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Why tolerance?

The world is not perfect, so is the optical fabrication. The final surface finish and geometrical shape of a final component always departs from the perfection. Although "best effort" can be considered as a qualitative specification of tolerance, it is not scientific and can result in unnecessary cost. A proper tolerance can minimize fabrication cost and lead time without compromising desired performance. 

What is tolerance? 

For optical components and systems, there are two kinds of tolerances: One is mechanical dimension tolerance, which is defined in the same way as general machined parts; another is optical specific, which may confuse some first-time customer. Here is a brief about optical tolerance. 

The tolerance is a quantitative description between perfection and fabrication reality. The fabrication process determines the final shape and cosmetics of the surfaces. Surface shape specifications of an optical element are defined in terms of the wavelength of light because it is the way a shop tests them. Cosmetics such as pits and scratches are expressed in accordance with military specification MIL-O-13830, where the width of defects is specified relative to a master set of standard scratches and digs. 

How is tolerance measured? 

Fringes: A comparison of the fabricated surface and a calibrated reference surface produces a pattern of Newton rings. This pattern shows the deviation of the fabricated surface from the surface of the calibrated reference. The resulting pattern consists of fringes that quantify the "power" and irregularity of the surface. The irregularity of the fringes represents the deviation of the surface from a perfect sphere. A sample interference test is shown in the Figure below, where a convex surface with a localized irregularity is compared with a concave reference plate. Two fringes of power and one quarter fringe of irregularity are shown.

Scratches: The thickness of a scratch is specified in microns and the diameter of digs, pits and bubbles is given in units of 0.01mm. For example, a 80-50 specification sets limits of the apparent width of the #80 reference for scratches (80mm) and a 0.50mm diameter for digs. The complete specification also limits the number of blemishes over the surface area. Refer to MIL-O-13830 for detailed information.

How to specify proper tolerance?

Here is the guideline for tolerance: