Ceramic Metal Halide CMH 315W Lamp Comparison Test Data & Review

Hey everyone—Nate with Growers House here. We just wrapped up a full comparison on 315W Ceramic Metal Halide (CMH) lamps. The whole idea was to tease out which 315W CMH bulb on the market is the best for growing plants—whether you care most about overall output, spectrum shape, UV contribution, or value. I went and hunted down every 315W ceramic metal halide bulb I could get my hands on (10 total), then ran them through the same test setup so the only real variable was the bulb.

We recorded spectral data and took a bunch of measurements to give you apples-to-apples info you can actually use. The “best” lamp depends on your goals—we’re just here to hand you the data so you can make an educated decision for your grow.

Quick compatibility note: sockets, reflectors, and adapters

315W CMH bulbs are smaller than a lot of HID lamps and they use a different socket than the usual mogul style. That’s why you typically need a purpose-built 315W CMH reflector/fixture or a compatible CMH bulb adapter to run them correctly.

CMH vs Metal Halide: what’s actually different?

CMH is not “just another metal halide”

First thing I want to clear up: CMH is not the same as a traditional metal halide lamp. Most folks hear “metal halide” and assume it’s automatically a veg-only type of lamp. Not quite. A traditional metal halide uses a quartz arc tube. A CMH lamp uses a ceramic arc tube component, and that ceramic can handle much higher operating heat.

That higher arc-tube heat is the big reason manufacturers talk about improved color stability and better lumen-per-watt performance. But for growers, the more useful way to think about efficiency is: how many usable photons are hitting the canopy per watt of input.

Why CMH spectrum looks “wider” than many HID options

CMH bulbs still work by heating salts inside the arc tube to create light—similar concept to other HID lighting. The difference is the ceramic arc tube can push those salts hotter, so they behave more like plasma. That higher-temperature plasma is what helps CMH output look wider and more balanced compared with a lot of traditional HPS or older metal halide setups.

Test method: how we kept it consistent

One variable only—the bulb

We ran 10 different 315W CMH bulbs through the same setup and held everything else constant—ballast, mounting height, reflector/fixture, and measurement approach. Then we layered in spectrum reads, relative UV comparisons, and PAR footprint readings to build a practical performance picture—not just a single hot-spot number.

Spectrum results: 3000K vs 4200K CMH

What we saw in the ~3000K category

In the ~3000K group, most of the intensity was concentrated around the ~600 nm range. That’s the “warm” part of the spectrum, which is why these lamps look more orangish/yellowish to the eye—similar vibes to HPS. We also saw a notable spike just north of ~800 nm, then output dropped off with not much past ~900 nm.

What we saw in the ~4200K category

When we looked at the ~4200K group, the spectrum generally appeared broader and more “full.” Side-by-side, what you can’t fully see from just looking at charts is that the 3K lamps often feel more intense, while the 4K lamps spread energy more broadly with a peak closer to the ~500 nm region. That’s why 4200K reads more like a white light (and why people often call it “bluer” than 3000K).

You’ll also usually see a bit more output creeping toward UV with a lot of 4200K lamps, although in our testing we also found some 3000K lamps that surprised us on UV performance too.

UV results: relative UVA/UVB comparison (250–400 nm)

How we measured UV for this test

Our UV measurement is relative. We measured intensity between 250 and 400 nanometers, which is primarily the UVA and UVB output these lamps produce. This isn’t meant to be a “UV specialty” evaluation—it’s a comparison inside the 315W CMH class.

Best UV performer in our lineup

The lamp that did best on relative UV was the Ushio 3000K CMH lamp, which was a little surprising. Next up was the Nanolux MaxPar 4200K CMH lamp.

Working down the list, the spread between the lowest and highest UV bulb wasn’t massive. These are also relatively low numbers compared with PAR, so I wouldn’t call 315W CMH a “max UV” solution—but you do get some UV. If your priority is the most UV within this group, our data points toward that Ushio.

PAR footprints: real-world distribution across the canopy

Fixture used for footprint readings

We built our PAR footprints using a Sun System 315W LEC fixture on 120V. One thing to note: the pebbled aluminum inside the fixture can scatter light and push it in different directions—so we don’t treat a single center number as “the truth.”

Why the 5x5 footprint matters most

We took readings across multiple footprint sizes—1x1, 2x2, 3x3, 4x4, and 5x5—then summed them to get a more complete performance story. If you’re testing lights at a lab level, you can measure thousands of points with serious instrumentation. We’re not doing that here (we’re closer to a few dozen points), so totals across larger footprints are the best way to avoid getting fooled by a single hot spot.

Also, higher arc-tube temperature and reflector design can influence perceived brightness and distribution—so the footprint totals are what we leaned on to compare bulbs fairly.

The highest center reading we saw across any bulb was the Philips MasterColor CDM-T Elite 315W CMH Agro lamp (3100K), and it was consistently strong across most of the footprint reads.

Conclusions: best 315W CMH bulbs by output (PAR)

Best overall PAR (based on 5x5 total)

This is the key result: the sum of all PAR points in the 5x5 footprint is the number I want growers paying attention to most, because it uses the largest sample size and reflects overall coverage better than any single reading.

From the 5x5 totals, the Philips MasterColor 3100K CMH lamp looked like the best overall 315W CMH bulb in our test set. It posted the highest PAR total across measurements.

Best 4200K performer

In the 4200K category, the Philips CDM Elite 315W bulb (4200K) performed the best in our lineup.

How big were the differences?

We saw the Philips ceramic metal halide lamps come in at roughly ~8% higher than the next best bulb, which was the SunPulse CMH 3200K bulb. Within the 4200K group, the Philips CDM Elite 315W (4200K) was just under ~5% ahead of the Nanolux MaxPar 4200K.

What this means for hydroponic growers

Practical takeaways

• If you want the highest overall PAR in this test: Philips MasterColor 3100K

• If you want the top 4200K output: Philips CDM Elite 315W (4200K)

• If you’re prioritizing relative UV within 315W CMH: Ushio 3000K CMH came out on top in our 250–400 nm comparison.

• 3000K vs 4200K spectrum: 3000K tends warmer and more HPS-like to the eye; 4200K tends broader and whiter with a more “blue” feel.

Final note

If you’re picking a 315W CMH grow light bulb, most ballasts in this wattage class are relatively similar—so the bulb choice is where you’ll see measurable differences. Use these results based on what matters to your garden: maximum PAR, spectrum character, or the best relative UV inside the 315W CMH category.

If you have questions, give us a call or send an email. This is Nate from Growers House—happy growing.