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"white" light LEDs or RGB?
9

"white" light LEDs or RGB?

"white" light LEDs or RGB?

(OP)
Hi, I'm trying to determine if it would be more efficient to build an LED array out of "white" light emmiting LEDs or use Red, Green, and Blue LEDs to make white light. Assume that I already know about the different voltage drops of the different colored LEDs and that I have a solution for that if I decide to take that route. Would any of you know of any papers written about LED efficiencies? surface mount vs. standard 5mm LEDs?

Thanks,
Windell

RE: "white" light LEDs or RGB?

Why not just buy some of each and measure whatever you mean by "efficiency" for yourself?

Mike Halloran
Pembroke Pines, FL, USA

RE: "white" light LEDs or RGB?

The LED specification sheets should give you the luminous intensity in mcd, for some specified current. There are several problems with combining RGB Leds.  The first is getting the correct proportions so that you actually get white light, and not some dirty tinged off white colour.

The second is that you will need approximately 39% red 59% green and 11% blue relative light outputs to get somewhere near a white colour.  The red LED will be by far the brightest, but you only need 39% red, so it will be running well below its full output capability. Green will probably be working flat out, and blue somewhat less.

Much better to just use some white LEDS, far less trouble.

Also be aware that most LEDs incorporate some sort of concentrating lens, and the light output may be highly directional, or fairly widely dispersed, depending on what part numbers you decide to buy.

RE: "white" light LEDs or RGB?

(OP)
Thanks Mike and Warpspeed! Your input is invaluable! My goal is to be able to change the light color by varying the brightness of each of the RGB LEDs. The brightness of each would be varied using a PWM circuit for each of the LEDs. Of course, in the begining there will need to be tweeking of the frequencies.

The one main disadvantages I see of the white LEDs is that they are pretty exspensive and I cannot change the color of the emitted light.

This LED array will provide lighting for the color camera of an underwater ROV at a depth of 30 feet below the water's surface so modulating the light frequencies might be something useful.

What do you guys think? Thanks again for your advice!

RE: "white" light LEDs or RGB?

First, for the "white" LEDs, be aware that they are not really white. It's a blue LED with yellow phosphor coating, so you basically get a spectrum with very little red, which might not be what you want for illumination for a colour camera.
Second, with RGB LEDs generating "white" light, you also have three distinct spectrum peaks, which need to be matched to your camera sensitivity.
In general, you probably need to do software colour correction to the video from your camera when using LED illumination.

I'd suggest using halogen bulbs instead. Perfect white continuous spectrum where you do not need to do any correction afterwards.
Contrary to popular belief, LEDs are not a lot more efficient than halogen lamps (30...35 lm/W vs. 15...20 lm/W). The main benefit from LEDs is that the light focusing is better.

Benta.

RE: "white" light LEDs or RGB?

Nice points you guys.

Generally underwater photography is starved for light and you want the brightest light you can get your hands on.  I think that is still MH followed by Halogen.  Video is even more light starved than photography.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: "white" light LEDs or RGB?

Finally, a question I feel uniquely qualified to answer ;)

"White" LEDs come in several flavors, the most common of which is a blue die surrounded by a yellow-emitting phosphour.  Our eyes see blue and yellow mixed to create white, but it has its drawbacks.  For instance, say you wanted to color the light using filters... this works fine on incandescent bulbs because they emit a wide spectrum.  Since the blue/phosphour LEDs only emit over two narrow spectrums, your filtered light will rarely have the color you desire.  for example, a red filter will actually show up as a pink color, a yellow filter will show up as a lemon-lime color, a green filter will show up as an aqua color, and an orange lens will show up as yellow.

A second (but less common) type has a set of RGB dice inside the same package, but there is no individual control... the output level of each color is set at the factory (when I say "set", I mean the process, materials, and quality control they use determine light output for each color).  With this type, you CAN use filters successfully.  But you'll pay more, sometimes a LOT more (sometimes even more than an LED which DOES have individual color control).

If you see any chance of wanting to change individual wavelength strengths (and this application sounds like the perfect example), you need to go with the individually controlled RGB type.  This can either be separate packages for each color, like 5mm LEDs (cheaper), or combined packages, like surface mount (lower board space requirements).

Also consider more than just a single R, G, and B... for example, you may want 2-3 different LEDs spanning the red spectrum to give you more adjustability, though I cannot say if you will need that level of control.  Scattering some IR or near-IR LEDs may also prove useful if you're using CCD cameras... they're quite sensitive to the near-IR spectrum, and you may get some nice shots not possible using visible light.

If this is a large array, heat dissipation will be a concern.  Consider attaching the whole shebang to a large aluminum heatsink that has the water flowing directly over it.

That's all i can think of for now... feel free to ask more questions as they arise.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: "white" light LEDs or RGB?

Radio Shack and Dollar Stores sell RGB LED light sticks for kids. They typically have a button and you can select various modes ('white' and various colours, some steady, some flashing). Might be useful for some intial proof-of-concept testing.

RE: "white" light LEDs or RGB?

I'll also add... if you intend to do mostly close-up shots, pick an LED with a low viewing angle (15-30 degrees), really put the light where you need it and not waste it on the sides.  For wider angle shots, you may need to go with 60+ degree.

The Radio Shack/$1 Store LEDs aren't going to be very strong at all, so don't expect any real distance from those, especially in murky water.  You can pick up bright R, G, and B  5mm LEDs for well under $1 for the three... I would suggest spending about $50 in LEDs and putting together a simple flat panel to try.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: "white" light LEDs or RGB?

My 'dollar store' suggestion was JUST for initial proof-of-concept testing (in dark lab, not in murky water smile ).

RE: "white" light LEDs or RGB?

While you are checking spectra, you should look at the sensitivity spectrum of the camera you intend to use.  If it's insensitive to the (very narrow) emissions of the LEDs you use, it won't matter how you modulate the source.

You're probably better off with the cave diver's standby, car headlights (tarred terminals) and gel cells.

Mike Halloran
Pembroke Pines, FL, USA

RE: "white" light LEDs or RGB?

Forget leds use lamps unless you want everything to come out the wrong colour.

RE: "white" light LEDs or RGB?

itsmoked,

Sharks are preferable to toilet paper... 3eyes

----------------------------------
  I don't suffer from insanity. I enjoy it...

RE: "white" light LEDs or RGB?

(OP)
All your advice is great! Thanks Mike, Keith, Scott, cbarn, VE1BLL, Dan, benta, and Warpspeed!

I've been looking at the LEDs assortment offered by Digi-key and there are so many to choose from! Are there any manufacturers that any of you would suggest I buy high brightness LEDs?

Also, the camera has a viewing angle of about 90 degrees. If I mount the LEDs in a simple plane I'm guessing to make things simple I should look for LEDs with a viewing angle of around 90 degrees too to get good coverage.

RE: "white" light LEDs or RGB?

Your best bet is probably to first get as much data as you can from the internet, and then phone around for best price and availability. Many suppliers have a minimum order, or there may be long delivery times for some particular types.  But you will never know until you ask.

 

RE: "white" light LEDs or RGB?

windell747;
You want bright LEDs?

Search not for LEDs!  Search for "light engines" this is industry speak for; multiple, ultra bright, LEDs, mounted on a single substrate for intense lighting systems.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: "white" light LEDs or RGB?

Hi Guys,

I'm after the same thing, I want to build a pannel for my camera too, but it's not for under water...

This is what I have in mind: http://www.litepanels.com

Any help will be much appreciated

RE: "white" light LEDs or RGB?

Shaolin, see my comments in your original thread.

windell, Luxeon makes some great emitters (their new K-2 series rocks!).  Also try Lamina Ceramics for more of the "light engine"-style emitter (though cooling will be more of an issue).

Dan - Owner
http://www.Hi-TecDesigns.com

RE: "white" light LEDs or RGB?

(OP)
Thanks Keith and Dan! Great advice!

Keith. Yeah, I've ran across those light engine configurations in the Digi-key catalog and wondered what they were. The one inconvinience about them is that they are already in clusters. I'm thinking about putting 100+ total Red, Green, and Blue LEDs on this panel depending on the brightness of the LEDs. And I'm predicting that since they are already in clusters, that might limit the quality of light distribution. I would like to put LEDs in rows on my panel so that the LEDs are close together.

Dan. As for cooling, since the ROV will be under water, the LED array will ba sealed by encasing it in in clear resin used especially for SONAR transducers and prevent water bubbles from forming within the resin. The lack of air bubbles in the resin will prevent light scattering in the resin.
Aside from that, the water could be used for cooling. I might mount some corrosive resistant aluminum heat sinks to the back of the array so that the heat generated by the high brightness LEDs is dispered immediately.

Thanks again for your help!

RE: "white" light LEDs or RGB?

Well, now we run into more issues.  What's the expansion coefficient of the resin?  If you heat it up with the LEDs, is it going to expand and apply undue pressure on the dice?  If the resin is truly melded around the LED face (what's the refractive index?), you may change the optical characteristics of the packages... to what degree, you'd have to either model with a raytracer or build a few prototypes.  You may have to shape the resin into a lens itself... a flat face would significantly increase the viewing angle, something that may be undesired.

# of LEDs is irrelevant, what you care about is total luminous flux.  For example, I have a board in front of me that has 64 red surface mount LEDs on it (a prototype brakelight, for those who care), each 2x3x3mm in size... it pulls  about 1/3rd of an Amp, and leaves spots in my vision during the daytime when viewed from 30 feet away (I really need to get some laser-safe goggles).  That's running them at 20mA, and they will handle up to 70mA continuous.

The clusters are very handy to work with, but you will probably not want the large viewing angle they give off (typically 110-120 degrees)... they would waste light off to the side.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: "white" light LEDs or RGB?

I am certain you are correct about mixing your leds over a large area.  Individual R,G,and B LED clusters will likely cause some fairly objectionable colour fringing around the edges of objects. Some sort of optical diffuser to scatter the light may also be an advantage, and also overcome the directionality problem giving a more soft and even lighting.

RE: "white" light LEDs or RGB?

(OP)
Thank you very much Dan and Warpspeed! The lenses of the LEDs are made of resin and the LEDs as well as the PCB would be encased in resin so I'm not worried about the coefficient of expansion because I don't believe that the resin temperature will get too high since it will be submersed in salt water. Also, the increase in viewing angle due to the flat resin surface of the LED panel might also be advantageous since majority of the high intensity LEDs seem to have low viewing angles of 30degrees.

RE: "white" light LEDs or RGB?

The resin you encapsulate the entire shebang in is most likely not the same resin used to encapsulate the dice, hence different expansion ratios.  Also, we're talking about quick localized heating around the dice, so you can't necessarily assume the cool water will help with expansion concerns... in fact, it may exacerbate the problem by cooling one face while the LEDs are heating the other.

If you expand the viewing angle too much, you're going to lose too much light to the edges where the camera never sees.   The viewing angle isn't a hard angle, it's merely the angle at which light output has dropped by 50%.  If you use a 60-90 degree viewing angle, that shoudl be more than enough to get a decent picture, maybe with some darker areas around the edge of the video.

Consider picking up the LEDs that use RGB dies in the same package, practically removing the fringing issue.  Luxeon has such a beast.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: "white" light LEDs or RGB?

(OP)
Thank you very much Dan! Your concerns are very legitimate and reasonable. I will look into the LEDs offered by Luxeon. Thanks again!

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