Hid, cmh, led...wth?!?!


Have you ever walked into the grow store and listened to the conversation going on at the counter? Did you find the banter between staff and some of the seasoned growers a little intimidating? Maybe the conversation sounded something like this:

GROWER: “I grew some killer flowers with these new CMH lights, bro! What do you think about the new 630 DE’s?”
HYDRO STORE EMPLOYEE IN THE COOL HAT: (always the expert, of course :wink:) “Dude, I switched to LED two runs ago and never looked back! I’m sold! You really ought to consider an upgrade, those 630’s run hella hot after all. What Kelvin spectrum are you rockin’?”

Did you run to the back of the store and cower with fear behind a pallet of soil? Did you reconsider every decision you’ve ever made in your life leading up to this very point?

It’s all good, because we are going to clear up some of the confusion with a great discussion about all the lighting options you have available to you.

HID (HPS & MH): Tried and true. These lamps deliver intense light needed to grow great herb. The downside is they draw a lot of power and can get very HOT. Available in DE and SE
CMH/LEC: Lower wattage with deep light penetration. Careful, they can get hot. Available in DE and SE
LED: Newer tech incorporating diodes to illuminate your garden. Can run cool. Many lamps have tubable light spectrum as well, allowing for greater control over your garden. Downside is these lights can be very expensive.
FLUORESCENT: Tried and true. These lights are cheap and easy to install. They rely on T5 bulbs which are easy to replace and come in a variety of spectra. Downside is the light penetration is minimal so they have to be hung very close to the canopy and the plants have to remain short. You will get smaller harvests than the lights listed here.

Whatever your choice, finding the correct light for your grow space is critical. You don’t want a light that runs too hot for your space or doesn’t supply sufficient light for your plants’ needs.

What lights are you considering and why? What issues have you run into with lights in the past? How did you overcome? Let’s have an illuminating discussion!


I think it’s well enough documented that you can grow good weed with any of them. All you need to do is have a good enough understanding of plant requirements, and what’s needed from each type of lighting to meet them. It can be very easy or quite complicated depending on what you want to take away from it. Breaking lights down to hps, cmh, led, or fluorescent is like saying cars, trucks, motorcycles, and boats. With exception of ceramic metal halide, there are several different types of each.

When discussing high pressure sodium lights, there are some pretty big differences in the efficiency of them depending on size and make. 400 watt and smaller usually have efficacy of around 1 umol/joule. 600-1000 watt hps lights seem to test out around 1.3 umol/joule, and the big cat de gavita 1.7-1.9 umol/joule depending on whether tested 400-700nm or 350-800nm. So a de gavita has twice the photosynthetic photon flux of something like a 400 watt hps on a per watt basis.

Ceramic metal halide doesn’t really have the gaps that hps does, and also has the 1.7-2.0 umol/joule efficacy that the big de fixture has. At least the Phillips bulb does, which was best I remember seeing. There’s not quite the amount of manufacturers for cmh, so pretty much is what it is in my opinion. You just choose the platform you want in 315, dual 315, or 630 performance is very similar between all of them to my knowledge.

Led’s are the most complicated. There’s so many different choices, theories, and a lot of bs in their advertising. There’s third party test data showing some very expensive fixtures performing at about the same efficacy as standard 600-1000 watt hps lights. There’s also independent third party sphere data showing several of them at 2.5 umol/joule and beyond, which absolutely crushes the de gavita and all of the cmh. I’d like to say you get what you pay for, but there’s some $2000+ fixtures that are garbage as far as performance goes. You really have to look for those independent tests and study the integrated sphere data to know what you’re getting.

Fluorescent is about the same as hps or leds. You have t5, t8, t12, compact fluorescent, and a few othe configurations too. I’m not familiar any of the photon efficacy data on fluorescent, mostly because I don’t think anyone cares enough to test them. But luminous flux is readily available on all of them and pretty consistent across each type. T5’s are most efficient coming in around 90ish lumens per watt. Cfl’s are usually in the 60-70 lumens per watt range, and t8 and t12 fixtures down from there.

All any of this means is that it takes or less power from each type of lighting to hit recommended daily light integral. If you hit recommended par levels to maintain target ppfd average within your space, there’s not a lot of difference in grows from one type of lighting to another. Despite what several light manufacturers claim, the emitted light spectrum doesn’t seem to change much within the grows when par levels are the same. Some subtle differences for sure, but very little in terms of weight and quality.

When shopping for a light, limit your choices to a fixture that you can find the total light output on. This is shown in ppf or total flux, not to be confused with a single point reading from directly below the fixture (shown in ppfd). Once you have the ppf, you can divide by space in m² to find your photosynthetic photon flux density average when run at proper height. Your target here should be about 600-800 umols/s/m² on ambient co2, the 800 umols/s/m² is considered point of diminishing returns in most spaces. If co2 is supplied you can obviously run higher intensity levels, usually exceeding ppfd average of 1000 umols/s.

Avoid gimmicks like led panel that has only red and blue diodes can replace 1000 watt hps fixture with 200 watt light, total bs. Even the most efficient leds in the world aren’t a 5-1 replacement. 2-3 to is possible, but get your checkbook out, because you won’t get that on Amazon for couple hundred bucks. Cmh has had tests where there was a 3-5% increase in thc. But a 3-5% increase is moving from 25% thc to 26% thc, not from 25% to 28-30% like most people seem to think. Moving from 25% thc to 30% thc is a 20% increase or 1/5, but most don’t think about that it much. I’ve yet to meet anyone that could tell me the difference between weed that tested at 20% vs 25%, so I’m sure 25 to 26% isn’t enough to justify running out for a new light.

Spend your money getting light intensity levels proper, and the rest is fine tuning from there.