Optical specifications are essential to designing and manufacturing an optical component or system to ensure it meets performance requirements within the allotted budget, including optical surfaces.
From spherical to plano or aspheric shapes, optical surfaces are essential to an optical system’s overall design and performance. Understanding surfaces’ role in optical systems is vital to designing a successful project.
What Is an Optical Surface?
An optical surface is an interface between different optical media that reflects or refracts light.[1] Lenses and mirrors are core components of an optical system, but from a conceptual viewpoint, the mechanics of an optical system rely on refraction and reflection.
Specifications for Optical Surfaces
Though optical specifications refer to materials, manufacturing, and surfaces, these specifications largely impact the optical surface:
Diameter Tolerance
Diameter tolerance is applied to a circular optical component. Though it’s primarily a mechanical specification and has no impact on the performance of the optic itself, it’s crucial to consider the mounting within the optical system.
Center Thickness Tolerance
The center thickness of a lens or similar optical component is the thickness of the material at the center point. This measurement is across the mechanical axis of the lens. The thickness of the lens can affect the performance because the center thickness and radius of curvature determine the optical path length of the rays that pass through the lens.
Radius of Curvature
The radius of curvature is the distance between an optical component vertex and the center of the curvature. This may be positive, negative, or zero, based on whether the surface is convex, concave, or plano. The radius of curvature is vital to understanding the optical path length of the rays that pass through a lens or mirror, but it also affects the power of the surface.
Centering
Centering, or decenter, of a lens is considered in terms of beam deviation. The amount of decenter is the physical displacement of the mechanical axis from the optical axis. The mechanical axis of the lens is the geometric axis of the lens as defined by its outer cylinder. In contrast, the optical axis is defined by the optical surfaces and the line between the center of curvature of the surfaces.
Angle Tolerance
With prisms and beamsplitters, the angles between surfaces are essential to the performance of the optic. This is measured using an autocollimator assembly with a light source that emits collimated light. The autocollimator is rotated around the optical surface to verify that the beam is hitting the surface at normal incidence, repeated throughout the optical surfaces. The difference in the angles between the measured positions reveals the tolerance between optical surfaces.
Parallelism
Parallelism measures how parallel two surfaces are to specify components like polarizers and windows. Parallel surfaces are necessary for optimal performance with minimal distortion that can negatively impact the image or light quality.
Bevel
Beveling reinforces and protects the fragile edges of glass components during handling and mounting. Bevels are defined by width and angle, which is often 45°, with a width determined by the diameter of the optic.
Smaller optics, such as micro-lenses and micro-prisms, are not beveled. Small components are more susceptible to chipped edges or other issues during the beveling process.
Clear Aperture
A clear aperture is the diameter or size of an optical component aligned with the specifications. With manufacturing constraints, the aperture can’t be exactly equal to the diameter of the optical component, so the ideal is 90% of the diameter.
Surface Quality
The surface quality of an optical surface is its appearance and any defects. In some cases, surface defects are only cosmetic and don’t impact performance, but defects like scratches and pits can significantly affect performance with certain surfaces. This is especially true of surfaces with image planes because these defects fall into focus and surfaces with high power levels that can cause increased absorption of energy.
Surface Flatness
Surface flatness is a surface accuracy specification that measures the deviation of a flat surface like a plano lens, window, prism, or mirror. This is measured with an optical flat, a flat reference surface with remarkable quality and precision for comparison. The optic is placed against the optical flat during the test to measure deviation.
If the deviations are straight parallel and evenly spaced, the test surface is as flat as the reference piece. If they’re not, there may be a flatness area that will impact performance.
Power
Power is a surface accuracy specification that tests surfaces with power or curved optical surfaces. This test is conducted using a similar reference piece as the flat reference piece, but the reference surface has a precisely calibrated radius of curvature. The principle of interference between the two surfaces reveals the deviation of the surface, creating a series of Newton’s Rings. The number of rings indicates the waves of error.
Irregularity
Irregularity is a surface accuracy specification that describes how a surface’s shape deviates from the form of a reference surface. This is tested using a similar method as the tests for power. A regular surface is measured by the sphericity of the circular fringes formed by comparing the test surface to the reference surface.
Surface Finish
Surface finish, or surface roughness, measures the irregularities on a surface. They are part of the polishing process. Rough surfaces will wear faster than smooth surfaces and may not be ideal for applications with intense heat or lasers because nucleation can occur with defects.
Types of Surfaces for Optical Components
Here are the common surfaces for optical components:
Flat or Plano Surfaces
Flat surfaces are used in virtually all optical systems. They have to meet rigid specifications, but they’re easily measured using interferometry with reliable results.
Spherical Surfaces
Spherical surfaces are often used for lenses and mirrors and are defined by their radius of curvature. Based on surface form error, these two quantities will affect the performance of the lens or mirror. Both of them need to meet the specifications to ensure optical performance, which is tested using a transmission sphere to generate an ideal reference wavefront.
Cylindrical Surfaces
Cylindrical surfaces are more challenging in optical testing than other types of optics. Testing these optics with a conventional interferometer requires a cylindrical wavefront that aligns with the curvature of the surface. Because cylindrical optics are more challenging to fabricate, it can be difficult to manufacture good interferometric references.
Conventional interferometry offers insights into a cylindrical optics surface. Still, a computer-generated hologram can compile a surface description and match the wavefront to the curvature of the test surface.
Aspheric and Freeform Surfaces
Aspheres can be more challenging to manufacture and test in optical systems, but advances in optical technology have allowed more reliable aspheric surfaces to meet performance and weight requirements.
Optical Design and Manufacturing at Apollo Optical Systems
Apollo Optical Systems is a leader in optical design and manufacturing. We believe that successful design comes from understanding the physical and technological principles that drive solutions to the problem. We’re committed to each process step from design to assembly for a successful finished optical system. Contact us today to discuss your custom optics!
Sources:
[1] https://www.sciencedirect.com/topics/engineering/optical-surface