Practical Guide for Optical Users on Surface Quality

Topics Covered

Introduction
Surface Quality
MIL-13830B: Practical Application
ISO 10110: Practical Application
Comparison of MIL-13830B and ISO 10110 Standards
Conclusion
About Optical Surfaces

Introduction

Optical Surfaces has created a new guide titled ‘A Practical Guide to Surface Quality,’ the second in a string of guides for optics users (Figure 1). The guide is useful for scientists and engineers to better understand an optics quoted surface quality because surface imperfections are generally the most misinterpreted and confusing of all optical specifications.

Figure 1. Practical Guide to Surface Quality.

Surface Quality

The term ‘surface quality’ represents surface imperfections on an optical component. There are two reasons to consider ‘surface quality': purely cosmetic reasons and the functionality of the component. If the surface of optical components contains defects, light can get scattered which in some cases may influence their performance. Additionally, undesired diffraction patterns due to scratches/digs can be detrimental to system performance and can result in component damage.

The two widely accepted standards that help defining and describing the required surface quality are:

  • The U.S. military surface quality specification, MIL-PRF-13830B
  • International Standard: ISO 10110.

MIL-13830B: Practical Application

The surface quality under MIL-13830B standard is specified by two numbers, for instance, 40/20. The first number specifies the maximum allowable width (in microns) of scratch; whereas, the second number specifies the maximum dig diameter in x10 microns. Scratch numbers under this standard specify the width of the reference scratch. The scratch designated to an optic is the product of visual inspection and a comparison of the scratch on an optic and the reference standard under specific lighting conditions.

As a rule of thumb, to attain good visibility, the collective length of the maximum allowable scratches on each surface must not go beyond one quarter of the diameter of the optic. If the surface of an optical component contains many scratches, then the sum of the products of the scratch numbers multiplied by the ratio of their length to the diameter should not go beyond half the maximum allowable scratch number.

A dig is a pit or a small crater and digs smaller than 25µm are generally ignored. Under this convention, digs are described by their apparent diameters under visual inspection and comparison to the reference standards.

Again as in the case of scratches, digs under this convention, are defined by their apparent diameters under visual inspection and comparison to the standards. A maximum size dig per each 20mm diameter section must not be larger than double the diameter size.

ISO 10110: Practical Application

This system applies codes and numbers to refer scratch and dig tolerances. The preface code number ‘5/’ is the ISO convention for surface imperfections. An example illustrating the general format is given below:

5/   N x A;
  ↑
  Digs
LN´´ x A´´

Scratches

The general format specific to scratches is illustrated as follows:

      5/LN´´ x A´´

ISO 10110 convention differentiates between long and short scratches. Scratches that are larger than 2mm are called long scratches. The header L specifies long scratches, while the number N´´ represents the count of allowed long scratches. The grade number A´´ is the square root of the area of the maximum size allowed imperfection in millimeters.

The general format specific to digs is as follows:

      5/N x A

Where, N is the maximum number of digs allowed and A is the grade number equivalent to the square root of the surface area of the maximum defect allowed in millimeters.

Comparison of MIL-13830B and ISO 10110 Standards

In MIL-13830B, polished optical surfaces are inspected for scratches and digs through visual comparison to the scratch standards, thus ensuring only the appearance of the scratch as compared to these standards not the actual width or diameter. Since the visual impact of the defect is the deciding factor, the method of illuminating the sample can be highly significant as certain types of marks that are readily visible in one light source may be so apparent in another. Marks of the same size can look visually different on different component materials. Moreover, the reliability of the ‘comparison standards’ is also questionable.

Conversely, ISO 10110 is an effort to make optical surface defects more objective through direct measurement of surface imperfections. Considering the subjective nature of the MIL-13830B method, a practical approximation of the values has been made by taking into account the equivalent dig values utilizing the ISO 10110 standard. The following table provides the practical comparison chart of the two standards:

SCRATCHES DIGS
MIL spec ISO equivalent MIL spec ISO equivalent
10/x 5/LN´´ x 0.010 x/5 5/N x 0.05
20/x 5/LN´´ x 0.020 x/10 5/N x 0.10
40/x 5/LN´´ x 0.040 x/20 5/N x 0.20
60/x 5/LN´´ x 0.060 x/40 5/N x 0.35
80/x 5/LN´´ x 0.080 x/50 5/N x 0.45

Using the aforementioned table as a guide, the values can be related from one standard to another. The following table summarizes the allowable tolerances and material imperfections in the absence of explicit indications as specified by ISO 10110:

  Range of maximum (diagonal) dimension of part (mm)
Parameter up to 10 over 10 up to 30 over 30 up to 100 over 100 up to 300
Diameter or Length (mm) ±0.2 ±0.5 ±1 ±1.5
Thickness (mm) ±0.1 ±0.2 ±0.4 ±0.8
Chamfer width (mm) 0.1 - 0.3 0.2 - 0.5 0.3 - 0.8 0.5 - 1.6
Wedge ±0° 30’ ±0° 30’ ±0° 30’ ±0° 30’
Centration tolerances 4/30’ 4/20’ 4/10’ 4/10’
Surface form error (fringes) 3/5(1) 3/10(2) 3/10 (2) (all Ø 30) 3/10 (2) (all Ø 60)
Stress birefringence (nm/cm) 0/20 0/20 - -
Bubbles and inclusions 1/3 x 0.16 1/5 x 0.25 1/5 x 0.4 1/5 x 0.63
Inhomonegeneity & striae 2/1:1 2/1:1 - -
Surface imperfection (digs) 5/3 x 0.16 5/5 x 0.25 5/5 x 0.4 5/5 x 0.63

Conclusion

The price of the component is higher if the surface quality spec is higher. Hence, careful consideration must be given to whether the higher standards are required in the optics applications. Achieving the most challenging surface specifications in highly aspheric surfaces is a difficult task because of the constantly changing surface slopes. These optics are expensive. With vast experience, Optical Surfaces is well positioned to satisfy and in most cases, surpass all demanded surface quality specifications.

About Optical Surfaces

Optical Surfaces Ltd, has been producing optical components and systems for more than 30 years and is now accepted as one of Europe's leading manufacturers of high-precision optics. Our list of products include:

  • Etalons and Windows
  • Off-Axis Mirrors
  • Laser Beam Expanders
  • Collimators
  • Lenses and Prisms
  • Sphericals and Flat Mirrors
  • Mounted/Custom Optics. 

The main workshops and test facilities are deep underground in a series of tunnels excavated in solid chalk where temperature remains constant and vibration is practically non-existent. With such stable conditions testing particularly with long path lengths become quantifiable and reliable.

Working with these natural advantages is a highly skilled team of craftsmen with a commitment to excellence in both product quality and customer service.

Interferometers routinely used include Zygo, Twyman Green, Scatterplate, Fizeau up to 600mm dia. Laser unequal path (LUPI) and point diffraction. They are able to test surface geometry, interfacing with our Wyko computer fringe analysis to confirm final wavefront.

This information has been sourced, reviewed and adapted from materials provided by Optical Surfaces.

For more information on this source, please visit Optical Surfaces.

Comments

  1. Stuart Roberts Stuart Roberts United Kingdom says:

    Anyone Know what 1/10 x 0.5 and 5/1 x C x1 is

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of AZoOptics.com.

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