Spectral Analysis for LEDs for use over planted aquaria; Graphs here! - Page 2
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Old 05-09-2011, 04:18 AM   #16
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Tom, I don't believe anyone can predict the PAR at all based on those graphs. There is no absolute value assigned to the vertical axis, and, without that, the graphs are just more illustrations of what the manufacturers provide in their specs for each LED. I would infinitely rather see PAR numbers at various distances, both from the LED on center, and at distances from the center, and at 3 different LED currents. With those numbers it is easy to predict the PAR for any current, any number and spacing of the LEDs.

Unfortunately it also requires a set of those numbers for each optic that would be used, a 60 degree and a 40 degree at minimum. I'm really looking forward to seeing that type of data.
Hence all the other variables that mattered added after that was stated
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Old 05-09-2011, 04:40 AM   #17
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Tom, which graph looks more compelling to you, in regards to spectral output. I know this varies from plant to plant, so assume a "general mixed tank" sort of situation.
Just in general, they are better than I expected, less high peaks and skewed spectral radiation.

Or put another way, as far as plants, a good graph if you are "a plant".

The energy(as watts) to PAR to growth rates seems pretty good based on the output. So we ought to get pretty good color, pretty good growth from these.

I'd likely use a mix of bulb type, and then mix and match to suit the user's preferences, aesthetics. Since these aesthetics vary person to person, hard to say. It's personal. I think over time, folks will have a general preference, but till things settle down more with the LED light, that's not going to happen anytime soon. Maybe 1-2 years or so for commercial marketed units at a fair widely available etc.
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Old 05-09-2011, 09:33 PM   #18
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If this isn't sticky it should be.

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Old 05-10-2011, 02:23 AM   #19
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Originally Posted by Hoppy View Post
Tom, I don't believe anyone can predict the PAR at all based on those graphs. There is no absolute value assigned to the vertical axis, and, without that, the graphs are just more illustrations of what the manufacturers provide in their specs for each LED. I would infinitely rather see PAR numbers at various distances, both from the LED on center, and at distances from the center, and at 3 different LED currents. With those numbers it is easy to predict the PAR for any current, any number and spacing of the LEDs.
The vertical axis is dependent on several things, such as the detector, grating, and A/D converter settings. Although it could be considered an arbitrary measurement, it gives a certain amount of information with respect to intensity as a function of wavelength.

I am actually quite familiar with the S2000, although I don't recall Ocean Optics being the manufacturer. This spectrometer used a PDA detector and had a choice of several different gratings depending on the desired application. If one assumes the integration time is adjusted appropriately, we have quite a bit of noise in these measurements. Some of these spectrographs have noise approaching 10% of the maximum intensity value, which is a little concerning. An interesting bit of experimental design info would involve how the light was collimated from the LED module into the spectrometer. This is a fiber-optic input (600um if I recall correctly) and would need to be lined up perfectly (machined fixture perfectly) if an external collimator was not used. Normalizing this data would make it a bit more useful, as it would become instrument-independent.

Different currents/distances and PAR measurements would yield some interesting results, however this assumes the LED performs in a linear manner, which many LEDs do not.
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Old 05-10-2011, 02:46 AM   #20
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Some of these spectrographs have noise approaching 10% of the maximum intensity value, which is a little concerning.
The 10-reading was ambient sunlight from the windows and it was very much indirect--- and resulted in a nominal value of 10 all across the board (no spikes). All artificial lights were turned off. The 10 was consistent across the board, which we verified about 30 times (glancing at it in between LEDs) and never once saw a spike.

The 10 ambient noise is quite consistent in every graph. How is this concerning? I admit a 0 would be perfect but I don't see how it would be necessary since it was consistent across the board.

Quote:
An interesting bit of experimental design info would involve how the light was collimated from the LED module into the spectrometer. This is a fiber-optic input (600um if I recall correctly) and would need to be lined up perfectly (machined fixture perfectly) if an external collimator was not used. Normalizing this data would make it a bit more useful, as it would become instrument-independent.
Once my finals are over for the semester in a few weeks I can upload a pic of the wooden jig I built to hold these steady. While I can't claim any sort of extreme accuracy, the wooden jigs held the LED's quite tight in their arrangement.

The wooden jig was placed under the probe to fit on all 4 sides along a marker-line that was drawn, so there would have been no more than 1/8" difference in the placement of the LEDs from one test to the next. At a 12" distance I see no reason to suspect this would corrupt the data enough to worry about, for our purposes.

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Different currents/distances and PAR measurements would yield some interesting results, however this assumes the LED performs in a linear manner, which many LEDs do not.
Agreed. They definitely drop in efficiency as current increases above their "sweet spot", which seems to be around 350mA for most of the 3-w types.

But taking a couple reference points would help make it predictable enough, I suspect. I did a spectral graph of the XML at 3 different currents and was glad to see how consistent the spectral output stayed.
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Old 05-10-2011, 04:25 AM   #21
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The 10-reading was ambient sunlight from the windows and it was very much indirect--- and resulted in a nominal value of 10 all across the board (no spikes). All artificial lights were turned off. The 10 was consistent across the board, which we verified about 30 times (glancing at it in between LEDs) and never once saw a spike.

The 10 ambient noise is quite consistent in every graph. How is this concerning? I admit a 0 would be perfect but I don't see how it would be necessary since it was consistent across the board.
I suppose a "dark" measurement taken when LED is off could eliminate the ambient sunlight. If it was consistent, I suppose it doesn't really matter. I guess I'm used to seeing maximum intensity around 68000 counts with approximately 100 counts for background noise, but we're talking about a whole different arrangement. Background noise of 10% would indicate a massive problem in our systems.


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Once my finals are over for the semester in a few weeks I can upload a pic of the wooden jig I built to hold these steady. While I can't claim any sort of extreme accuracy, the wooden jigs held the LED's quite tight in their arrangement.

The wooden jig was placed under the probe to fit on all 4 sides along a marker-line that was drawn, so there would have been no more than 1/8" difference in the placement of the LEDs from one test to the next. At a 12" distance I see no reason to suspect this would corrupt the data enough to worry about, for our purposes.
I suppose at 12" it wouldn't really make much difference for this application. I guess I'm too picky here also, as slight misalignments (a few um) cause me major problems.
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Old 05-10-2011, 04:29 AM   #22
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I suppose at 12" it wouldn't really make much difference for this application. I guess I'm too picky here also, as slight misalignments (a few um) cause me major problems.
But I greatly appreciate your criticism. There are always things we can improve on.

If I could afford one of these, I'd buy it. I'd rather have this than a PAR meter since, if one is clever, you could probably produce some sort of way of getting PAR numbers based on the intensity levels given by the machine. Besides, you can find PAR meters all over the place if you live near a larger city.

Then again, I am just an average hobbyist with very NOT-average interests. Most people would do better with a PAR meter if they can afford it.

I look forward to the day we can get them more affordably. I've thought about buying the sensor and hooking it up to my multimeter, which should be accurate enough (it measures down to 0.0 millivolts) but I can't justify the expense right now.
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Old 05-10-2011, 04:39 AM   #23
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great post
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Old 05-10-2011, 08:55 PM   #24
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But I greatly appreciate your criticism. There are always things we can improve on.

If I could afford one of these, I'd buy it. I'd rather have this than a PAR meter since, if one is clever, you could probably produce some sort of way of getting PAR numbers based on the intensity levels given by the machine. Besides, you can find PAR meters all over the place if you live near a larger city.

Then again, I am just an average hobbyist with very NOT-average interests. Most people would do better with a PAR meter if they can afford it.
Actually, if you still have access to the S2000 you could calibrate the unit to ambient sunlight, then use the spectrometer to collect a power measurement of the LED. If you are using a fiber in the arrangement, the type of fiber chosen will impact the results. Fused silica would be my recommendation if taking this route. Collimation of light is not really necessary, as it will not be indicative of the conditions of LED use within a planted tank.

In terms of a PAR meter... I'd love to play with one as well, but I am limited to spectrometers. One bit of info a spectrometer yields that a PAR meter does not is the distribution of the light across the UV/Vis spectrum. This may help match light sources to the specific needs of the chromophores in the plant, but as said before, different species of plant have slightly different wavelength needs.
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Old 05-10-2011, 09:14 PM   #25
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For those interested (and who haven't already seen it) here is a study of PAR calculations from a MH bulb. Maybe not directly related to this topic, but it has some nice background and gives me a few ideas... although the wife will probably kill me.
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Old 05-10-2011, 09:34 PM   #26
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Even ambient sunlight is highly variable as to the time of day and year, atmospheric conditions, latitude, etc.
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Old 05-10-2011, 09:49 PM   #27
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VEry nice information! Thanks for sharing. Your graphs literally cemented my 100% confidence with LED's for our planted tanks. Excellent job!
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Old 05-11-2011, 12:29 AM   #28
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Actually, if you still have access to the S2000 you could calibrate the unit to ambient sunlight, then use the spectrometer to collect a power measurement of the LED. If you are using a fiber in the arrangement, the type of fiber chosen will impact the results. Fused silica would be my recommendation if taking this route. Collimation of light is not really necessary, as it will not be indicative of the conditions of LED use within a planted tank.

In terms of a PAR meter... I'd love to play with one as well, but I am limited to spectrometers. One bit of info a spectrometer yields that a PAR meter does not is the distribution of the light across the UV/Vis spectrum. This may help match light sources to the specific needs of the chromophores in the plant, but as said before, different species of plant have slightly different wavelength needs.
I don't have access to it, at least not easily. My friend Joe has taken a job with SeCor and will be leaving soon and he's the only access to a spectrometer I have.

PAR meter is a different story, lots of those around. I just need a good benchtop power supply to keep the numbers dead constant from one LED to the next.
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Old 05-11-2011, 12:30 AM   #29
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VEry nice information! Thanks for sharing. Your graphs literally cemented my 100% confidence with LED's for our planted tanks. Excellent job!
Thank you, and glad I could be of help.

LEDs make good reading lamps too , the neutral whites are very soft on the eyes. Warm whites are too orange for my eyes.
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Old 05-22-2011, 06:36 AM   #30
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Excellent post! Thank you.
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