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Lanmark

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I am hoping someone who understands lighting better than I do can help me with what should be some rather simple calculations. I could use a light meter except for the fact I don't have the actual LEDs in my possession to test them. I am trying to decide which lamps to buy based on these calculations. I have already taken into account photosynthetically active radiation. All I am looking for here are the illuminance numbers. All numbers I give are assuming that the lamps' given specifications are fairly accurate.

I am trying to figure out how many LUX or footcandles -- either number will work for me since I understand how to convert one to the other -- will be reaching my plants' leaves. Just be sure to specify what specification you are giving -- LUX or footcandles.

First scenario: 1240 lumens (luminous flux) with a 40° spread with the distance between the light source and the top surface of the plant leaves at 28 inches (.7112 metres) centered directly under the lamp.

Second scenario: 920 lumens (luminous flux) with a 40° spread with the distance between the light source and the top surface of the plant leaves at 28 inches (.7112 metres) centered directly under the lamp.

I hope there's someone here who can figure this out. I should be able to do it, but I'm having a brain fart today. :p I'll keep working on it in the meantime. It shouldn't be that difficult.
 

naoki

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The calculator in the first web page is probably assuming that the light is distributed evenly within the beam angle. I get the similar number from my calculation under this assumption, but slightly different number. They may have some error or they might be making some different assumption.

The coverage area should be (using metric):

pi * (tan(pi * 29/180) * 0.7112)^2 = 0.2105063 m^2

Since 1 lux = 1 lumen/m^2, 1240/0.2105063 gives 5890.56 lux (547fc).

One thing to note is that LED doesn't have the homogeneous spread. Also, when they define beam angle, quite a bit of light spills outside of the beam angle. I think they use the angle at which output becomes 50% of the maximum (in the middle). Maybe the calculator is considering these factors. But I think you get the ball-park number from your calculation.
 

MorandiWine

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Why focus on lumens when that is such a narrow band of light within the larger band of light that plants use? Wouldn't PAR be the better measurement to determine what would be best to grow?


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Lanmark

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Thanks, naoki :)

Indeed, MorandiWine, PAR measurements would more accurately determine the quality of light being provided by the lamp for plant use. I was hoping, however, to keep it more simple by finding a lamp or pair of lamps with the proper Kelvin color output and with sufficient light output to successfully grow one very compact plant in a dark corner without either bleaching it or starving it for light.

I've been successfully growing a wide variety of orchids under LED lamps for several years now, but there has definitely been a learning curve for me in the process. Mostly I have used the educated guess/trial and error method up until now to find which LED lamps at which distances would grow healthy plants. More recently I have been trying to learn more about lighting so I can make smarter lamp choices in advance and rely less on trial and error.
 

naoki

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I agree, MorandiWine, and I wish all light specified the PPF values! I understand why most white LEDs don't come with this specification, but it is super lame that lots of "grow light" makers don't publish this specification. I know that the special equipment (integrative sphere) to measure the total PPF is super expensive, but a serious grow light maker should be able to test their products!

If they publish the emission spectrum (e.g. spectral power distribution; SPD), we can calculate the conversion factor, though. We can even convert the lumen or fc to yield photon flux (which puts higher weight to the more useful red light than typical PPF), too!

With white LEDs, we roughly know how to covert lumen/lux to PPF/PPFD. So lumen data can give some rough estimate of plant efficiency.
 

naoki

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I've been successfully growing a wide variety of orchids under LED lamps for several years now, but there has definitely been a learning curve for me in the process. Mostly I have used the educated guess/trial and error method up until now to find which LED lamps at which distances would grow healthy plants. More recently I have been trying to learn more about lighting so I can make smarter lamp choices in advance and rely less on trial and error.
Yes, with the purple grow light, it is difficult at first to judge the distance. I started to write about how to use fc/lux meter to get the rough estimate of PPFD, but I haven't had time recently...
 

Lanmark

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Yes, with the purple grow light, it is difficult at first to judge the distance. I started to write about how to use fc/lux meter to get the rough estimate of PPFD, but I haven't had time recently...
The lamps I was asking about in this particular thread aren't the purple (red + blue) grow lights, but rather they are natural white cob LED lamps with lenses to focus the center beam at 40°. They have a very smooth light dispersion pattern. They are 14W Par38 4000K lamps with a CRI of 90+ and an R9 value of >50 ...so the reds are enhanced. I'm wondering if I might need something with more blue values in it.

I have found in my own experience that there is something about these types of directed/focused LED beams of light which seem to penetrate and illuminate my plants much better than expected. Sometimes lamps which I expected to be too dim ended up turning my leaves purple and I had to either increase the beam angles or the distances between the lamps and the plants.
 

naoki

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Oh, I see. I thought that it was difficult to gauge the distance because of the purple light.

There are some variation, but 4000K CRI 90 seems like a good spectrum. There are theoretical aspects about the effects of light spectrum on plant morphology. But I do wonder what difference it makes for orchids in reality. With LED, I just go with anything 3000-5000K. 3000K has more red, but I haven't seen any problems.

If there is a choice, you might want to consider lower CRI. High CRI is nice for human eyes, but plants don't care, and you usually get a little more efficiency with lower CRI LEDs.

If you are comparing the lumen values of fluorescent vs LEDs (to draw your expectation), there are two things which might be helpful. With fluorescent bulbs, if it says 5000lumen, the actual lumen value is usually lower than that. Different ballasts have different efficiencies (ballast factor), and most of them are probably around 90%. So actual lumen becomes 5000 * 0.9 = 4500. Also, since florescent bulbs emit to 360 degrees, there are loss due to reflection.

If you are judging the amount of light by eyes, could the different light color be the cause of the unexpectedly strong light from LED? Generally, higher K florescent or LED (such as 6500K) looks brighter than 3000K or 4000K LEDs to human eyes.

With the beam angle and distance, there are two interesting aspects to think about.

1. variation in light density. Let's say the plant is 30cm tall. If you use narrow beam and place it far away (say 2m away from the top), then the top leaves are 2m away and the bottom leaves are 2.3m away. So the difference in the light density is not so different. But if you use a wide angle beam, and place it 30cm away from the top leaf, then the bottom leaves are 2x longer distance from the light. This means that the bottom leaves receive 1/4 of the light compared to the top leaves.

2. shadowing effects. If the light source is far away with narrow beam pattern, the top leaves can put shadow, and lower leaves can't receive enough light. If you place the light close to the plant, The light can illuminate the lower leaves of the neighboring plants. Basically it is more similar to diffused light (one big advantage of linear light like fluorescent light).
 

Ray

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1. variation in light density. Let's say the plant is 30cm tall. If you use narrow beam and place it far away (say 2m away from the top), then the top leaves are 2m away and the bottom leaves are 2.3m away. So the difference in the light density is not so different. But if you use a wide angle beam, and place it 30cm away from the top leaf, then the bottom leaves are 2x longer distance from the light. This means that the bottom leaves receive 1/4 of the light compared to the top leaves.
Naoki, please correct me if I'm wrong but in the first case, the lower leaves, being 15% farther from the lamp are going to get about 25% less light, which I would hardly say is "not so different", and beam spread aside, for the same lamp intensity, the lower leaves of the plant that's 30 cm from the light are going to get a helluva lot more light than the one that's 200 cm away!

In fact, I would think that an array of the wider spread chips would be an advantage, helping illuminate the foliage that might have been shadowed more by near vertical, narrow beams.
 

Lanmark

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Beam angle/spread is indeed important to take into consideration, and I always factor it and the plant's structure and form into my calculations. Most of the time I like to mount my lights further away from my plants -- it gives me plenty of headroom to work with them -- hence my usual choice of Par38 lamps using track lighting with sockets which can swivel and be angled.

Sometimes I find it better to mount multiple lamps, angling them in from above and somewhat off to the sides for better penetration of the leaf canopy, rather than using a single lamp directly overhead.

Given my space restrictions, linear lighting is simply not an option for me, although I can't deny its practicality and usefulness for many growers. Also, I often grow plants which require far more light intensity than can be provided by most linear systems. This is where focused-beam LEDS really come in handy for me.

The focused-beam LEDS are also very good for lighting a single small plant from a distance and doing so within the confines of a darker living or office space without creating a lot of light pollution throughout the remainder of the space.

In some situations, Ray's LED vivarium lamps have proven themselves to be indispensable. I already have three of them, and I'm pretty sure I will be getting more in the future.
 

naoki

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Naoki, please correct me if I'm wrong but in the first case, the lower leaves, being 15% farther from the lamp are going to get about 25% less light, which I would hardly say is "not so different", and beam spread aside, for the same lamp intensity, the lower leaves of the plant that's 30 cm from the light are going to get a helluva lot more light than the one that's 200 cm away!

In fact, I would think that an array of the wider spread chips would be an advantage, helping illuminate the foliage that might have been shadowed more by near vertical, narrow beams.
Well, you are right; 25% less is quite a bit less. So there is a limit to this variation reduction argument. But it is better than 50% less.

I wasn't clear, but I was thinking very narrow beam, so the coverage area becomes same. Then the light density at the top leaf is the same. Here is more concrete example. We can make a simplifying assumption that the light falls within the beam angle homogeneously. Then let's assume that the low mount bulb (30cm from the top of the leaf) has 90 degree beam angle. Then the diameter of coverage area is 60cm. If the high mounting bulb (2m) has the beam angle of 17 degree, the diameter of the coverage area becomes same. I think that it is 2 * arctan ( 0.3 m/ 2 m) * 180 / pi, where 0.3cm is the radius of the coverage area, and 2m is the distance between the top leaf and the light.

But in practice, I also like the closer distance with more diffused light as long as the light doesn't leak too much outside of the grow area (e.g. by surrounding them with reflective materials).

As a side note, this principle is used for photography, too. As you put the light source closer to the subject, you can get more dramatic effect (e.g. right side of the flower or face bright, but the other side sink to the shadow etc).
 

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