Handling Rules
- Never touch optical surfaces with bare hands. Skin oils etch coatings permanently, especially AR coatings. Always wear powder-free nitrile or latex gloves.
- Hold optics by the edges only. Even with gloves, avoid contact with the coated surface. Use lens tissue or a clean lint-free cloth as a barrier when setting optics down.
- Work in a clean environment. A laminar flow bench is ideal. At minimum, work away from air vents, fans, and open windows. Airborne particles scratch coatings during cleaning.
- Keep optics in their containers until ready to install. Replace protective caps and containers immediately after use.
Inspection
Before cleaning, inspect the optic under a bright light at a shallow angle. This reveals dust, fingerprints, and coating damage. If the optic looks clean, do not clean it -- every cleaning cycle risks introducing new scratches. The cleanest optic is one you have not touched.
Cleaning Methods (Least to Most Aggressive)
1. Air Blow
Use filtered, dry compressed air or a rubber bulb blower to remove loose dust. Never use canned air directly on coated surfaces -- the propellant can leave residue. Hold the optic at an angle so particles fall away from the surface.
2. Drag Wipe
Place a fresh sheet of optical-grade lens tissue flat on the surface. Apply a few drops of spectroscopic-grade acetone or methanol to the tissue (not directly to the optic). Slowly drag the tissue across the surface in one direction only. Do not rub back and forth. Use a new tissue for each pass.
3. Drop and Drag
For stubborn contamination, place the optic on a clean surface coated side up. Drop a small puddle of reagent-grade acetone on the surface. Immediately drag a lens tissue slowly across, pulling the puddle edge to pick up contaminants. The key is that the solvent dissolves the contaminant and the tissue edge sweeps it off without pressing particles into the coating.
4. Solvent Bath (Last Resort)
Submerge the optic in spectroscopic-grade solvent (acetone, then isopropanol rinse). Use an ultrasonic bath at low power only if the coating manufacturer confirms it is safe. Many coatings, especially soft coatings, can delaminate in ultrasonic baths. Always test on a sacrificial optic first.
What NOT to Do
- Never use household glass cleaner (ammonia attacks coatings)
- Never use paper towels, cotton swabs, or regular tissues (too abrasive)
- Never blow on an optic with your breath (moisture and acid)
- Never rub dry -- always use solvent as a lubricant between the tissue and surface
- Never apply pressure -- let the solvent do the work
- Never reuse lens tissue -- fresh tissue for every pass
Storage
Store optics in their original containers with protective caps in place. If original packaging is not available, wrap each optic individually in optical-grade lens tissue, then place in a rigid container. Store in a low-humidity, temperature-stable environment. For long-term storage, a desiccated cabinet or nitrogen-purged container is ideal.
Avoid stacking optics. Contact between coated surfaces and other hard surfaces causes scratches. Use individual compartments or separators.
Supplies Checklist
- Optical-grade lens tissue (e.g., Thorlabs MC-5, Kimwipes alternative)
- Spectroscopic-grade acetone
- Reagent-grade isopropanol
- Powder-free nitrile gloves
- Rubber bulb blower (not canned air)
- Filtered compressed air (if available)
- Bright inspection light