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Old 02-28-2004, 07:49 PM   #1
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Yaksplt - My Error in LED brightness (mcd) in earlier post

In an earlier post several of us were discusing the possibility of using arrays of LED's to light a complete aquarium. We were looking for ways to overcome a few problems that LED's present 1.) Limited bandwidth, 2.) narrow radiation beam and 3.) Luminous Flux (brightness)

In this thread I erroneously posted that LED brightness is rated in micro-candela when in fact they are rated in milli-candela. For those that care, I wanted to correct this. Yaksplat you were correct in your assumption, it is milli-candela. I've been playing arround with 6000 mcd Blue LED's and yikes - they are pretty bright, an array of 100 (10x10) will provide considerable output, however - still very tightly focused.

I know this will be of particular intrest to Yaksplat and a few others.

Sorry about that.
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Old 02-28-2004, 09:31 PM   #2
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Thanks for the correction
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Old 02-28-2004, 11:06 PM   #3
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I was supposed to get back to you guys with some related information I believe but I had just finished building my computer a few weeks ago and forgot about it...

December 2003 vol 9 num 11 issue of Compound Semiconductor (III-V).

As a note lumens/m^2 is equivalent to candela/m^2 and both measure luminosity.

Watts/m^2 is a measure of radiance (I remember someone had a question about this before but I didn't have a chance to answer).

You can now buy 183,000 mcd LEDs (183 lumen or candela) at digi-key but at a price of $24.63 per unit unless you buy a large number. 60 degree viewing angle though it is green: http://dkc3.digikey.com/pdf/T041/1271-1272.pdf
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Old 02-29-2004, 09:44 AM   #4
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Thanks Patryuji,

Those LEDs at digikey look pretty interesting (the 183,000 mcd units are really arrays as far as I can tell - hence the power and wider view angle). Still, the 60 deg is mighty narrow compared with the output of a MH lamp. Also, it was not specific as to the output at the 60 degrees, i.e. there was no graph showing the units actuall dispersion (i.e. everything beyond 30 degrees might be very minimal).

Also, I'm not sure but seems you have an error in your conversions. I believe A lumen and candela can not be directly converted as they measure two different types of luminosity. However 1 Lumen / m² = 0.092903 Footcandle.

As I recall (and this is going back some time) Candelas are a measure of luminous intensity where Sr (Steradian - three dimensional degrees) must be figured in. So a Candla is a Lumen per Steradian Where as a Lumen is the total quantity of radiated light. Basically they are not both measures of Luminous intensity - one is a measure of Luminous flux and the other a measure of Luminous intensity. For example if you take a 40 watt light bulb (approximatly 500 lumens - lm) and measure it's candelas based on its dispersion let's say about 75 candelas now you physically focus the light produced by the 40 watt bulb into a very narrow intense beam, it's candelas will jump but its lumens will remain the same. I.e. the amount of light the bulb produces will be the same but if focused narrowly enough you will not be able to look directly at the beam - it will be too intense.

This site has many light conversions but you will notice that Lumens and Candela appear in two seperate conversion windows. I have seen the formula you have posted but it only applies (not 100% sure about this) if the Steradians of the light sources are equal - this is rarely the case so it is sort of a rough conversion assuming similar styled radiators.

http://www.onlineconversion.com/light.htm

Anyway - just some additional thoughts.
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Old 02-29-2004, 06:59 PM   #5
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Thanks for the correction

okay, back to my array of 100 12000 mcd led's. That would be roughly 1200 candela. Comparable to a bulb of decent wattage...

I'm thinking that this could be interesting in a long strip... Well, i still have to do some testing. So amybe I'll get to this over the summer. Although this feels more like a winter project.


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Old 03-01-2004, 12:39 AM   #6
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hey mad,
just ordered some 4000mcd blue led's for my moon light project. just a bit curious where you got your 6000mcd at and how much were they ? the 4000mcd are pretty bright, 2 of them make a perfect night light .

last week, i read on some website of a technique that can be used to increase the lighting angle. i haven't tried it yet, doing that tomorrow, so don't blame me for ruining an led.
the process involes using a dremel or low rpm grinding/cutting instrument, and grinding the bulb portion of the led down, to just above the metal parts inside the led. it should only take about 15seks, and after it's finished the light pattern that's emmitted should be a bit wider.

i am trying it tomorrow (too late now to try it) so, i will post the outcome good or bad tomorrow sometime.
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Old 03-01-2004, 12:47 AM   #7
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as long as you don't touch the metal portion of the led it shouldn't be a problem. Look close though, there is a small wire in there. the ones i used in my nightlight were only 2600 mcd and they were plenty. I haven't seen any above 4000. The 4k ones were from lsdiodes.com and the 2600 were from radioshack.


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Old 03-01-2004, 01:28 AM   #8
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i got my 4000mcd from there too, side note: they now offer 12000mcd (12k) white leds for 90cents a peice. i have seen 6000mcd back in college, but back then i was more worried about finishing labs and not experimenting (my priorities were all screwed up..) the Instuctor of the class bought a few hundred online and was giving them to other teachers, and playing with them.. they are very bright, lite up the classroom when the lights were off.
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Old 03-01-2004, 01:29 AM   #9
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My first double post...
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Old 03-01-2004, 07:47 AM   #10
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I bought a few of the 12k ones from there too, but i haven't seen any that bright in blue.

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