Scratch-dig 60-40 vs 40-20—what's the difference in practice?
When comparing Scratch-Dig 60-40 vs 40-20 surface quality specs, it is important to know what these grades actually mean and how they affect your optical system and cost.
What is Scratch-Dig?
Scratch-dig is an industry standard for describing the surface quality of optics. Scratch refers to thin, linear marks. Dig refers to small pits or spots. These numbers appear as 'X-Y':
- First number (X): Maximum allowed scratch grade (visibility).
- Second number (Y): Largest allowed dig (in hundredths of a millimeter).
60-40 vs 40-20: What's the Practical Difference?
| 60-40 (Standard Quality) | 40-20 (Precision Quality) | |
|---|---|---|
| Allowed scratches | Brighter and more visible | Fainter and less visible |
| Allowed digs | Up to 0.40 mm | Up to 0.20 mm |
| Cost | Lower | Higher |
| Typical use | General imaging, lighting | Lasers, demanding optics |
Why Does Surface Quality Matter?
- Light scattering: Bigger or brighter scratches and digs scatter more light, lowering image quality and increasing stray light.
- Laser damage risk: Flaws can act as failure points in laser systems—tighter specs lower this risk.
- Cost impact: Tighter specs (lower numbers) cost more. Over-specification increases cost without benefit if not needed for your system.
How to Choose Between 60-40 and 40-20
- 60-40: Sufficient for most imaging, lighting, and general uses.
- 40-20: Best for laser optics, low-light work, or critical applications.
- Match your specification to your system's needs—don't pay for precision you do not benefit from.
Key Takeaways
- 60-40 allows brighter scratches and larger digs—standard for most jobs.
- 40-20 ensures finer, less visible flaws—for high performance and lasers.
- Only specify as tight a grade as you need for the best balance of cost and performance.
Frequently Asked Questions
https://www.kupooptics.com/en/blogs/q-a/scratch-dig
What does scratch-dig mean in optical filter specifications?
Scratch-dig is a surface quality specification for optical components. "Scratch" refers to linear surface defects; "Dig" refers to pits or small circular defects. In MIL-PRF-13830B notation (e.g., 60-40), the first number rates scratch severity and the second gives maximum dig diameter (in hundredths of mm).
What scratch-dig values are typical for optical filters?
Common scratch-dig specifications: 80-50 (lowest quality, for non-critical use), 60-40 (standard for general optics), 40-20 (precision optics, laser use), 20-10 (high-precision, sensitive detector windows), 10-5 (research grade, extreme quality). Lower numbers mean higher quality but higher cost.
When does scratch-dig matter most for optical filters?
Scratch-dig is most critical: near focal planes (where defects appear in focus), in high-power laser systems (surface defects can initiate laser damage), and in fluorescence microscopy (surface scatter creates background noise). For filters in collimated beams away from focal planes, 60-40 is usually sufficient.
How do I specify scratch-dig for my optical filter drawing?
Use MIL-PRF-13830B notation (e.g., S/D 40-20) or ISO 10110-7 notation. State the required quality separately for each optical surface. For filters, specify scratch-dig on both the entrance and exit surfaces. More stringent specs on the surface closer to the focal plane.
