"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

 

[windell747]

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!

 

[MacGyverS2000]

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

 

[Warpspeed]

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.

 

[windell747]

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.

 

[MacGyverS2000]

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

 

[windell747]

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