Spherical Singlet, Best Form, and Achromatic Lens Comparison Guide
Singlets are the most basic form of lens and are available in several types: plano convex, biconvex, plano concave, biconcave, meniscus, and aspheric. Singlets are the most cost-effective lens and perform well. For instance, our standard plano-convex lens will perform nearly as well as a best-form version when the convex side is facing the infinite conjugate.
Best Form lenses, however, are engineered to a higher standard than normal singlets, tending to have better surface quality and flatness than standard singlets. Unlike singlets, these lenses are fabricated using a different radius of curvature on each side of the lens. By doing so, spherical aberration, a common drawback of spherical lenses, can be corrected. Additionally, best form lenses are capable of producing smaller waist sizes than singlets as well as realizing diffraction-limited performance for small input beam diameters. While slightly more expensive than singlets, the spherical aberration correction and improved performance properties make them an alternative to singlets.
Compared to singlets or best form lenses, achromatic lenses are the most complex in that they are actually composed of two or more lenses (e.g., an Achromatic Doublet is comprised from 2 lenses and an achromatic Triplet is designed from 3 lenses). These lenses are either cemented together or have an air gap between them and typically include both positive and negative elements with different indices of refraction. Achromatic lenses are by far the superior lens choice, achieving a tighter focus that is often free of coma and spherical aberration and can also achieve near-diffraction-limited waist sizes. Additionally, achromats are less susceptible to “centration,” meaning that off-lens-axis beams are focused to the same spot as those on axis (simply put, the lens cannot “steer” the beam). The largest benefit of achromatic lenses, however, is their ability to correct for chromatic aberration. Whereas with singlets and best form lenses the focal length is dependent upon wavelength, achromats provide nearly constant focal length across a wide range of wavelengths. Thus, the achromat is ideal for broadband imaging applications and beam shaping.
Choosing a Proper Lens
In general, there are no hard and fast rules as to when you should choose one lens over another. Function mimics necessity, and often times a singlet can serve just as well as a best form or achromat. For instance, with intensity measurements, such as fluorescence measurements, where one may be using a PMT or photodiode, spherical aberration is not a large concern so long as the beam can be focused onto the active area. In this case, the much more economical singlet is preferred. Even in simple systems in which one must correct for spherical aberration, a proper choice of biconvex lens or best fit lens will suffice (such as one-to-one imaging).
For measurements with much higher constraint, however, one may find that a singlet is not sufficient for the given application. Precise imaging and magnification measurements will need to correct for spherical aberration and coma effects. In this case, the achromatic or best form lens is the proper choice. In many single-frequency imaging techniques, a best form lens and appropriately sized beam will suffice. The best form lens will be able to handle high-power concerns and remove spherical aberration. Imaging systems with large magnifications can benefit significantly from the employment of a best form lens over the standard singlet. Additionally, in some cases, best form lenses offer an economical solution to air-gap achromatic lenses (discussed below).
Any broadband technique that utilizes a large wavelength range would benefit from the use of achromatic lenses. In applications where waist size is critical (such as in far off resonant trapping of neutral atoms), achromats are again the proper choice, as they can realize a tighter, near-diffraction-limited waist size. Additionally, the isolation from centration effects will ensure the waist size is in the proper position as the laser beam is moved. In general, achromatic lenses will outperform singlets and best form lenses. While in the past laser power has been a limitation to the use of achromatic lenses, the advent of air-gap achromatic lenses has alleviated that concern. Air-gap achromatic lenses are capable of handling the higher power light sources once reserved for best-form lenses. While significantly more expensive than their best-form counterparts, air-gap achromatic lenses will provide the highest performance, handle high-power applications, and correct for coma, spherical, and chromatic aberration.
|Lens Type||Pricing||Spherical Aberration||Chromatic Aberration||Diffraction Limited||Wavelength Range*||Max Damage Threshold|
|Singlet||$|| || || ||0.18 - 12 µm||7.5 J/cm2 |
10 J/cm2 (V-Coat)
|Best Form||$$||X|| ||X**||350 - 1620 nm||7.5 J/cm2|
|Cemented Achromat||$$$||X||X||X||0.4 - 12 µm||5 J/cm2|
|Air-Gap Achromat||$$$$$||X||X||X||650 - 1050 nm||10 J/cm2|
*Wavelength Range and Damage Threshold are dependent upon lens material and AR coating
**for small input beam diameters