Deep Dive into Optics: Optical Tolerances

Sept. 12, 2022
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Manufacturing optical tolerances are important factors that impact both performance and cost of an optical component or system. Optical components commonly require tighter optical tolerances than mechanical components.

Cost Drivers for Optical ComponentsEach of the factors below will affect the cost for an optical component:

• Quantity

• Material

• Surface Shape

• Size

• Dimension/Diameter Tolerances

• Surface Figure Accuracy

• Surface Quality (Scratch & Dig)

• Coating Requirements

• Delivery Time

Optical tolerance classes:

Base: typical, no cost impact for reducing optical tolerances beyond this

Precision: requires special attention, but easily achievable in most shops, may cost 25% more

High Precision: requires special equipment or personnel, may cost 100% more (double)

Below are the definitions of typical optical parameters and their optical tolerances.

Dimension/Diameter Tolerance provides the acceptable value of the diameter of a lens, filter, or other optical component. Though diameter tolerance does not have any effect on optical performance, it must be considered for any component that requires mounting. Typical manufacturing tolerances are shown in the following table.

Base    Precision    High Precision

100 μm    25 μm        6 μm

Center Thickness Tolerance is the allowed manufacturing error of the lens or filter thickness, measured at the center of its diameter. Because center thickness determines the optical path length of rays passing through a lens, it can affect optical performance. Typical manufacturing tolerances are shown in the following table.

Base    Precision    High Precision

200 μm    50 μm        10 μm

Radius of Curvature is the distance between an optical component’s vertex and the center of curvature. Knowing this value allows one to determine the optical path length of rays passing through the lens or mirror and plays a role in determining the power of the surface. Typical manufacturing tolerances are shown in the following table.

Base    Precision    High Precision

0.2%        0.1%        0.01%

Angular Tolerance in components such as prisms and beamsplitters is the maximum allowed deviation of angles between adjacent sides of an optical prism. Typical manufacturing tolerances are shown in the following table.

Base            Precision            High Precision

6 arc min        1 arc min            15 arc sec

Bevel protects fragile glass corners from being chipped. Chipped edges lead to problems during the lapping/polishing processes and could result in scratches. Beveled edges also protect anyone handling the components – handling of machined glass could lead to cut fingertips without bevels. The typical manufacturing tolerances are provided in the following table.

Base        Precision    High Precision

0.2 mm        0.1 mm        0.02 mm

The Surface Quality of an optical component includes describing defects such as scratches and pits or digs. In many cases, these defects are purely cosmetic and do not affect optical performance. The most common specification for surface quality is scratch/dig. The scratch designation is determined by comparing scratches on the test surface to a set of standard scratches under controlled lighting conditions, thus this designation does not describe the scratch, but compares it to a standardized scratch. The dig designation does directly describe the dig and can be calculated at the diameter of the dig in microns divided by 10. The typical specifications are provided in the following table.

Base    Precision    High Precision

80/50        60/40        20/10

Surface Irregularity refers to the shape and sphericity of an optical component and how the surface being tested compares to a reference. The comparison between the test and reference surfaces forms circular fringes. The consistency (or inconsistency) of the fringes indicates regularity and irregularity. Unlike surface quality, surface irregularity has a great impact on the performance of an optical system. . The typical manufacturing tolerances are provided in the following table.

Base    Precision    High Precision

1 wave    λ/4                λ/20

Surface Finish or surface roughness measures small scale irregularities on the surface of an optical component. The typical manufacturing tolerances are provided in the following table.

Base            Precision        High Precision

50 Å rms        20 Å rms        5 Å rms

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