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  Topic Review (Newest First)
07-13-2011 12:04 AM
Hoppy I updated the graph, with my Ebay LED light data. For cone angle I used my data from a single LED, using the point where the PAR drops to half of the peak value. That gave me an 80 degree cone angle. The two points for that light fall pretty close to the same line.
07-12-2011 12:42 PM
redfishsc Now it does make sense.

The Ebay LEDs will probably have a 140 degree cone angle compared to the 120 for the XML and the 90 for the XRE.

Honestly the XREs, even though they are slightly older technology and slightly less efficient (not by a huge amount though) are actually more useful for us right out of the box since the primary optic is so tight.


When you put a PAR meter up to an XRE and to an XPG at the same drive current, the XRE will give a surprisingly higher PAR reading because the optic is tighter.... even though the XPG is actually brighter. It's just scattered more.


Since most of us won't need 60 degree optics (the widest optic commonly available for XP series), the included 90 of the XRE makes it a pretty good choice for tanks up to about 20" deep.
07-12-2011 04:45 AM
Hoppy The two angles I assigned were done that way to get the data to follow the equation. I could have worked out a different way, but I wasn't that sure the cone angle for the bare LED was determined the same way as with the optics. And, by then I was getting tired of trying to find another rational way to reconcile the data. Every other method I tried failed. The angle I'm looking for is the half angle of the cone, 30 degrees for a 60 degree cone, for example. Then, since most of the light is in the central half of that angle, I'm using 1/4 of the cone angle in the equation. That, when squared is proportional to the area of most intense light from the LED. The correlation doesn't work nearly as well using 1/2 or one times the cone angle. (The tangent goes negative with angles larger than 90 degrees.)

If you look at the Cree data in their pdf, the shape of the light intensity vs angle is considerably different for the XM-L and XR-E. And, I have no idea what the shape with the optics looks like, but I understand that it is best to work with only the inner half of the angle.

I may try to see what it takes to fit the data I have from Ebay cheap 3 watt LEDs to that equation, just to get a better feel for whether or not the equation will work with other LEDs.

One problem with trying to derive an equation from a bunch of data is the errors in the data. The PAR meter isn't accurate below 10, since it doesn't read in decimals, only whole numbers. And, my current measurements are probably riddled with errors since it was very hard to keep the multimeter probes in the right locations - probably a +/- 10% error at best. Then, when you square things the errors double. And, human errors in writing down data enter into it too.
07-12-2011 02:44 AM
redfishsc
Quote:
Originally Posted by Hoppy View Post
In order to get everything to cluster around the line I had to assign a number for the cone angle for bare LEDs, which seems a bit small, but it isn't completely unreasonable.
When you say "cone angle" for the bare LEDs, I'm not sure what you're referring to.

At first I thought you were talking about the primary optic angle (the silicone dome over each LED), but when I saw that you had assigned them a 72 (XML) and 48 (XRE), I realized that you were either giving a half-angle, or a number that I'm just not familiar with.

The XML has a 120 degree primary optic which should give you a 60 degree half angle.... if that is indeed what your numbers represent.
07-12-2011 02:18 AM
Hoppy Here is what I have come up with. In order to get everything to cluster around the line I had to assign a number for the cone angle for bare LEDs, which seems a bit small, but it isn't completely unreasonable.



I think this will work for a variety of LEDs and LED layouts. But, the rows of LEDs will need to be about the same distance apart as the LEDs in a row are. And, I suspect that at large distances from the light, maybe twice the length of the light, the correlation will break down completely. But, aquarium lights are very rarely that far from the substrate.

Anyone else have any luck with this?

EDIT: Added data from my Ebay 3 watt LEDs light, which still follow the curve pretty well. The cone angle of these LEDs I got from my own measurements, using the point where the PAR is down to half of the peak value to get the cone angle.
07-11-2011 03:58 AM
redfishsc
Quote:
Originally Posted by audioaficionado View Post
Do some Excel scatter plots and see if there are some linear correlations.
Not a bad idea, and if I remember anything from calculus, if you can get a scatter plot with a linear pattern, you should be able to find some sort of equation that roughly matches it. Like the "reg eq" function on the old TI85 graphing calcs.
07-11-2011 03:50 AM
audioaficionado Do some Excel scatter plots and see if there are some linear correlations.
07-11-2011 03:01 AM
Hoppy I am thinking that I am cheating a lot of people by working on my own to develop the "Grand Unified Theory" of PAR vs LEDs. I hate to be that stingy. So, here is the data I have taken from LED lights I have made:



Now, those of you who are grieving because you can't participate - please give it your best effort too. Right now I am stalemated on this. I can see lots of hints that a single equation can be developed to predict the PAR from any of these configurations, but it escapes me for now.
07-11-2011 02:58 AM
Hoppy
Quote:
Originally Posted by Robbie1990 View Post
got another question for you hoppy,

I am going to be running 2 rows of xp-g led's on a freshwater setup

each row has 9 led's spread out over a 44" length. Led's will be ran at about 750mA so half power.

Will i be able to use some aluminium flat bar that i have lieng about for the heatsinks for each row or will i have to shell out for heatsinks?

led's are from ledgroupbuy so are 20mm base stars the flatbar is 6mm thick 40mm wide and as i say will be cut to 44" long with 9 leds on. i may also have some 50mm wide stuff
I don't think I would try to use thin flat aluminum bars as heatsinks for that high a current, even though it is half power. In order for the heatsinks to get rid of the heat they need enough surface area to transfer the heat to the air. Finned heatsinks give lots of surface area for that purpose. Aluminum channel extrusions add the surface area of the "legs", and the wider the heatsink the more basic surface area they have.
07-11-2011 01:16 AM
cggorman I can't find an optic with that star pattern from Carclo, Fraen, or L'edil. The overall shape of the lens looks similar to a L'edil Eva-W, but that one has a honeycomb pattern and the holders are all wrong.

Either way, even that bad lenses are still around 75% efficient. The good ones are in the mid 90s.
07-10-2011 10:53 PM
Robbie1990 got another question for you hoppy,

I am going to be running 2 rows of xp-g led's on a freshwater setup

each row has 9 led's spread out over a 44" length. Led's will be ran at about 750mA so half power.

Will i be able to use some aluminium flat bar that i have lieng about for the heatsinks for each row or will i have to shell out for heatsinks?

led's are from ledgroupbuy so are 20mm base stars the flatbar is 6mm thick 40mm wide and as i say will be cut to 44" long with 9 leds on. i may also have some 50mm wide stuff
07-10-2011 05:56 AM
Hoppy I'm using 40 degree optics from ledgroupbuy, which look just like the illustration on their website. Their performance is acceptable, but the method of attaching just seems too crude, and I suspect they lose a lot of light in transmission through them. I would never have gotten the PAR I wanted without using them.

I suppose the best way to look at them is to consider how cheap they are, given what they do. And, they aren't simple lenses either, not compound lenses, but the profile is more complex than I expected.
07-10-2011 05:33 AM
cggorman Hoppy, what lens are you using? I've been pretty happy with each of the three different L'edil lenses I've used. They all drop right on and provide nice even light across the beam.
07-10-2011 05:26 AM
cggorman
Quote:
Originally Posted by audioaficionado View Post
Sounds good to me. Holds well even when hot (unlike hot glue) and easy to remove (unlike epoxy).
Even low-temp hobby-grade hot glue melt around 120 C (250 F). If the stars and heatsink are getting that hot, there is a major design flaw.
07-10-2011 05:07 AM
Hoppy

This is the list of parts and costs for the light, a little higher than it should be, because of the mistakes and need to rebuild the control box twice. But, still not a bad cost for what it is. If I had been able to get XP-G LEDs, and had made fewer mistakes the cost would be even better. It isn't really necessary to use 3 amp capable LEDs, although they do add the abilty to jack up the light intensity a lot, and using the LEDs at a fraction of their maximum power should make them last forever.
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