Originally Posted by
MadJohn
I've done some reading - others with more grow experience may disagree.
There's a number of factors that drive how you have your lights.
1. Light PAR capability
2. Length of time the light is on
3. The size of the space you're lighting
4. Your light spread
5. The distance the light is from the camopy
6. The plants
So it's all a balance between all of those.
Moving the Light
As you raise your light, the inverse square law kicks in and your PAR decreases so you need the light stonger or the light on for longer so your plants get enough light. If you can't provide enough light this causes stretching and can cause "airy" bugs during flower.
If you lower the light your PAR goes up provided you keep the light output the same. This can lead to too much light and you get light burn (very very easy with LED's)
Changing the PAR of the light
Exactly the same as moving the light, the higher the PAR, the further away you can have the light but high PAR runs the risk of light burn. Lower the PAR the closer you can have it without risking burn
Spread of Light
So every light and every manufacture publish PAR charts that map out the PAR ratings in the typical use case of the light in question. So if the light is sold as a 2*2 light, you'll generally get a chart that shows PAR testing in a 2*2 space. With a perfect light, you'd get an absolutely perfectly uniform PAR measurement for the entire area. This means all parts of the plant are getting exactly the same PAR. Very few lights provide this and generally, the cheaper the light, the more inconsistent the light uniformity is. Shane over at Mygro has a fantastic set of light comparisons where he's measured the PAR and shows how good/bad certain lights PAR uniformity is. That said, even though lights like the Mars TS1000 don't have a very uniform canopy, they are cheap and still grow some fantastic bud for many 1000's of people.
The spread is also affected by the height of the light. Most lights have a 50 or 60 degree wide beam so ids the same is large and the light is low, it's not going to get out very far.
Which leads to...
Size of Space
The smaller the light, the tighter the spread and the less light you'll get at the edge. You can run a 150W light in a 4*4 and right in the middle 1*1m you'll get great light, but at the tent walls you'll get very little light at all. So your light needs to have enough PAR and enough spread to ensure it reaches out to the edges of your space. The more uniformly, the better.
Lastly
The plants
Every plant has a different capacity to deal with light and it also depends not only on the strain, but also the stage it's at in the grow.
DLI
The preferred way to measure lighting these days is DLI - Daily Light Integral. This is a number that is derived from the reading of the PAR at the canopy to and the length of time you have the light on. The "ideal" DLI changes through the grow (roughly 10-15 seedling, 25-40 veg, 35-45 flower) but it's VERY dependant on the plant as to how much light they can take. This should always be your preferred way of measuring your lights for your grow because it factors in how long you have light on for.
Bringin it all together
Everything is a balance. All that's really important is that your plants have enough light for the stage of grow they're at. I've not read anything about whether it matters whether you change the height or the intensity (dimming) accepting that your lights are appropriately sized for the space you have. I prefer to have the lights closer and the intensity lower as that saves me money. Depending on all the other factors YMMV though.
MJ
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