A practical engineering guide to the MIL-PRF-13830B scratch-dig standard — what the numbers mean, how to specify correctly, and how to avoid paying for surface quality you do not need.
Scratch-dig is the most widely used surface quality specification in the optical industry, defined by the US military standard MIL-PRF-13830B (formerly MIL-O-13830). It provides a quick, standardized way to communicate the maximum allowable size of surface defects on an optical element.
A scratch-dig specification is written as two numbers separated by a dash, for example: 60-40, 40-20, 20-10, or 10-5. The first number is the scratch number; the second is the dig number.
Importantly, scratch-dig is a comparative standard. Inspectors compare observed defects against calibrated reference standards under controlled illumination (typically a long-wave UV lamp or diffuse white light against a dark background). It is not a direct dimensional measurement — it is a width comparison against certified scratch and dig standards.
The scratch number indicates the width of the widest allowable scratch, expressed in units of 0.001 mm (1 μm). A scratch number of 80 means the widest visible scratch must not exceed 0.08 mm (80 μm) in width.
| Scratch Number | Width (μm) | Width (mm) | Visibility | Typical Use |
|---|---|---|---|---|
| 80 | 80 | 0.080 | Clearly visible under inspection light | Rough industrial, non-critical |
| 60 | 60 | 0.060 | Visible under inspection light | General purpose optics |
| 40 | 40 | 0.040 | Moderately visible | Imaging systems, moderate quality |
| 20 | 20 | 0.020 | Faint under inspection light | Laser systems, precision optics |
| 10 | 10 | 0.010 | Very difficult to see | High-precision, lithography, space |
The scratch number also governs the cumulative length of scratches. Per MIL-PRF-13830B, the total length of scratches at the specified width must not exceed certain limits based on the optic diameter. Narrower scratches (lower number) are allowed to be longer; wider scratches must be shorter. The standard provides specific formulas for this tradeoff.
The dig number indicates the diameter of the largest allowable pit (circular defect), expressed in units of 0.01 mm (10 μm). A dig number of 10 means no pit may exceed 0.10 mm (100 μm) in diameter.
| Dig Number | Diameter (μm) | Diameter (mm) | Appearance | Impact |
|---|---|---|---|---|
| 100 | 1,000 | 1.00 | Large, obvious pit | Unacceptable for most optical use |
| 50 | 500 | 0.50 | Large pit | Rough industrial use only |
| 20 | 200 | 0.20 | Moderate pit, visible | General purpose optics |
| 10 | 100 | 0.10 | Small pit | Laser, imaging, precision |
| 5 | 50 | 0.05 | Very small pit | High-precision optics |
| 2 | 20 | 0.02 | Microscopic | Precision, lithography, space optics |
Per the standard, the total area of all digs on the optic must not exceed a specified fraction of the clear aperture. For most specifications, the allowable number of digs of the maximum size is limited to prevent clustering.
The following table provides typical scratch-dig specifications for common optical applications. These are guidelines — your specific system requirements may differ. For reference, our optical flats are held to 20-10 or better surface quality, while our precision optical windows and high precision mirrors are available across the full scratch-dig range.
| Application | Typical Scratch-Dig | Why This Level |
|---|---|---|
| General purpose (windows, mirrors for alignment) | 60-40 | Defects have minimal impact on beam quality or transmission |
| Imaging systems (camera lenses, microscope objectives) | 40-20 | Surface defects scatter light, reducing contrast and creating artifacts |
| Laser systems (beam steering, focusing, windows) | 20-10 | Scratches act as stress concentrators and laser damage initiation sites |
| Precision optics (interferometer flats, reference surfaces) | 10-5 | Any surface defect degrades wavefront quality and measurement accuracy |
| High-power laser (fusion research, industrial cutting) | 10-5 or better | Surface defects cause localized absorption leading to catastrophic damage |
| Space optics (satellite instruments) | 20-10 to 10-5 | Cannot be cleaned or replaced; contamination and defects accelerate degradation |
| Consumer optics (eyepieces, simple magnifiers) | 80-50 | Human eye tolerance; cost is primary driver |
| Fiber optic collimators | 40-20 | Back-reflection and insertion loss sensitivity |
Tighter scratch-dig specifications increase cost through multiple mechanisms:
| Specification | Relative Cost Factor | Why It Costs More |
|---|---|---|
| 80-50 | 1.0× (baseline) | Standard polishing; most blanks meet this easily |
| 60-40 | 1.0–1.2× | Minimal cost increase; still within standard process capability |
| 40-20 | 1.3–1.5× | More careful handling, additional inspection time, potential rework |
| 20-10 | 1.5–2.0× | Extended polishing, clean-room handling, 100% inspection under calibrated light |
| 10-5 | 2.0–3.0× | Precision polishing, class-100 cleanroom, multiple inspection passes, high yield loss |
The cost drivers are not just the polishing time. Tighter specs require:
Surface roughness (Ra or RMS) and scratch-dig measure different things, and both may need to be specified:
| Parameter | What It Measures | Unit | Typical Values |
|---|---|---|---|
| Scratch-Dig | Discrete defects (scratches, pits) — localized damage events | Dimensionless (comparative) | 60-40 to 10-5 |
| Surface Roughness (Ra) | Average height variation of the entire surface texture | Nanometers (nm) or Ångströms (Å) | 0.5–5 nm RMS typical |
| Total Integrated Scatter (TIS) | Total light scattered from surface roughness | Percentage or fraction | < 0.1% for polished optics |
An optic can have excellent scratch-dig (no visible defects) but poor surface roughness (high scatter), or vice versa. For laser and imaging applications, both parameters matter. Roughness controls diffuse scatter; scratch-dig controls discrete scatter sites.
For most polished optical surfaces, standard polishing produces Ra < 1 nm RMS. When scatter is critical, specify both scratch-dig AND surface roughness separately.
When writing scratch-dig into your optical drawing or purchase order, follow these best practices:
Scratch-dig inspection per MIL-PRF-13830B uses the following standard approach:
For automated or quantitative inspection, some manufacturers use dark-field microscopy with digital image analysis. This provides objective, repeatable measurements but requires correlation to the visual comparison standard for acceptance/rejection decisions.
Our optical engineers can review your application and recommend the appropriate scratch-dig specification — balancing performance requirements with manufacturing cost.
Request a Quote View Quality StandardsA 60-40 specification means: the widest allowable scratch has a width of 0.060 mm (scratch number 60 = 60 × 0.001 mm), and the largest allowable pit has a diameter of 0.40 mm (dig number 40 = 40 × 0.01 mm). This is the most common specification for general-purpose optics and provides a good balance between quality and cost.
Not necessarily. In imaging systems, tighter scratch-dig reduces scatter and improves contrast. But for a simple window used in a non-imaging alignment system, 60-40 and 20-10 will perform identically. The improvement only matters if surface defects are in the signal path of a scatter-sensitive application. Always tie the specification to a system-level requirement.
Both standards define surface quality for optical elements, but they use different numbering systems. MIL-PRF-13830B uses the traditional scratch-dig numbers (e.g., 40-20). ISO 10110-7 uses a numerical code (e.g., 5/40×20) where the first number indicates the total allowed defect area and subsequent numbers describe scratch and dig limits. When ordering from international suppliers, specify which standard you are referencing to avoid confusion.
Yes, significantly. Surface scratches and digs act as stress concentrators and local field enhancement sites where laser damage can initiate at lower fluence than the bulk material would otherwise support. For high-power laser optics, specify 20-10 or tighter, combined with high-quality polishing (Ra < 0.5 nm RMS) and IBS-deposited coatings. Many laser damage events originate at surface defects, not coating or substrate bulk.
Typically 50–100% increase in fabrication cost for the same optic. The cost comes from extended polishing time, more careful handling (clean-room conditions), 100% inspection under calibrated illumination, and lower yield. For a standard Ø25 mm BK7 plano-convex lens, the difference might be $10–20 per unit. For large optics (>Ø100 mm), the cost difference scales significantly.
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