Is there a reason why every LED has to have its own discrete lens, as opposed to arranging a bunch og LEDs onto, say, a hemispherical substrate and then covering that with one contiguous layer of lens material? Could the inside of a shaped substrate be used to dissipate heat from the LEDs?
I'm just wondering whether there is a physical (in the sense of, "related to physics") that all the "LED light bulbs" I've seen are basically small plastic plates dotted with discrete lenses.
TIA!
- Kris
They are not really lens, but waveguides. There is no air inside the device. Its simply a matter of getting the proper alignments and using one is much simpler. Heat goes everywhere, but much of the normal led's heat goes out through the leads.
greg
Think about what an LED chip does, and what a lens does.
Then your question answers itself.
-- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
Depends on your optical requirement. The lenses are for focusing the light. I am working on a project with no lenses and the optical requirements are met using a reflector.
LEDs gst hot, so a multi-led assembly can be an aluminum plate - the heat sink - dotted with led chips. The chips are separated to spread out the heat. So if you want directional light, it works better to have one small lenslet per led. Optically, to direct the light from the cluster, a single lens would have to be very big, and use a lot of plastic.
Something like that.
John
The curved lenses help the extraction efficiency of the light. Light comes out of the LED's active area in all directions. Because the refractive index of the chip is so high--usually n=3.4 to 3.5--almost all the light undergoes total internal reflection and so fails to leave the chip. Coating the chip in high refractive index plastic (n~1.6-1.7) makes the angular range from which light can actually make it out of the LED chip. However, that doesn't help so much if the light undergoes TIR at the plastic/air interface, and that's where the curvature comes in. If the lens were a large hemisphere centred on the LED die, all the light would come out perpendicular to the lens surface, which would minimize the loss. The actual package shape is a compromise between efficiency and illumination pattern.
In order to an array with a single lens, it would have to be very big--it would be a hemisphere much bigger than the array. It's much easier and cheaper to use individual lenses.
Much of the improvement in LED efficiencies in the last several years has been due to increased extraction efficiency and decreased absorption of light in the active region, using surface relief and heterojunctions, respectively.
Cheers,
Phil Hobbs
Kris Krieger wrote: > Is there a reason why every LED has to have its own discrete lens, as opposed to arranging a bunch og LEDs onto, say, a hemispherical substrate and then covering that with one contiguous layer of lens material? Could the inside of a shaped substrate be used to dissipate heat from the LEDs? > I'm just wondering whether there is a physical (in the sense of, "related to physics") that all the "LED light bulbs" I've seen are basically small plastic plates dotted with discrete lenses. >
(Resending--first one dropped into the bit bucket. PH)
The curved lenses help the extraction efficiency of the light. Light comes out of the LED's active area in all directions. Because the refractive index of the chip is so high--usually n=3.4 to 3.5--almost all the light undergoes total internal reflection and so fails to leave the chip. Coating the chip in high refractive index plastic (n~1.6-1.7) makes the angular range from which light can actually make it out of the LED chip. However, that doesn't help so much if the light undergoes TIR at the plastic/air interface, and that's where the curvature comes in. If the lens were a large hemisphere centred on the LED die, all the light would come out perpendicular to the lens surface, which would minimize the loss. The actual package shape is a compromise between efficiency and illumination pattern.
In order to an array with a single lens, it would have to be very big--it would be a hemisphere much bigger than the array. It's much easier and cheaper to use individual lenses.
Much of the improvement in LED efficiencies in the last several years has been due to increased extraction efficiency and decreased absorption of light in the active region, using surface relief and heterojunctions, respectively.
Cheers,
Phil Hobbs
osed
Yes. Essentially by putting the chip at the focal point of the small hemispherical lens you can collimate the LED beam so that most of the light is emitted in a relatively tight cone. This is usually beneficial in a lot of applications.
You could use a bigger lens and have several LEDs packed at its focus but it would require a lot more plastic to make it.
nside
If you did this then the light would go in every direction. A lens has to be the right shape, or rather the air to lens surface angle has to be right to refract light how you want it to go. A few Fresnel lens type assemblies exist.
to
You can get bare or nearly bare LEDs with just diffuser or something to protect the semiconductor from damage for surface mount applications. It all depends what you want to do.
Regards, Martin Brown
Thank you to all the folks giving informative answers to my question - Phil's was especially informative :) , but all were helpful in (dare I say it?) shedding light ;) on the reasons why LED light bulbs are made the way the way they are.
- Kris
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.