"white" light LEDs or RGB? 9

[windell747]

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

 

[MikeHalloran]

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



Mike Halloran
Pembroke Pines, FL, USA

 

[Warpspeed]

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.

 

[windell747]

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!

 

[benta]

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.

 

[itsmoked]

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

 

[MacGyverS2000]

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

 

[VEBill]

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.

 

[MacGyverS2000]

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

 

[VEBill]

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

 

[itsmoked]

Murky, shark filled, water...

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MikeHalloran]

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

 

[cbarn24050]

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

 

[ScottyUK]

itsmoked,

Sharks are preferable to toilet paper...

----------------------------------

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

 

[itsmoked]

Indeed. And 6 pack holders!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[windell747]

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.

 

[Warpspeed]

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.

 

[itsmoked]

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

 

[ShaolinWood]

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

 

[MacGyverS2000]

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