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 (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 |
- 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.
- 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.
- 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.
