Needed for calculation of minimum spot size in visible light. Can somebody recommend how to choose?
More information is needed to answer the question: Monochromatic or polychromatic light? I the light propagating on the optical axis?.
In general: An objective performs better for polychromatic light and has a good performance on the focal plane and with a flat focal plane and usually at higher NA than a doublet.
an asphere corrects for spherical aberations and has a good performance in the focal point, but may work for monochromatic light only.
An ashperical doublet corrects for chromatic aberations at two wavelengths like an achromat with the additional advantage of having a better correction for spherical aberations. Making it perform better, in principle, at a larger NA than a standard achromat. Expect coma and other aberations when working off axis
To decide what works best for you some optical modelling in ZEMAX may be necessary.
Best Regards
Thank you for your response, Christian and Rolf.
First assumption was that light is polychromatic (general case), but monochromatic light also can be used.
I'm interested in understanding the influence of the aberrations in addition to diffraction.
Thank you again.
A Combination of optical refractive index and Eddy's coefficient in a lattice crystal can be a good method !
Because this is practical questions, it would be nice to know the application. Also, as it has been said, NA is very important. What is the NA you need? If your NA is very small, you do not need expensive optics to reach diffraction limited performance
You said you need the info for calculation. I am not sure if you need to consider some practical constrains, such as working distance, lens mount, cost, etc.?
Thank you for your response, Alex and Ivan.
In the discussed setup the NA is large and required spot size needed to be about 9 microns including aberrations.
To resolve a 9 micrometer spot size, NA is not a huge concern with microscope objectives - the diffraction limit according to Abbe's criterion isn't reached until until the NA is less than 0.04 (using the equation cited by Punckt above and 700 nm as the wavelength of light). Almost any standard low magnification microscope lens (10X lenses usually have an NA around 0.25) will have this. But you also need to make sure your condenser NA is set to that of your objective for the equation above to apply with trans-illumination. Most medium-price level condensers mark the NA on the condenser aperture for the different settings of the aperture, but if it does not you need to look down the tube of the microscope and be sure that the aperture of the objective - the light coming through - is not limited by the aperture of the condenser (in a Koehler-illuminated microscope). For low magnification microscope and photographic lenses, their field of view is so large and the technology of production difficult enough so that aberrations are indeed a severe problem and pose a greater problem for resolution than diffraction. Smaller objects are actually resolved at a certain higher f-number (smaller apertures) than the lowest f-number of the lens. This is called the sweet spot of the lens. However, most photographic lenses will only resolve about 12-20 micrometer spots, even with great aberration correction. If you are referring to an aspheric doublet photographic lens and a standard microscope objective in your question, then you should go with the microscope objective (even if it isn't a plan lens, i.e., corrected for flatness-of-field).
I think that you should consider tolerances. As mentioned above, the objective will be corrected over a larger field. This means that you likely won't have to align the objective as well as you would have to align the doublet. I would go with the objective unless cost were an issue. If cost is an issue, you'll need to model the performance of the lens with fabrication and alignment tolerances to determine if the doublet will work.
Thank you for your response, Zhou,Griffing and Matthew .
In may calculations I used the spot size calculated in the Zemax as a resolution limit. Does it is really good criteria for this?
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