Guidelines for Surface Quality Control of Optical Components——Standards Analysis, Cleaning Procedures, and Inspection Solutions
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Publish time:
2026-01-29
Guidelines for Surface Quality Control of Optical Components
——Standards Analysis, Cleaning Procedures, and Inspection Solutions
With the deep integration of precision laser technology in industrial processing, medical devices, aerospace remote sensing, and other fields, the surface quality of optical components has become a core factor determining the upper limit of system performance—microscopic scratches or digs may lead to localized energy concentration in high-power lasers, beam distortion, or even permanent damage to components. This article systematically outlines internationally mainstream surface quality assessment standards, details key cleaning and inspection technologies, and provides enterprises with standardized, high-precision quality control solutions.
1. Standards for Assessment of Optical Surface Quality
Currently, the appearance inspection of optical components is primarily based on four core standards. Among them, the Chinese national standard GB/T 1185-2006 aligns closely with the international standard ISO 10110-7 in technical requirements, both originating from the German DIN 3140-7 standard, ensuring mutual compatibility within the technical framework. The following section focuses on the widely adopted U.S. military specification MIL-O-13830 and the international standard ISO 10110-7.
（1）.U.S. Military Standard MIL-O-13830:
This standard defines surface defects using dual parameters: "S (scratch) - D (dig)." Visual comparison is achieved through the Brysen surface finish standard plate, offering operational convenience and low cost, making it suitable for inspecting most industrial-grade optical components.
①Defect Definition and Measurement
- Scratch (S): A slender defect with a length-to-width ratio > 4:1. The grade is indicated by "#" (industry practice typically associates the number with width, e.g., a 60# scratch generally refers to a width of 60μm. In CASTECH quality control system, a conversion factor of 0.5μm is applied, meaning 60# corresponds to 30μm). Assessment is based solely on comparison with the standard plate, with no direct specification for depth.
- Dig (D): A point-like defect with a length-to-width ratio ≤ 4:1. The unit of measurement is 0.01mm (e.g., a 50# dig corresponds to 0.5mm). For irregular shapes, the equivalent diameter is calculated as "(maximum length + maximum width) / 2."
Scratch
Dig
②Typical S/D Grades and Acceptance Criteria
Taking the industrially common "60/40" grade (i.e., Scratch 60#, Dig 40#) as an example, the assessment logic is as follows:
Inspection Item | Acceptance Criteria | Inspection Tool |
Scratch (60#) | a. If the scratch grade on the component is > 60#, the component is rejected. b. If the scratch grade is = 60#, and the sum of the lengths of all scratches of this grade does not exceed 1/4 of the component's diameter, the component is accepted. c. If the scratch grade is < 60#, and the sum of the products of each scratch's grade multiplied by the ratio of its length to the component's diameter does not exceed half of the maximum scratch grade, the component is accepted. d. If the spacing between two or more scratches is < 0.1 mm, they are combined and treated as a single scratch for assessment using the methods above. | Visual Inspection + Brysen Surface Finish Standard Plate |
Dig (40#) | a. If the dig grade on the component is > 40# (i.e., > 0.4 mm), the component is rejected. b. If there is one and only one 40# dig per φ20 mm area, the component is accepted. c. If the sum of the diameters of all digs within any φ20 mm area does not exceed twice the maximum dig (0.4 mm), the component is accepted. | Visual Inspection + Brysen Surface Finish Standard Plate |
(2) ISO 10110-7:
This standard does not differentiate between scratches and digs but quantifies surface defects through dual dimensions of "quantity and size." It supports precise measurement using instruments like microscopes, offering greater objectivity, making it suitable for demanding applications such as high-power lasers and precision optical instruments.
① Drawing Indication and Parameter Definitions
The complete indication format is: 5/N×A; CN'×A'; LN''×A''; EA'''
The meanings of each parameter are as follows:
- "5/": Fixed code for surface imperfections (mandatory on all drawings).
- N×A: Base number of defects (N) and their size grade (A), where A is the square root of the defect area in mm. Example: "2×0.016" means 2 defects, each with an area ≤ 0.000256 mm².
- CN'×A': Coating imperfections. Number of coating blemishes (N') and their size grade (A'). Example: "C1×0.02" means 1 coating blemish of grade 0.02 mm.
- LN''×A'': Long scratches (slender indentations > 2 mm). Number of long scratches (N'') and their width grade (A''). Example: "L1×0.005" means 1 long scratch with a width ≤ 0.005 mm.
- EA''': Edge chips. The maximum permissible extent grade for edge chips (A'''). Example: "E0.03" means edge chips ≤ 0.03 mm.
②Defect Substitution Rules
A single surface imperfection of a larger size grade may be substituted by several smaller ones, provided the sum of their areas does not exceed the area of the original, larger imperfection. A conversion table for size grades is provided as below:
N | 1 | 2.5 | 6.3 | 16 | 40 |
Grade numbers A (mm) | 0.004 |
|
|
|
|
0.006 | 0.004 |
|
|
| |
0.01 | 0.006 | 0.004 |
|
| |
0.016 | 0.01 | 0.006 | 0.004 |
| |
0.025 | 0.016 | 0.01 | 0.006 | 0.004 | |
0.04 | 0.025 | 0.016 | 0.01 | 0.006 | |
0.063 | 0.04 | 0.025 | 0.016 | 0.01 | |
0.1 | 0.063 | 0.04 | 0.025 | 0.016 | |
0.16 | 0.1 | 0.063 | 0.04 | 0.025 | |
0.25 | 0.16 | 0.1 | 0.063 | 0.04 | |
0.4 | 0.25 | 0.16 | 0.1 | 0.063 | |
0.63 | 0.4 | 0.25 | 0.16 | 0.1 | |
1 | 0.63 | 0.4 | 0.25 | 0.16 | |
1.6 | 1 | 0.63 | 0.4 | 0.25 | |
2.5 | 1.6 | 1 | 0.63 | 0.4 | |
4 | 2.5 | 1.6 | 1 | 0.63 | |
6.3 | 4 | 2.5 | 1.6 | 1 | |
10 | 6.3 | 4 | 2.5 | 1.6 |
Example: Conversion Explanation for ISO 10110-7 Surface Defect Grades
To clarify the conversion logic of surface defect grades under ISO 10110-7, the following example uses the specification "5/ 2×0.016" for step-by-step illustration.
Conversion Steps:
- In the conversion table, first locate the baseline defect size of 0.016 mm corresponding to N=1.
- Within the same row, find the N value of 2.5 corresponding to the target size of 0.01 mm.
- Multiply the original defect quantity (2) by this N value (2.5) to obtain the equivalent defect quantity under the new size: 5 defects.
→ Final conversion: 2 × 0.016 mm = 5 × 0.01 mm.
Key Considerations:
- Converted defect quantities are rounded down to the nearest integer without rounding up. For example, 2×0.016 mm may also equate to 12×0.006 mm.
- Defects smaller than 0.16×A (where A is the baseline size) are negligible. In this case, defects below 0.0025 mm are excluded.
- If coating blemishes or long scratches are not separately specified on the drawing, they default to the general surface defect criteria.
- Special rules for long scratches: The total width of all long scratches must not exceed N×A, and individual scratches narrower than 0.3×A may be disregarded .
(3)Core Differences Between the Two Standards:
Aspect | MIL-O-13830 | ISO 10110-7 |
Assessment Logic | Distinguishes scratches/digs; visual comparison (subjective) | Quantifies defects by size/number (objective) |
Cost & Efficiency | Low cost, rapid, no specialized equipment | Requires microscopes; time-consuming and costly |
Applicable Scenarios | Industrial optics (e.g., common lenses, prisms) | High-power lasers, precision optical systems |
2. Cleaning and Inspection:
Residual contaminants may be misjudged as defects, while improper cleaning can cause irreversible scratches. Standardized cleaning procedures and professional inspection schemes are critical to accurate quality assessment .
(1). Cleaning of Optical Components:
The primary goal is to remove oil and dust without compromising the surface. Refer to instructional videos for detailed cleaning protocols to balance efficacy and safety .
(2). Professional Inspection Solutions: Equipment + Lighting
For varying precision requirements, a "multi-equipment + multi-lighting" inspection system ensures reliable technical grounding for surface quality assessment :
Equipment:
- CCD Microscopes: High-magnification observation of micro-scratches/digs.
- Stereo Microscopes: Low-magnification overview for rapid defect localization.
- Olympus Microscopes: High-precision optics compatible with ISO 10110-7 quantification requirements.
CCD Microscopes
Stereo Microscopes Olympus Microscopes
Lighting Configuration: Flexible options (e.g., incandescent, cool white/warm yellow LEDs, green high-intensity lamps) adapt to specific technical standards .
③Inspection Methods:
- Transmitted Light Inspection: For double-sided transparent components (e.g., lenses, windows). Defects invisible under transmitted light are excluded.
- Reflected Light Inspection: For substrates like silicon wafers or single-side polished elements, using 60° angled lighting to highlight defects via contrast .
-
Transmitted Light Inspection
Reflected Light Inspection
For industrial applications, prioritize MIL-O-13830 to maximize efficiency; for high-precision scenarios, adopt ISO 10110-7 to ensure accuracy. Combining standardized cleaning, professional equipment, and context-aware standards safeguards product performance and credibility .
Reference：
[1] ISO10110-7：1996 Optics and optical instruments Preparation of drawings for optical elements and systems Part 7: Surface imperfection tolerances.
[2] GB/T 1185-2006：Surface imperfections of optical elements.
[3] MIL-PRF-13830B: 2000 Performance specification optical components for fire control instruments; general specification governing the manufacture, assembly, and inspection of.
