wavelengths – this can be beneficial to plant health as it can promote the defense response and increase the synthesis of secondary metabolites.


What are MH lamps?

MH lamps contain metal halides that when heated up emit light.  The lamps have two electrodes, that when voltage is supplied, electricity is passed through from one electrode to another.  This heats up the mercury inside causing it to vaporize causing the electrical current to grow. When things heat up the metal halides turn into gas – then the metal halide atoms begin to move away from the arc and in doing so a light is created.  MH lamps can emit various shades or colours of light depending on the types of metal halides used – for growing the most common colour emitted is a blue-white colour.  This spectrum is mostly used for vegetative growing stages, which require a higher blue spectrum to promote bushy growth habits and compactness.   Quartz glass is often the material used to make the arc tube, which can erode over time diminishing the output of light. This can diminish the overall lifetime of the lamp and will require re-lamping sooner than with other types of HID lamps.  This design can make MH lamps more unstable in terms of maintenance, longevity and spectrum output than other types of HID lighting, including CMH lamps.

CMH luminaires being used to grow cannabis in an indoor growing application

What are CMH lamps?

The largest difference between MH and CMH lamps is the Ceramic or “C” in CMH.  This refers to the arc tube material, which is made of a ceramic material – similar to what is used for HPS lamps.  This material is more stable than the quartz glass used in MH lamps, along with being able to withstand much higher temperatures and is more resistant to the aggressive salts.  By using a higher temperature inside the arc tube, the efficiency, colour stability and overall light maintenance are greatly improved.

With CMH lamps, because of its design, you get a more balanced spectrum of light, similar to sunlight, which helps your plants grow and develop like they would outdoors. In addition, when it comes to people and work environment, CMH lamps have a higher colour rendering index (CRI) making for a more pleasant work environment, along with being able to assess plant health easier in sole source lighting conditions.

With the broad spectrum emitted by CMH lamps, you can also see many advantages in the morphogenesis of the plants, including the production of terpenes, flavonoids and other secondary metabolites from the UV and blue wavelengths.  However, because of the lower emittance of red wavelengths compared to HPS lamps, it is not recommended as a flowering or production spectrum (unless you are growing leafy greens or herbs). A good option, if you want the added benefits of CMH lamps in your production cycle, would be to install a hybrid HPS-CMH system so you are getting the most balanced spectrum including that higher red for the flowering response.

Another advantage when using the CMH lamp, is the lower wattage – it is only 315 watts, compared to a metal halide lamp which is often 600-1000 watts.  Having a lower wattage means you can get closer to the crop canopy without burning and you will have greater energy efficiencies.

CMH luminaires being used to grow leafy greens in a vertical growing system

Which should I use?

One thing to make clear is you cannot simply replace an MH lamp fixture with a CMH lamp. CMH lamps require a specific ballast for ignition, and is why we offer our 315W NXT-LP CMH as a stand alone luminaire. That could be a disadvantage to you if you are looking at using the same growing space for both vegetative and flowering cycles. In this case, MH lamps would be your best option as they can be housed with the same ballast as HPS lamps.

However, if you have separate vegetative and flowering rooms, or are growing solely greens or herbs, CMH lamps will make more sense.  Having a lower wattage than MH lamps, means that you can achieve greater energy and space efficiencies since you can mount the lights close to the crop.  In addition, because of their design, you will require less frequent re-lamping than with MH lamps. You will also have greater spectral efficiencies with more wattage of electricity available in the PAR wavelengths for growing.  If CMH lamps have caught your attention, check out our latest 315W NXT-LP CMH luminaire and don’t miss our grower feature on Aqua Greens who use our legacy HSE Daylight luminaires with CMH lamps to grow high quality leafy greens.