Cleaning Bare Gold Coatings

Dust and stains on optics can cause scattering, and impurities on the optic surface can react with incident laser light to damage optical coatings. With proper handling and cleaning of your optics, you can prevent damage and ensure their continued performance. If it’s not dirty, don’t clean it! Although this tip applies to all optics, it is particularly important with optics that are coated with relatively soft metals such as Gold. Handling optics increases their chances of getting dirty or damaged, so you should clean optics only when necessary.

General Tips

You should handle optics in a clean, low-dust environment while wearing powder-free acetone-impenetrable Gloves or Finger Cots. Since oil and debris from your hands or from used lens tissue can stain or damage optical coatings, you should not touch any transmissive or reflective surface of your optic and never reuse a lens tissue. Remember that lens tissues are inexpensive compared to the price of an optic.

Inspect an optic for dust and stains by holding it near a bright visible-light source. Viewing the optic at different angles allows you to see scattering from dust and stains.

Step 1: Use a clean-air duster

Dusting is always the first step in cleaning your optics. Wiping a dusty optic is like cleaning it with sandpaper. So always dust with either a canned air duster, or low-pressure compressed and filtered air or nitrogen before wiping any optic. If the dusted optic has no visible stains after you dust it, remember: “If it’s not dirty, don’t clean it.” If it’s still not clean, proper use of solvents and lens tissue can often do the trick.

Step 2: Use solvent and lens tissue

The way to use them depends on the optic, but always wipe slowly and clean the edges first.

Glass-cleaning solvents will streak, and tissue paper or a t-shirt will scratch, so always clean bare gold coated optics with reagent or spectrophotometric grade methanol and low-lint tissue manufactured for cleaning optics. Methanol will absorb water from the air so only use methanol that has been stored in a tightly-sealed container and that is relatively fresh. Cleaning your optic’s edges before cleaning its faces prevents dirt from being drawn up onto the face. Wiping slowly allows the solvent to evaporate without streaking. Remember, slow and steady cleans the optic. The method that follows was specifically developed for cleaning of bare-gold coated retroreflectors but the basic process can be adapted for cleaning of other types of optics.

Solvent Cleaning Method

NOTE: Minor sleeking resulting from cleaning is almost unavoidable on soft metal coatings. Severe contamination may not be removable.

  1. Take fresh piece of optics cleaning tissue and fold several times so there is a square corner with an edge at least the width of the surface to be cleaned.
  2. Generously wet the edge of the tissue with methanol.
  3. Place the corner of the tissue in the center of the reflector parallel to one facet, and without exerting direct pressure on the surface of the facet through the tissue, slowly drag the edge of the tissue outward and over the edge of the facet.
  4. Repeat this process for each facet, each time with a fresh, wet tissue.
  5. Always use a fresh wet tissue for each wipe on each facet. (Do not be tempted to turn the tissue over in order to use the other side!)
  6. Do not use a scrubbing motion at any time. Do not bear down on the coated surface at any time.
  7. Keep repeating the wiping until the surface meets the cleanliness requirement, and blow off any remaining lint from the tissue.
  8. It may be necessary to adjust the wetness of the wiping tissue, and the pressure used in order to minimize solvent streaking. The least amount of wiping is safest for the coating.