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Figure 1: Illustration of Spherical Aberration
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Optical aberrations are deviations from a perfect, mathematical model. It is important to note that they are not caused by any physical, optical, or mechanical flaws. Rather, they can be caused by the lens shape itself, or placement of optical elements within a system, due to the wave nature of light. Optical aberrations are named and characterized in several different ways. For simplicity, consider aberrations divided into two groups: chromatic aberrations (present when using more than one wavelength of light) and monochromatic aberrations (present with a single wavelength of light). For more information on aberrations, view Chromatic and Monochromatic Optical Aberrations and Comparison of Optical Aberrations.
One of the most common types of monochromatic aberrations is spherical aberration. Spherical aberration is the result of light focusing at different locations based on its radial distance from the lens center resulting in poor system performance (Figure 1). Though spherical aberration is present in all spherical optics, an innovative way to correct for it is by employing spherical aberration compensation plates to reduce or remove known quantities of spherical aberration in a system.
WHAT ARE SPHERICAL ABERRATION COMPENSATION PLATES?
By compensating and correcting for a known amount of spherical aberration, spherical aberration compensation plates are single-element optical components that can be easily inserted into a system, reducing spot size and drastically improving image quality (Figures 2a – 2b). These corrector plates signify a change in the way aberration correction can be handled. By correcting for known amounts of spherical aberration, they save in design time, reduction of system weight as well as manufacturing costs.
Spherical aberration compensation plates are designed to be used in collimated space near a pupil. They should be used for systems that have small fields of view such as laser systems or applications imaging point-like objects. These corrector plates can be combined to induce the desired amount of compensatory spherical aberration. Negative sign plates create over corrected spherical aberration, while positive plates create under corrected spherical aberration.
Spherical aberration compensation plates are optically flat windows with low wavefront distortion that have been magnetorheologically polished to impart a mild aspheric surface. Aspheric surfaces are traditionally defined with the following surface profile (sag):
Where Z is the sag of the surface parallel to the optical axis; s is the radial distance from the optical axis; C is the lens curvature, inverse of radius; k is the conic constant; and A4, A6, A8 are 4th, 6th, 8th… order aspheric terms.
However, in the case of spherical aberration compensation plates, there is no optical power (curvature) to the surface (i.e. C = 0). The corrector plates achieve their known amounts of spherical aberration by having at least one non-zero aspheric coefficient. Equation 1 then reduces to:
Figure 2a: Spot Diagram of an Optical System with Uncorrected Spherical Aberrations
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Figure 2b: Spot Diagram of an Optical System with Spherical Aberration Compensation Plate [View Larger Image]
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