Optical specifications are utilized throughout the design and manufacturing of a component or system to characterize how well it meets certain performance requirements. They are useful for two reasons: first, they specify the acceptable limits of key parameters that govern system performance; second, they specify the amount of resources (i.e. time and cost) that should be spent on manufacturing.
An optical system can suffer from either under-specification or over-specification, both of which can result in unnecessary expenditure of resources. Under-specification occurs when not all of the necessary parameters are properly defined, resulting in inadequate performance. Over-specification occurs when a system is defined too tightly without any consideration for changes in optical or mechanical requirements, resulting in higher cost and increased manufacturing difficulty.
In order to understand optical specifications, it is important to first review what they mean. To simplify the ever-growing number, consider the most common manufacturing, surface, and material specifications for lenses, mirrors, and windows. Filters, polarizers, prisms, beamsplitters, gratings, and fiber optics also share many of these optical specifications, so understanding the most common provides a great baseline for understanding those for nearly all optical products.
Figure 1: Decentering of Collimated Light [View Larger Image]
Figure 2: Test for Centration [View Larger Image]
The diameter tolerance of a circular optical component provides the acceptable range of values for the diameter. This manufacturing specification can vary based on the skill and capabilities of the particular optical shop that is fabricating the optic. Although diameter tolerance does not have any effect on the optical performance of the optic itself, it is a very important mechanical tolerance that must be considered if the optic is going to be mounted in any type of holder. For instance, if the diameter of an optical lens deviates from its nominal value it is possible that the mechanical axis can be displaced from the optical axis in a mounted assembly, thus causing decenter (Figure 1). Typical manufacturing tolerances for diameter are: +0.00/-0.10 mm for typical quality, +0.00/-0.050 mm for precision quality, and +0.000/-0.010 mm for high quality.
Center Thickness Tolerance
The center thickness of an optical component, most notably a lens, is the material thickness of the component measured at the center. Center thickness is measured across the mechanical axis of the lens, defined as the axis exactly between its outer edges. Variation of the center thickness of a lens can affect the optical performance because center thickness, along with radius of curvature, determines the optical path length of rays passing through the lens. Typical manufacturing tolerances for center thickness are: +/-0.20 mm for typical quality, +/-0.050 mm for precision quality, and +/-0.010 mm for high quality.
Radius of Curvature
The radius of curvature is defined as the distance between an optical component's vertex and the center of curvature. It can be positive, zero, or negative depending on whether the surface is convex, plano, or concave, respectfully. Knowing the value of the radius of curvature allows one to determine the optical path length of rays passing through the lens or mirror, but it also plays a large role in determining the power of the surface. Manufacturing tolerances for radius of curvature are typically +/-0.5, but can be as low as +/-0.1% in precision applications or +/-0.01% for extremely high quality needs.
Centering, also known by centration or decenter, of a lens is specified in terms of beam deviation δ (Equation 1). Once beam deviation is known, wedge angle W can be calculated by a simple relation (Equation 2). The amount of decenter in a lens is the physical displacement of the mechanical axis from the optical axis. The mechanical axis of a lens is simply the geometric axis of the lens and is defined by its outer cylinder. The optical axis of a lens is defined by the optical surfaces and is the line that connects the centers of curvature of the surfaces. To test for centration, a lens is placed into a cup upon which pressure is applied. The pressure applied to the lens automatically situates the center of curvature of the first surface in the center of the cup, which is also aligned with the axis of rotation (Figure 2). Collimated light directed along this axis of rotation is sent through the lens and comes to a focus at the rear focal plane. As the lens is rotated by rotating the cup, any decenter in the lens will cause the focusing beam to diverge and trace out a circle of radius Δ at the rear focal plane (Figure 1).
where W is the wedge angle, often reported as arcminutes, and n is the index of refraction.