Hemispherical lens

[TheArcLight]

I am new here and have been doing some learning. I see that an aspherical lens when used in conjunction with an LED produces quite a powerful very focused beam, i am curious if a hemispherical lens was used in the aspherical lens's place what would be the outcome. Also one other question. Is it possible to use multiple lens's spaced correctly to magnify the light of the LED even further? Is there such a lens that combines a fresnel lens with an aspherical lens? I am new here and find everything very interesting so much so that i actually purchased a new mill and lathe from grizzly with the hopes of learning more and possibly having something to contribute one day.

 

[Throwjunkie]

Welcome to CPF this should help you a bit.
http://www.candlepowerforums.com/vb/showthread.php?143017-Optic-theory


Joe

 

[Walterk]

And this is what there is to say about aspheric lenses:
Formula for calculating throw using aspheric lens

I don't know much about hemispherical lenses.
I have played with aspherics with adjustable focus in the two led lights in my signature line:

 

[HarryN]

Hi, to save you some time, a hemispherical lens is theoretically perfect, for a perfect light source and a single wavelength of light. (single color)

An aspherical lens is very similar, but it is squished a bit to help correct for the real world of slightly imperfect light sources, and that most people use white light, which is a mixture of colors.

The result, is that most lenses, both conventional and fresnel, are built as aspheres.

 

[Benson]

Hi, to save you some time, a hemispherical lens is theoretically perfect, for a perfect light source and a single wavelength of light. (single color)

No, spherical aberration is a geometric defect. Spherical surfaces are not theoretically perfect in any way, they're just cheap to make, and close enough for some applications. (Since the aberration increases at short focal ratios, and our aspheric flashlights are typically f/1 or so, it's rather bad for our purposes.)

An aspherical lens is very similar, but it is squished a bit to help correct for the real world of slightly imperfect light sources, and that most people use white light, which is a mixture of colors.

Again, nothing to do with "imperfect light sources", geometric optics is enough to show spherical lenses cannot image any source correctly. And it looks like you're now confusing chromatic aberration, which affects single-index aspherics the same as any other lens; you need a multiple-element or variable-index lens system to fix that.

 

[HarryN]

Hi Benson, Thank you for those corrections.

It looks like perhaps you should have answered the original posters question instead of my more simplistic attempt to help him.

My main point, which I incorrectly articulated, is that he should just skip hemispherical lenses and go directly to aspherical ones if he wants to go down a lens path.

Harry