APPLICATION NOTE

Surface Quality Specifications: Understanding Scratch-Dig and Beyond

Q: What does '60-40 scratch-dig' mean?

Per MIL-PRF-13830B: max scratch width = 0.6mm (60 × 0.01), max dig diameter = 0.4mm (40 × 0.01). Lower = finer. 80-50 economy, 60-40 standard, 40-20 precision, 20-10 high precision.

Q: How is surface quality inspected?

Visual comparison against calibrated reference standards under dark-field illumination at 45°. Inherently subjective — specify quantitative Ra for critical applications.

Q: When to use ISO 10110-7 vs MIL-PRF-13830B?

ISO 10110-7 for European customers or ISO quality systems. MIL-PRF-13830B for US military/aerospace. Many modern specs reference both.

Q: Does surface quality affect laser performance?

Yes — scratches reduce laser damage threshold. A 20-10 surface can have 2-3x higher LDT than 60-40 at the same coating.

Q: How much does tighter surface quality cost?

80-50 to 60-40: +10-15%. 60-40 to 40-20: +30-50%. 40-20 to 20-10: +100-200%. Always specify the loosest grade your application allows.

Q: Can I specify surface quality without scratch-dig?

Yes. ISO 10110-7 offers 'N' classification. You can also specify surface roughness (Ra) or total integrated scatter (TIS) instead.

Complete guide to optical surface quality — from MIL-PRF-13830B scratch-dig to ISO 10110-7. Learn to specify the right quality without over-specifying cost.

QualitySurface FinishScratch-DigISO 10110

Published: August 23, 2026By PhotonEdge Engineering Team~8 min read

What Are Scratch-Dig Numbers?

MIL-PRF-13830B uses two numbers: scratch width (× 0.01mm) and dig diameter (× 0.01mm). '60-40' means max scratch width 0.6mm, max dig diameter 0.4mm.

First Number: Scratch

Scratch number × 0.01mm = max width. "60" = 0.6mm max width.

Second Number: Dig

Dig number × 0.01mm = max diameter. "40" = 0.4mm max diameter.

Common Grades & Cost Impact

Grade	Use	Cost	Applications
80-50	Economy	Base	Windows, beam steering
60-40	Standard	+10-15%	Imaging (f/8+), mirrors, prisms
40-20	Precision	+30-50%	Imaging (f/4+), laser optics
20-10	High precision	+100-200%	High-power laser, lithography

Rule of Thumb: Every step finer adds 30-100% polishing cost. 60-40 to 40-20 is the most common over-specification trap.

When Does It Matter?

High impact: High-power laser (scratches reduce LDT), precision imaging at f/2 or faster, interferometric surfaces.

Low impact: Non-imaging windows, mirrors at f/8+, prisms in fold mirrors, any defect smaller than the Airy disk.

Beyond Scratch-Dig

Surface roughness (Ra/RMS): Measured with profilometer. 0.5-2nm standard, <0.2nm superpolish. Directly correlates with scatter.
Total Integrated Scatter (TIS): Ratio of scattered to total reflected light. TIS < 0.1% for precision optics.
ISO 10110-7 'N' count: Specifies max defects per area. More objective than scratch-dig for production QC.

Practical Advice

Recommendation: For most commercial applications, 60-40 with visual inspection per MIL-PRF-13830B is sufficient. Only move to tighter grades when your application specifically requires it. For production consistency, add quantitative Ra or TIS alongside scratch-dig.

Frequently Asked Questions

Common questions from optical engineers and procurement teams.

What does '60-40 scratch-dig' mean?+

Per MIL-PRF-13830B: max scratch width = 0.6mm (60 × 0.01), max dig diameter = 0.4mm (40 × 0.01). Lower = finer. 80-50 economy, 60-40 standard, 40-20 precision, 20-10 high precision.

How is surface quality inspected?+