Difference between Aspherical Lenses types/materials

[Yurirt94]

Hi everyone,

I'm building an optical system in which I need a focusing lens of a certain diameter and focal lenght. I decided to use an aspheric lens since it reduces aberrations in comparison to the spherical ones.

I found many aspheric lenses with the specifications I need, nevertheless, there are many types/materials of aspheric lenses and I didn't find a clear explanation about the advantagens/disadvantages between using one type or another.

The types I found are: Molded Glass, Plastic, Plastic Hybrid, Acrylic, Precision Aspheric and Aspheric Condenser.

Does anyone know the difference between some of them?

Thanks,

Yuri

 

[Drakkith]

What will you be doing with this lens? Do you need to worry about chromatic aberrations? In reality, a single lens is very limited in what it can do to correct aberrations. However, if your f-ratio is slow enough (long focal length compared to lens diameter) the aberrations inherent to the lens are lessened, sometimes to an acceptable amount.

The most important properties of the materials used to construct the lens is the index of refraction and dispersion inherent to the material. Very low dispersion materials are generally very expensive compared to higher dispersion, low cost glass.

 

[Khashishi]

I think the aspheric condensers are lower optical quality and intended for illumination or light transfer, but not for images. The precision aspherics are probably higher quality and more expensive. Something from a mold won't have good optical quality but might be good enough for a condenser.

The material will depend on what you need and cost. I think you can only remove the aberration at one wavelength, and some materials have chromatic aberration than others. See http://en.wikipedia.org/wiki/Abbe_number

 

[Yurirt94]

I'm using a laser with very precise wavelenght (so chromatic aberration would not be a problem) to focus a beam with a certain waist. The laser light comes from a fiber, then I use a pair of spherical lenses as a telescope and finally focus the beam with the aspheric lens.

About the money, I'd say no more than $100 for an aspheric one...

 

[brianhurren]

have you tried freds Supplus shed. he has a huge range of cheap suplus optics. a lot of quality x militry stuff. I bought a 4 inch two element acromat from him for about 90 bucks. made a good telescope. The distortion youget through a sperical lens is spherical aberition, this is iliminated by using a aspherical lens. In my case I was using a lens with a focal lengh of f9. When I compared the the chromatic distortion between a 3 element apochromat and a 2 element achromate and also considering that 200x would be the highst usfull magnification of the system. There wasnt much difference between the two (somthing like 0.001% dif) So I went with the achromat.
I wouls say compare you aspherical choice of lens with its spherical equvalent and if the difference is less than 1/4 wave legnth of the light you are using, then go for the cheaper spherical lens. I would chose class over plastic.

 

[sophiecentaur]

Flare can be a significant problem. Have you considered that a coated lens may be needed for your application?

 

[Yurirt94]

- Yes, sophiecentaur. I'm looking for lenses with coating!

- brianhurren, but why glass over plastic, do you know the difference between them?

 

[Drakkith]

No one here could give you a complete explanation of the differences between the different types of lenses. That's going to depend entirely on the specific material each lens is made of. At best we could give you a few general guidelines, and it would be immensely useful if you told us exactly what you are using the lens for. What are the specifics of your setup? What are you trying to achieve? So far all we know is that you are focusing light. Is it monochromatic? Do you need to form a high quality image for something like a CCD detector? Etc.

 

[Yurirt94]

Drakkith, I need the system with the best image quality as possible; that's exactly the goal here: I'm trying to develop a high-resolution imaging system for an experiment in which we visualize a cloud of cold-atoms.

I need to simulate and test this system. To do that I need to start with a laser of λ=780nm, pass this beam through two spherical lenses that will work as a telescope and then focus this beam to a waist of 1 μm with the aspherical lens.

In the real system, the focus of the beam represents the position of my atom cloud and the size of the waist is directly related to the resolution of the system. The light emitted by the atoms would pass through the aspherical lens, then through the telescope and then reach a CCD camera.

The function of the telescope here is to simply adjust the beam diameter so that we can obtain the wanted waist at the focus for a certain input diameter.

Please, ask anything you might need to answer! :D
Thanks for your help.

 

[sophiecentaur]

- Yes, sophiecentaur. I'm looking for lenses with coating!

- brianhurren, but why glass over plastic, do you know the difference between them?

Sorry - but someone needed to state the obvious, I think - if only for the casual reader's benefit.

 

[Drakkith]

Yurirt, I don't think you need to worry about anything but the shape and focal length of your lens. Whether it's glass, plastic, or some other material isn't going to matter if the lens meets all of your needs. Honestly I'd recommend contacting a company or individual who designs/sells lenses and telling them what you need.

 

[Yurirt94]

Ok. My biggest concearn was if a specific material was more succeptable to some kind of optical aberration than other. But it seems to me that's not the problem.

Thanks, everyone! :D

 

[sophiecentaur]

The best way round this is to come up with your essential specification and see what's available. The way it's achieved need not matter - except in terms of the price. If you want good performance, it will cost you, unless there's something already available on the mass market (photography, perhaps). As with electronics, it may be cheaper to buy a whole system than to build using components. There are some really cracking 'kit' lenses available for a few tens of GBPs.

 

[brianhurren]

the difference between an ashperical and a spherical lens is that aspherical is hyperbolic in shap while spherical is, well, sherical. that is a aspherical lens surface is a section of a hyperbaloid while a spherical lense is a section of a sphere. I recommend glass over plastic because it is chemical and scratch resistant and has better thermal characteristics than plastic (you find a high quality telescope or camera lens made of plastic.). you will need to design your optical system as a whole and match all your lenses up so that they complement each other...this is from experience building telescopes.