What are best practices for cleaning optics?

Technical Discussions
1

I have a aspherical lens made from fused silica with an aluminum based anti-reflective coating. How do I clean fingerprints without damaging it? What materials / cleaning chemicals are best to use?

2

Thanks for the question. The specific substrate and any optical coating, size of the optic and type/degree of damage is crucial to consider to determine the appropriate cleaning of optics. The chemical resistance and durability of the optic differs depending on its substrates and optical coatings.

A couple of general principles always apply:

  1. Clean the work area prior to cleaning
  2. Always wear gloves when handling optics
  3. Always use specific lens tissues recommended by the manufacturer
  4. Always use appropriate cleaning solution / glass cleaning solvent.

AR coatings are extremely thin and depending on the type, vary in hardness. The cleaning solution may consist of a mixture of distilled water, acetone , isopropanol and methanol. The addition of methanol slows down the evaporation of the solvent and thus increases the cleaning effect but the thin nature and microstructure of AR coatings calls for a gentle cleaning solution to avoid that the harsher solvents attack the film structure and begin to stress the film over time. In the first instance, I would advise to mix 1 part isopropyl alcohol to 3 parts clean distilled water.

The cleaning technique will depend on the degree of dirt.
First use compressed air to remove any dust and loose particles.
It may be sufficient to use a ´drag and drop´ technique. In this method the lens is placed on a lens tissue on a flat surface. A second tissue is placed over the lens and a few drops of cleaning solution is used to moisten it. Gently drag the lens tissue of the lens and repeat with a clean lens tissue until clean.

If a more rigorous cleaning is needed a brush technique can be used. Here the lens is again placed on a lens tissue on a flat surface. A pair of tweezers is used to fold a second lens tissue and hold it perpendicular to the optic. Moisten the folded lens tissue with cleaning solvent and apply light pressure to brush the folded tissue over the optic. For smaller optics the use of special cleaning swaps may be required.

After the substrate is dried, examine the coated optic with standard ambient lighting against a black background. To remove any streaks, repeat the cleaning process using isopropyl alcohol. Hope it helps!

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3

If, I recall cleaning solutions can be :
DI tap with 2% mild soap
Methanol
Acetone
Alcohol
Methods can be with:
lint free wipe- drap and drop
Lint free swabs for smaller optics
If you have the right vessels ultrasonic cleaning
Preferably in a clean room environment while wearing powder free gloves.

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4

I agree with you. This wa a more elaborate explanation than my own…lol