I’ve grown in a bunch of environments and with a variety of lighting, but I’ve actually got limited experience running LED for anything over a few plants. I’d love to see what everyone’s take on differences in growing from LED to HPS to MH to double ended etc.
Here’s a study on the effects of LED spectrums on cannabis/hemp as an example. It showed a reduction in leaf size and height if your spectrum skews too blue/red and not enough full spectrum.
In my opinion LEDs have the highest variability between manufacturers and generations, as they compare to PAR spectrum and efficiency. Higher prices often lead to ideal mixes of spectrum. Penetration is medium to medium low depending on LED type.
HPS I tend to think of as the least variable type with even output of spectrum, focusing on the far red side, although not necessarily efficient. Good overall penetration, depending on bulb wattage, but usually higher than LEDs controlling for distance/heat tolerance. Used to help penetrate thick layers of foliage/canopy in flowering.
Metal Halides I think of quite similarly to the HPS, although favouring a fuller spectrum with a lot more blue light (and unusable green). These tend to help a vegetating plant keep internode space shortest, and thusly are favoured before flowering.
Double ended lights help produce a more concentrated light pattern on your grow space. They are also used to decrease distance to a canopy of said lights. 2x600W HPS can be set closer to the canopy than a single 1000w lamp. This increases light intensity, and as 600w are the most efficient HPS, I hypothesize that using double ended HPS is the most efficient way to produce the highest light intensity over even coverage during flowering.
Reflectors, plant training and orientation (like vertical grows) all manipulate your bulb to plant interface. A reflector will set your pattern on the floor (or walls/ceiling) around it. Larger reflectors spread the light wider, covering a larger area with less intense light. Where it gets complicated is the angles of the reflector, whether a glass tube encases the bulb and even the texture of the reflector all impact light dispersal. For large even coverage, you have to be aware of the pattern your light(s) reflectors disperse.
Training plants vertical, on a horizontal screened plane or in an arena/stadium type setup allows your plant canopy to match the light, instead of the other way around. Maximizing the canopy density at this point ensures that the most light that reaches the plant is absorbed as possible without being reflected. Very little light should reach through the canopies to the floor, pots or walls. They also place the plants equidistant from the light source allowing it to be as close as possible.
Vertical grows train plants all around a vertically placed bulb, utilizing 100% radiated light efficiency, with decreased intensity at any one point.
No love for CMH