Spherical vs Aspherical Mirrors

The main difference between spherical and aspherical mirrors is that spherical mirrors are simple and uniform in shape, whereas aspherical mirrors are more complex and have a greater variation in curvature. Aspherical mirrors can better correct aberrations and distortions, resulting in better image quality.

 

Spherical lenses are, without a doubt, one of the most commonly utilized types of lens in optical systems. Their ease of manufacture and analysis stems from their simple shape. Specifically, the design of this type of lens is a spherical surface that maintains a constant curvature from the center to the edge of the lens. Conversely, aspherical surfaces appear to vary in curvature with proximity to the optical axis. Thanks to technological advancements, aspherical lenses are now prevalent in modern systems including medical technology and laser material processing. Furthermore, both types of lenses can be further optimized by using sophisticated CNC technology, intelligent control software, and interferometry. In this article, we will delve into the geometry, advantages, and applications of spherical and aspherical optics.

 

Spherical and Aspherical Optical Geometry

A spherical mirror is a rotationally symmetric optical element whose shape corresponds to the cross-section of a spherical surface. This is because its surface is part of a sphere. The center of the sphere is called the center of curvature and the radius of the sphere is called the radius of curvature. The radius of curvature is constant in distance from the geometric center. There are two types of spherical mirrors, concave and convex. They have simple shapes and are easy to fabricate and analyze.

Aspherical lenses have a more complex shape and are not spherical in nature. This shape allows them to correct aberrations caused by spherical aberrations in spherical lenses. The geometry of an aspherical lens is described by mathematical equations, unlike a spherical lens which has a natural geometry.

The most commonly used expression for aspheric surfaces is a conic surface as a datum then iterated with a series of higher order polynomials constituting the expression:

Z = surface profile parallel to the optical axis

s = radial distance to the optical axis

c = curvature, reciprocal of radius

k = cone constant

A4, A6... are the 4th, 6th ... aspheric coefficients.

When the aspheric coefficient is zero, the aspheric surface is then a quadratic surface (conic surface), the shape of the conic surface depends on the size and sign of the conic coefficient k.

 

Advantages of spherical and aspherical lenses

The manufacturing process is simple and spherical lenses are relatively easy to produce compared to other shapes such as aspherical or cylindrical lenses. The process, which includes grinding and polishing, can be accomplished using standard tools and techniques; the simplicity of the manufacturing process means that spherical lenses are cheaper to produce compared to other lens shapes. The manufacturing process for spherical lenses is highly repeatable and precise, allowing for consistent quality at a lower cost. This also applies to the optical inspection and measurement process, as measurements can be made uniformly across the entire surface and results can be produced quickly. One of the disadvantages of spherical mirrors is the presence of spherical aberration, i.e. distortion of the image produced by the mirror. However, this can be minimized by using a plurality of mirrors of different curvatures or by using other corrective optical elements such as lenses.

Aspherical lenses have several advantages over spherical lenses. They can minimize the spherical aberration introduced by spherical lenses in collimation and focusing systems by adjusting the surface constants and aspheric coefficients, resulting in better image quality. As shown below, the aspherical lens on the right (light rays converge to the same point, providing optical quality) essentially eliminates the spherical aberration created by the spherical lens on the left (light rays converge to different points, resulting in blurred imaging).

 

Aspherical lenses also reduce the overall size and weight of optical systems, making them more compact and portable. For example, in a zoom system, where 10 or more lenses are normally used (plus: high mechanical tolerances, additional assembly procedures, improved anti-reflective coatings), similar or better optical qualities can be achieved with 1 or 2 aspherical lenses, thus reducing the size of the system and minimizing the number of surfaces where internal reflections can occur, increasing the luminous flux of the system while avoiding the introduction of stray light. This reduces the size of the system and minimizes the number of surfaces where internal reflections may occur, increasing the luminous flux of the system while avoiding stray light.

 

Applications

Spherical mirrors have many applications, including microscopes, telescopes and automotive mirrors. They are also used in projectors, optical test equipment and laser systems. They are also widely used in areas such as metrology, aerospace or medical technology. Spherical mirrors are also used in solar concentrators, which concentrate the sun's rays on a small area to generate electricity. Due to low manufacturing costs, short production times and a wide range of optical applications, spherical surfaces are an integral part of the optical market and offer a very good price/performance ratio.

Aspherical mirrors have more complex shapes and curvatures, which allow them to focus light to a single point or form an image with minimal distortion. Aspherical mirrors have applications in laser systems, microscopy, medical imaging, and aerospace, such as in laser technology and advanced medical imaging, where they eliminate aberrations and produce high-quality images. Aspherical mirrors can be customized to produce specific shapes, for example, parabolic mirrors are used to focus beams of light in telescopes and satellite communication systems, while ellipsoidal mirrors are used in headlamps, projectors, and astronomical instruments, among other things, depending on the desired application.