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I want to focus a 300W LED into a small area, about 5mm in diameter, much like focusing the sun's rays with a fresnel lens.

I have to be like 2 meters away from the LED to focus the LED light with a fresnel lens. Why is that?

Is there a way to focus the all of the LED's light somehow onto a small area?

If it's possible, would it be akin to having a 300W laser? Mostly likely not, but why?

Thank you in advance for your answers.

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  • $\begingroup$ Depends on the light source. Many high power LEDs are made up of many discrete LEDs and thus constitute an extended source. Those would be difficult or impossible to focus to a small point. $\endgroup$ – Eric S Oct 25 '19 at 18:11
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If you just want a highly focussed light source, consider a halogen car headlight bulb. The bulb itself contains a parabolic reflector to give a focussed beam. The Fresnel lens in the front of the actual headlight acts as a diffuser, rather than to focus the beam.

I have used a benchtop-sized test rig where a couple of those bulbs (12V 55W each) can heat up a small object (1mm diameter) to about 1000C in a few minutes.

The individual semiconductor structures in an LED tend to be point sources which produce an omnidirectional radiation pattern (except where the rest of the structure physically gets in the way and blocks the radiation), which probably isn't what you want.

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  • $\begingroup$ Key point: start with reflectors before you get to lenses. Like a torch... $\endgroup$ – Jonathan R Swift Oct 26 '19 at 1:04
  • $\begingroup$ Incandescent bulbs have very low radiance, and cannot be focused tightly like LEDs. Halogen bulbs have higher radiance, but are still a much worse choice. Individual diodes are not point sources, and are also not isotropic, with a typical extent of at least a few hundred wavelengths and emitting most of their energy into a solid angle of 2-3 steradians. $\endgroup$ – user1850479 Dec 1 '19 at 5:20
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I have to be like 2 meters away from the LED to focus the LED light with a fresnel lens. Why is that?

You are building an imaging system that images the LED onto another plane: http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lensdet.html

The distance you need to be away from the LED depends on the focal length of your lens and the distance from the lens to where you are forming the image. If you want to have the source closer, either get a shorter focal length lens, or move the plane you're focusing on further away.

Is there a way to focus the all of the LED's light somehow onto a small area?

It depends on your definition of small. The above link gives the equation for magnification of your image. To have a lot of energy focused tightly, you'll need a magnification << 1, which means putting the lens far from the source and close to the image. Note that the smaller the source of the light, the smaller the area a given lense will be able to focus it down to.

If it's possible, would it be akin to having a 300W laser? Mostly likely not, but why?

It would not because lasers emit energy more densely than LEDs, meaning for a given lens, you will have a more intense spot than for an LED. For focusing energy, there are two classes of laser that are relevant, single mode and multimode. A single mode laser acts as if all of its energy is emitted from a single infinitely small point, and therefore when you image it down you can form an image a wavelength wide. A multimode laser appears to emit energy from a series of adjacent points, thus when focused down it behaves more like an LED, and cannot form a spot as small as a single mode laser.

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