Spherical PCX lens? (Not aspheric)

[LukeA]

I've been thinking: can spherical aberration be used to advantage with a non-point light source? My reasoning is that aspheric lenses are optimized for a point source that the LED die doesn't provide, while at a given distance less than or equal to the focal length of a spherical lens, parallel rays are focussed into a small and finite plane.

Any insights?

 

[CampingLED]

Re: Spherical lens? (Not aspheric)

I have been thinking of the term Aspherical lens that we all use in these forums (that includes myself). My understanding of an Aspherical lens is a lens that has an irregular curvature that corrects spherical aberration (mainly used in wide-angle camera lenses to ensure that the blue and red light rays focus at the same point). The lenses that we use are according to me actually plano convex lenses.

 

[LukeA]

Re: Spherical lens? (Not aspheric)

An aspheric lens is a lens with a parabolic curved shape. It works slightly better than a spherical lens. The common flashlight lens is both PCX and aspheric.

 

[McGizmo]

Re: Spherical lens? (Not aspheric)

I have always assumed that an aspherical lens was designed as suggested by Camping LED to reduce spherical aberration and the actual shape of the curve was a function of the refractive index of the lens material in addition to the overall redirection of the light intended. In other words, one might start with a convex/concave or plano surface combo that bends the light as desired but then the curves are adjusted based on minimizing the refraction likely to occur with the various wave lengths of light.

I believe convex and concave typically refer to a constant radius or spherical surface. Aspherical I take to mean as an adjustment to the surface where you no longer have a constant radius. I believe the deviation from the initial radius increases as you approach the perimiter of the lens but I wasn't under the impression that surface was parabolic? I assumed that the assymetry of the surface curve was a function of the index of refraction of the lens material.

 

[LukeA]

Re: Spherical lens? (Not aspheric)

This thread is geting off track. It doesn't matter at all what an aspherical lens's curvature equation is. The spherical aberration of a spherical PCX lens might be advantageous for light projection with LEDs. I was wondering if anyone had tried such a lens. I guess no one has.

 

[CampingLED]

Re: Spherical lens? (Not aspheric)

Sorry LukeA, the intention was not to side track your thread. I was wondering about it for a week or two now and thought that this was an appropriate thread to ask my burning question. :duck:

 

[McGizmo]

Re: Spherical lens? (Not aspheric)

Well obviously I didn't or don't understand your question and please forgive me for taking your thread off track. I believe PCX refers to plano/convex and all of the lenses I have played with are spherical (constant radius) in terms of their convex surfaces and not aspherical. I have a light called a SunDrop that uses a plano/convex lens of sapphire. I am quite pleased with the partial condensing or convergence of the light I get from this lens. As to how the spherical abberation is manifest either in an advantage or disadvantage, I am clueless and would think I would need to make a comparison against a similar lens with an aspheric convex surface to make a call.

With the sapphire lens, at the perimiter of the round projected beam, there are some thin bands of different color or tint and I attribute this to spherical abberation but that is just a guess. In use, with proper coverage of the target, these rings are outside my area of focus and neither here nor there. I see no advangate obviously in these rings. If the tint or consistancy of tint is improved or degraded within the actual area of interest by virtue of the lens having aspherical abberation, I have no idea.

Forgive my intrusion in your thread and I leave it to you and the other experts to find answer to your question.

 

[snarfer]

Re: Spherical lens? (Not aspheric)

Actually I think it is the other way around. The spherical lens is optimal for focus of an ideal point source, and various types of aspheric lenses have been designed to compensate for real world conditions, such as sources that are not really points, chromatic aberration, and so forth.

Also it seems kind of confusing to use the term "aspheric lens" to refer to a particular type of flashlight lens, when in fact the term refers to just about any lens whose surfaces have a profile that is neither a portion of a sphere nor of a circular cylinder.

 

[LukeA]

Re: Spherical lens? (Not aspheric)

Actually I think it is the other way around. The spherical lens is optimal for focus of an ideal point source, and various types of aspheric lenses have been designed to compensate for real world conditions, such as sources that are not really points, chromatic aberration, and so forth.

Also it seems kind of confusing to use the term "aspheric lens" to refer to a particular type of flashlight lens, when in fact the term refers to just about any lens whose surfaces have a profile that is neither a portion of a sphere nor of a circular cylinder.

The aspheric lens focuses parallel rays to a point better than a spherical lens. The reverse is also true. Aspheric lenses focus rays from a point source into parallel rays. But our lumens don't come from a point source, they come from small but finite planes, which it seems like spherical lenses may be able to take advantage of better. This wikipedia image is an excellent representation.

The description that goes with the image:

Spherical aberration. A perfect lens (top) focuses all incoming rays to a point on the optic axis. A real lens with spherical surfaces (bottom) suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the axis. It therefore does not produce a perfect focal point. (Drawing is exaggerated.)

 

[thegeek]

Re: Spherical lens? (Not aspheric)

I think an LED can be reasonably approximated by a point source located behind the LED. I wouldn't expect much of a difference with a spherical lens vs. aspheric. The Shoppe has some acrylic balls if you want to test it out.

 

[Nos]

aspheric lenses do usually have a shorter backfocal lenght, that why its used so often in flashlight mods. The other advantage is they dont produce spherical abberations.
The 2nd point isnt too important for us, since flashlights are no optical high precision products.

 

[saabluster]

Re: Spherical lens? (Not aspheric)

I was wondering if anyone had tried such a lens. I guess no one has.

Yes I have. And you can have it. When I was developing the DEFT I did not want to have to make my own lenses so searched pretty hard for something that would work off the shelf. One of them was a spherical PCX and the image it threw was not a pretty one. It was a fairly high quality piece too with a nice AR coating. You could make it work but it would require additional optics to correct it.

 

[LukeA]

Re: Spherical lens? (Not aspheric)

Yes I have. And you can have it. When I was developing the DEFT I did not want to have to make my own lenses so searched pretty hard for something that would work off the shelf. One of them was a spherical PCX and the image it threw was not a pretty one. It was a fairly high quality piece too with a nice AR coating. You could make it work but it would require additional optics to correct it.

This is the definitive answer I was hoping for. It was just a thought I was bouncing around. Thanks for the info, even if it is a little disappointing.

 

[i2optics]

I have found PCX lenses work quite well with LED's and are less expensive than aspheric.