A Guide to the Importance of Surface Quality in Optics

A Guide to the Importance of Surface Quality in Optics

“Surface quality” is a term that refers to surface imperfections on an optical component. These imperfections appear as localized blemishes such as scratches and digs.

There are two reasons why these have to be taken into consideration:

Firstly, for purely cosmetic reasons, is the surface quality consistent with a quality optic, i.e. is it cosmetically acceptable?

The subsequent, and most important reason is related to the functionality of the component. The appearance of defects on the surface can cause the scattering of light which can severely disrupt the component’s performance.

This is of particular importance in laser applications because of the high-intensity of incident illumination. In addition, diffraction patterns caused by unwanted scratches or digs can lead to the degradation of system performance and cause further component damage or ruin.

Surface imperfections can be the most misconstrued and ambiguous of all optical specifications. The two most widely accepted and common conventions that help define and communicate the required surface quality (and other optical standards), are:

a) The U.S. military surface quality specification, MIL-PRF-13830B (often referred to as “MIL spec.”).

This is the most widely used convention based on a set of ‘Master’ standard scratches and digs stored at the Frankford Arsenal in the USA. There are numerous companies that commercially offer copies of these standards for use in the laboratory and workshop. Using a visual comparison as a guide, surface quality of the optic is tested under specific lighting conditions against the standard set in order to define the size of the imperfection.

b) International Standard: ISO 10110.

This standard is based on the German standard DIN 3140. Increasingly adopted across industry because it is less subjective and in principle, requires the direct measurement of every scratch and dig on the component’s surface.

MIL-13830B: Practical Application

Surface quality under this convention is identified by two numbers e.g. 40/20. The first number indicates the maximum width (in microns) of scratch allowed. The subsequent number indicates the maximum dig diameter in x10 microns. For example, a spec of 60/40 would allow for a scratch width of 60 microns (0.0024”) and a dig diameter of 400 microns (0.0158”). In that respect, a ‘perfect optic’ would be represented as 00/00.

a) Scratches

Scratch numbers under this convention refer to the width of the reference scratch. The scratch assigned to an optic is the result of visual inspection and a comparison between the scratch on an optic and the reference standard under particular lighting conditions.

As a rule, to ensure good visibility the combined length of the largest permissible scratches on each surface cannot exceed one quarter of the diameter of the optic. If multiple scratches are visible (e.g. several 40 scratches on a 40/20 mirror) the sum of the products of the scratch numbers times the ratio of their length to the diameter cannot be more than half of the maximum scratch number.

b) Digs

A pit or a small crater (a bubble is considered a dig in this convention) is what is meant by the term dig. Those which are smaller than 25 micrometers are dismissed. Again, as in the case of scratches, under this practice digs are determined by their apparent diameter under visual inspection. When compared to the standards a maximum size dig per each 20 mm diameter section can be determined not to be greater than twice the diameter size.

ISO 10110: Practical Application

Codes and numbers are used in this system to specify scratch and dig tolerances. The preface code number “5/” is the ISO convention for surface imperfections. To illustrate this the general format is displayed below:

a) Scratches

The general format specific to scratches is shown below:

5/LN´´ x A´´

ISO 10110 convention, recognizes a distinction between long and short scratches. Long scratches are those that are in excess of a length of 2 mm. The header L is used to mark and identify long scratches, whereas the number N´´ is the number of long scratches that can be tolerated. This limitation in the number of scratches tolerated is only applicable to the long scratches and must be specified separately.

The grade number A´´, is equal to the square root of the area of the maximum size allowed imperfection and is thus expressed in millimeters.

A typical example found on a drawing would be:

5/L5 x 0.040

This specifies that each part may contain scratches of a maximum scratch width of 0.040 mm. In addition, this indicates that a maximum number of 5 long scratches of the same maximum width are permitted. An extra note can be added e.g. “Maximum length allowed 4 mm” in which case the whole specification can be read as follows:

“Allow 5 scratches, no wider than 40 um (0.040 mm) and no longer than 4 mm over the clear aperture”

b) Digs

Similarly, digs follow the same format i.e.

5/N x A

Where: “N” is the maximum number of digs permissible and, “A” is the grade and equals to the square root of the surface area of the maximum allowed defect in millimeters.

A typical example found on a drawing would be:

5/5 x 0.10

This would read: “Allow a maximum of 5 digs; each dig may have a grade of 0.10 mm over the clear aperture”

Comparison Between Two Standards

Although the aim of both systems is to ensure a method of defining consistently and in an understandable manner a standard throughout the industry, there are probably some issues that need to be carefully considered.

In the case of ‘MIL spec,’ it should be noted that any inspection of polished optical surfaces for scratches and digs is carried-out by a visual comparison to the scratch standards.

It is not the actual width or diameter that is identified, but the appearance of the scratch in comparison with these standards. Due to the fact it is the visual impact of the defect that is the determining factor, the illumination of the sample is extremely important - particular types of marks that ‘stand out’ on one light source may not be so clear on another.

Blemishes and marks on different component materials can also have a visually different appearance, although they may in fact be of the same size. Moreover, there is the question of how dependable the ‘comparison standards’ really are, those which are manufactured in plastic can certainly appear different to the more common chrome on glass reticules. Also, the test being subjective may have a bearing influence.

ISO 10110 is an attempt to make optical surface defects more objective by directly measuring the surface imperfections.

The question must be asked, in practice, “would a workshop measure every tiny mark on a large optic which may take hours to determine if an optics is passable or not?” Interestingly, there are two methods that can be used in the ISO standard; the one previously described (method 1); and a visibility test (method 2), which judges purely from a subjective cosmetic perspective.

A comparison chart of the two standards can be helpful since some may be familiar with one type and not the other. Due to the subjective nature of the “MIL spec” method, a practical approximation of the values has been determined when considering the equivalent dig values using the ISO 10110 standard. The comparison chart is illustrated below in the following table:

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

 

The above table facilitates the possible relation of values from one standard to another. For example, a 60/40 scratch-dig should translate to the following:

5/N x 0.35; LN´´ x 0.060

Numbers N and N´´ can be selected according to what may be considered suitable.

As a guide, the table shows the default tolerances/values for a number of parameters. This includes surface imperfections during manufacturing in the absence of explicit specifications given under the guidance of ISO 10110. The following table illustrates permissible tolerances and material imperfections in the absence of explicit indications given under 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

 

Surface Quality Application Guide

Below is the ‘rule of thumb’ guide to surface quality, (described in MIL spec) in order for the optic to fulfill its operational function.

S/D APPLICATION
80/50 Commercial grade, commonly acceptable standard for imaging applications.
60/40 A good standard for most scientific research applications.
40/20 For low to moderate laser applications where scattered light has to be considered in regards to system performance.
20/10 For moderate to high power laser systems where scatter is very important.
10/5 High precision quality, used for the most demanding very high power laser applications.

 

In general, the higher the surface quality specification, the higher the price of the component, therefore careful consideration should be given to whether the higher standards are necessary in the optics application and use.

Where extremely aspheric surfaces are concerned, the most challenging surface specifications become difficult to attain due to the constant changing surface slopes - these optics often carry a price premium.

On a practical level, there is a special case where very large or very small components are concerned. For example, on a one meter mirror a 5 mm scratch would be tiny with little impact in proportion to its size, but the same scratch would be significant and costly on a surface 10 mm in diameter.

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

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

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