Lumens don't add. The reason the t-5 setups your looking at say 20,000 lumens at 5,000 lumens per bulb is because the 5,000 lumens is spread out over a larger area. If you took those four bulbs and put them next to each other it wouldn't make the area directly below any brighter. Multiple bulbs allows you to cover a larger area more effectively. If you want brighter light though you will need more powerful bulbs or better reflectors.
I think this principal applies to PAR as well but I'm not entirely sure.
This is wrong. And really, it's a glaringly obvious contradiction. How will a reflector improve anything but more light emitting sources themselves won't? Ugh!
Don't you just love it!
Of the six, definitive, "Limiting Factors" light is (without a doubt) the most complicated and hardest to understand. I think that's why most people just "accept" a few "basics" and pretend to understand the rest. Yes, what you heard is true - "Lumens are for Humans" and they're a piss poor measurement for a grow light. However - - - -
Lumens measure the appearant brightness of a light source as percieved by the human eye. Our eyes are most sensitive to green light, so the green portion of the light spectrum produces a high ratio of "lumens". The problem with lumens is that plants, in general, don't absorb green light they reflect it - thats why they're green in appearance. Plants actually use more red light and blue light, which both produce a lower ratio of lumens. Now we get to PAR light:
In the "Illumination" business lumens are important because they can indicate how bright a lamp is (even though it puts a premium on green light). PAR stands for "Photosynthetically Active Radiation, it is only interrested in those parts of the light spectrum that a plant actually uses. So, basically, you can think of green light as non-PAR light - as opposed to LED grow lights which are virtually 100% PAR lights!
Like BigBudBalls said "They do and they don't" (add up).
"They do" in the sense that 2 identicle lights will allow you to grow twice as big of an area.
And now for some nonsense:
"They don't" in the sense that 2 identicle lights wont produce twice the intensity, they will just give you "more of the same". Think of it this way - if you have a quart of water @ 106 degrees f. and you add another quart @ 106 degrees f., you don't end up with boiling water! You just end up with twice as much water at the same 106 degree f. temperature (intensity). OK?
That analogy is horrible, and does not make any damn sense! Intensity can very well increase, but adding another bulb is not going to magically make the original bulb brighter.
Big Bud Balls also said "Light diminishes further it travels". This can't be stressed enough!!! Most growers (well, at least, some growers) generalize this to mean "Keep the light "close" to the plants, if you can, sort of". The harsh reality is "The Inverse Square Law (of light)", this is pure Psysics and it's right out of the book, it goes something like this: As the distance between the light source and the plant doubles, the energy of that light is reduced by a factor of 4 (double the distance equals one fourth the light energy!). Think about that for a moment! That is a huge drop in the amount of light your plants will receive because the formula starts at a rediculousely small distance.
He was doing so well.... Sigh. But apparently he thinks(according to the analogy about water temperature) that adding more bulbs won't increase heat what so ever? Shocking! it's a bad analogy anyway -moving on.
Lumens do add. That's what's called total lumens. Lumen output is a measure of a bulb. A bulb is the same amount of lumens whether next to another bulb, or it's on another planet all by itself
. Distance doesn't mean anything, more or less bulbs don't change this. A bulb's lumen rating is the bulb's lumen rating no matter what.
Lux, lumens per sq meter, will change depending on how many bulbs are used. This is obvious. If you take multiple flash lights, shine them into one spot, it's obviously a brighter spot than one of them alone. Does this make any of the flash lights brighter? Of course not! Take a mirror or something else highly reflective, shine your room's light(I assume your room has a light bulb, otherwise use the sun) rays onto the wall, there's a brighter spot, and it moves around the room according to the angle of the reflective material relative to the light source. Ever burn something with a magnifying glass? Again, focusing light, MULTIPLYING it several fold.
Ok, we've proven light can be additive/multiplicative. Moving on...
We're talking about WHERE LIGHT IS STRIKING, not WHERE LIGHT IS COMING FROM. Does the magnifying glass make the sun magically brighter? No, it does no, only SELECT STRIKING rays emanating from it(that hit the magnifying glass, of course!) -not the sun itself. Where multiple sources are all STRIKING becomes brighter, it doesn't 'travel backwards' to the source and magically make the source brighter -that's ridiculous! But this is apparently the argument used to claim light is not additive.
Lux is a more complex notion than lumens. But basically, two equal light sources, equidistant from the striking point(and equal/opposite incident angles) your lux would double. Does this make either of the bulbs brighter? No, of course not -ridiculous. Because at 1 foot away, the bulb is already 16 times less intense than it is at 3 inches(approximately). And at 3 inches it's 16 times less intense than it'll be at 0.75 inches(approximately). However, having two bulbs, equidistant and equiangular from these 'test points', these measurements would be very near doubled(especially the further away).
Most bulbs aren't considered true point sources, so the mileage will vary. Moving on.
Nothing is going to make the bulb any brighter. It's not a matter of WHERE LIGHT IS COMING FROM, but WHERE LIGHT IS GOING TO. Are two fluorescent tubes capable of twice the lux? Yes! But not at 1 millimeter away! That's the peak lux zone of the bulb. No amount of bulbs are going to produce a lux beyond that of the bulb itself(measured at the surface of the bulb)! Of course this doesn't matter, do your lights touch the plants? Of course not! Or at least probably not. And by the time it travels from .075 inch to 1 foot, it's now 16*16 times(that's 256 times) less intense, anyways. But with two bulbs, that could be nearly 128 times(half the decay), 4 bulbs, 64 times(nearly 4 times brighter at 1 foot!), which is why fluorescent tubes are often used in 2 or 4 tube fixtures.
What more bulbs will do is increase the volume of usable light, and increase the intensity(density of photons) within that volume, assuming they're all illuminating the same volume.
