I’m pretty sure I’ve been keeping my lights way too close all these years…I noticed a GSC that I did outside and inside looks way better outside and the buds are actually more dense too.
I found this chart online, does it seem accurate? I know foot candles aren’t necessarily the best measure of light for plants, but it could still be useful as a general reference.
From what I read direct sunlight is about 10,000 FC so I have definitely been overblasting them lol.
Any insight you smart(er) people have into this would be very useful, and if anyone has a similar reference chart that might be more accurate that would be awesome too. Also, I’d love to see numbers like this for different LEDs including SILs.
Some plants are light hungry some are not,
I’ve found og’s to be light hungry, and cookies to be not so much… they always do better around the edges of my gardens.
Came here for the science on indoor lights vs sunlight and now I’m left pondering if there really isn’t a way to at least approximate optimal light distance without a par meter. I know lux can be helpful to a degree. It doesn’t help when you want to calculate invisible light efficiency I’m pretty sure though.
A thought though is to get a extremely sensitive plant to act as a light meter of sorts in the tent. Something that shows a strong response when light levels change slightly. A quick and dirty method I guess could be starting seeds at regular intervals and observing how much they stretch or don’t.
For example in my tent the lights are way too close if a seedling is below any lights directly. All 3 even the bigger one. All of them showed a very strong response once I moved them to indirect light.
I’ve compared lux readings from an HPS 12" away against a bright sunny afternoon and the sun was a good 15-20% brighter than the HPS.
Obviously the suns intensity drops the closer to the horizon it gets due to the amount of photon-blocking atmosphere so theres that to consider as well. That same atmosphere also diffuses and scatters photons everywhere which provides better “side lighting” to plants.
And not that I condone such experiments, but it’s much easier to look directly into a grow light than directly into the sun.
All I know is I run about the most potent light you can indoors…the EXACT SAME PLANTS outside, will make my BEST inside ones…look anemic! They are 2-3 times the size, buds much larger, and more stick n smell too.
My light , regardless of what any meter says…is no match at all for the Sun.
@Jetdro I’ve definitely noticed that too, I always thought it was more due to the perfect complete spectrum of the sun vs our crappy knockoffs. Maybe it is also intensity too though
I’ve run plants on my balcony in the summer at the same time as having a few inside, all in 2 gallon fabric pots, all the same strain and the ones outside always get bigger than the ones inside under LEDS.
Crazy. According to that chart the hps at that distance should be 4x as intense as the sun, at least in regards to foot candles. I would have guessed the hps would be way brighter at that distance too.
All the different scales of measuring light intensity are so confusing…
Here is a spectrum capture of the sun in the Northeast in, ummm, June I think:
The captured energy in the PAR range was around 1580 umol / m^2 s.
A Fluence VyprX Plus at 18 inches measured around 997.1 umol / m^2 s at 18 inches.
The cool thing is, the sun is really really far away. Around 150,000,000 km.
This means that the intensity of the sun light is not going to vary much at the scale of a plant or trees. Whether we make a measurement at the top of the plant or at the bottom of the plant, it will receive practically the same 1580 umol / m^2 s on the unshaded portions.
This is unlike the lighting fixture which will show a great deal of variation depending on the distance to the fixture. Moving from 18 inches to 36 inches away from the fixture will produce 1/4 the intensity. For 1000 umol / m^2 s, we’d see 250 umol / m^2 s.
In the case of the sun, we are already at 150,000,000 km. We’d have to be at 300,000,000 km to see the intensity drop by 1/4 (under similar atmospheric conditions). A couple of feet does nothing much of anything to the intensity. That’s kinda cool.
The standard is umol/m^2 per sec. umol essentially means the number of photons in this context. For this it is usually for a square meter per second but it could also be mols/m^2 day, e.g. DLI (daily light integral).
You’ll typically see something like a range attached to the measurement such as 400-700nm that denotes the frequency ranges measured (color). Or, they may just say PAR. 400-700nm is the standard PAR ranges, though the values can vary to account for UV or far red, for instance.
PAR is still an evolving measure imho as more is learned about how the different wavelengths and ratios initiate and support metabolic processes within the plant. This is obviously not unique to what we are doing… Some of the most fanatical people when it comes to lights are tropical fish keepers… I have seen the craziest controlled light setups that will vary the ratio of wavelengths over the daylight period… so deep reds in the morning and afternoon and a gradual transition to more complete spectrum towards mid day. I have read various reports of the benefits of far red for a period at lights on and lights off, but have not experimented myself…
This simplest way to look at PAR is to take a step back and look at the relative spectral distribution graph… here is an example from CREE.
So basically PAR in simplistic terms is little more than am upper and lower bracket on such a chart that indicates the spectrum range that is utilized by the plant for photosynthesis.
Here is one I ripped off the interwebs… note the claim that green is “unusable by plants” … this I would suggest is a controversial claim, hence the question marks…
The difference between the quality of lights and the sun is more complex than simply intensity… from memory I think the maximum intensity of the sun is about 120, 000 lux… this is supposedly as bright as it gets at midday in the middle of summer in the desert… It’s easy to produce more ligth that this indoors in a square meter, though it’s probably not going to gain you a lot and also can cause a whole lot of other issues… such as burn and bleaching and heat etc…
One common measurement to represent the accuracy of the light spectrum relative to sun is CRI… or the colour rendering index… loosely speaking the closer it is to 100 the closer it is to the spectrum output by the sun…
One informal measurement you will hear get thrown around a lot is light penetration… which is a bit vague… What it refers to is “Radiant Intensity” and is typically measured in watt per steradian (W/sr), the relative intensity of the spectral wavelengths within the light source is the watt per steradian per metre (W·sr−1·m−1) or watt per steradian per nanometre (W·sr−1·nm−1). I have never seen these measurements displayed on a commercial grow light however… and again the impact of high point intensity vs diffuse light on plant response is more complex that any simple measurement…
could be spectra 
