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While First Contact is a great tool, it is by no means the only way to clean optics, and some optics can actually be damaged during the application of First Contact if the optic is really dirty. Additionally, If not allowed to dry  properly, First Contact can leave a thin film that is invisible without magnification, at least until you hit it with a multi- watt laser beam. 

I use laser systems that are both hermetically sealed and also laser systems that have fans blowing right into them. 

High purity solvents such as High Resolution Gas Chromatograph grade ( or better) Acetone and Methanol are still the best way to clean precision optics using one of a few methods.  I am sure I will get a lot of replies telling me I'm an idiot and I should just use First Contact, but I will say in advance that those of you that have cleaned a laserscope High Reflector under a microscope will vouch for what I am saying.

The first step in cleaning any optic is to gently blow the loose dust off, and this is where a good squeeze bulb works best, as most "canned air" is not air, and in fact often contains corrosive halogen gases!!!

The key thing to remember with high purity solvents is that they tend to be hygroscopic and can pull moisture out of the air. I do not recommend using the same bottle of solvent for more than 4-6 months. After that time, I tend to use the older bottles for bounce mirrors and other non critical components. 

Thousands of brewster windows, PCAOM crystals, and other high precision optics have been cleaned using these methods for many decades. 

Here is the text out of the instruction manual for my optics cleaning kits- Sections of this were printed in the ILDA publication, "The Laserist", back in the late 90's. 


Before beginning to work on any optic, make sure that you have a clean work space. It is a good idea to put a lens tissue on the surface in case the optic is dropped or if you need to set the optic down before cleaning is complete. 

 There are three basic methods for cleaning an optical surface:
 1. Drag and Drop
 2. Hemostat and tissue
 3. Cotton swab

Prior to handling any optic, be familiar with all components in the cleaning kit. Some components may cause damage if used improperly.

Always be sure to check the specs or consult the manufacturer before applying any solvent. Some specialty coatings may not be compatible with solvents such as Acetone or Methanol.  For these optics, high purity water is necessary.

Never handle an optic by its surface. Always hold by the edges or preferably while the optic is mounted. 

Before cleaning any optic, you must first evaluate its condition.  Examine the surface of the optic under a bright light to see if there are any scratches or abrasions.  If the optic has visible dust or dirt on it, use a rubber puff bulb to gently blow the dust off. 

Each method of cleaning optics can use either Acetone, Methanol or Water.  Methanol is used most often, as Acetone is usually only needed on especially dirty or hard-to-clean optics.  Evaluate which solvent is necessary before beginning any cleaning procedure.   

Once you have decided what solvent to use, estimate how much of it you will need based on number of optics, how dirty they are, etc. Carefully dispense a small amount of acetone or methanol at a time into their respective dropper bottles. This is to prevent the main bottles from getting contaminated from constantly being opened.  Be careful not to store any solvents in these eyedropper bottles as they could degrade the rubber dropper part if stored for long periods of time.  Never pour unused solvents back into the main bottles! 


 1. The Drag and Drop method:

Lay down at least one lens tissue on a flat, secure surface to form an optically-safe ( dirt free ) work surface
While holding the optic, use the rubber blower to gently blow off any dust or dirt that may be on the optic’s surfaces. This is to prevent the lens tissue from dragging any dirt particles across the surface of the optic. 

Then gently place the optic face up on the lens tissue. For some optics that both reflect and transmit,such as beamsplitters and dichroics,  both sides will need to be cleaned. Always begin by cleaning the first surface side  before gently flipping the optic over and cleaning the secondary surface. 

After the optic has been placed flat on the tissue covered surface, gently place a lens tissue on top of the optic.  Place the tissue so that the edge facing you is mostly hanging off the other end of the optic, so it can be gently dragged towards you to clean the surface. This is the reason it is imperative that the work area near the optic is covered with tissues. When handling the tissues, be sure to use the hemostat or to only touch the material on the edges. 

Once the tissue is properly placed on the optic, use the small eyedropper to place 1-2 drops of methanol on the tissue, centered on the optic. 

Then proceed to gently drag the lens tissue straight towards you, over the optic.  Repeat as necessary and then repeat for the opposite side of the optic if necessary. 


 2. The Hemostat and tissue method:

Lay down at least one lens tissue on a flat, secure surface to form an optically-safe work surface
While holding the optic in hand, use the rubber blower to gently blow off any dust or dirt that may be on the optic’s surfaces. This is to prevent the lens tissue from dragging any dirt particles across the surface of the optic. 

Handling by the edges or using a hemostat, fold a lens tissue in half longways twice to form a long strip. Then fold the tissue strip over to shorten the length of the strip. Take the hemostat and gently clamp it into the tissue so that the tissue covers the end of the hemostat. This is to ensure that the hemostat cannot slip and damage the surface of the optic. 

To clean the optic, place a few drops of either acetone or methanol on the end of the tissue on the portion that will contact the optical surface.  Gently drag the tissue over the surface to be cleaned while taking care to avoid putting too much pressure on the optic.  Repeat as necessary.

This method is most commonly used on critical cleaning applications such as laser cavity optics and brewster windows. 



 3. The Cotton Swab method:

Lay down at least one lens tissue on a flat, secure surface to form an optically-safe work surface
While holding the optic in hand, use the rubber blower to gently blow off any dust or dirt that may be on the optic’s surfaces. This is to prevent the cotton swab from dragging any dirt particles across the surface of the optic. 


Using a small eyedropper, place 1-2 drops of acetone or methanol onto the tip of the cotton applicator swab.  Never dip a swab into the solvent bottles! Then while gently rotating the swab counterclockwise pass it over the optic away from you. Apply very little pressure to the surface of the optic.  The rotation is important as it prevents the same spot on swab from being dragged over the entire surface, which could trap dust particles between the swab and the optic’s surface.  Never pass a cotton swab over an optic more than once. Always use a fresh swab for each pass over the optic. It is normal to use a lot of swabs when using this technique cleaning an entire laser projector.  A good practice to avoid reusing a swab is to snap it in half when setting it down to get a new swab.

Karl Rothweiler 
Laser Wizardry

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 mixedgas :Steve's mods to Karl's most excellent post:

When dropping and dragging, you wait until the drop of solvent on the optic is starting to evaporate.  Then you slowly pull the tissue across the lens, watching the white line that forms where the solvent is rapidly evaporating. If you don't get the "white line" you slow down the dragging The "Interface" between the wet tissue and the drying tissue is where the cleaning occurs. The "crud" does not stick to wet tissue, it sticks to dry tissue. The solvent wets, dissolves, and "picks up" the crud.

I know that sounds like a art, but if you do it one or two times on a dry day, as practice, it becomes obvious.


 Steves other mod:

Use cotton swabs on wood sticks, without glue. Do NOT use VCR cleaning swabs with the foam.
Sticks with foam heads and swabs that have glue, cardboard sticks, or plastic sticks can leave plasticizers on the optic.  Standard throat swabs from a pharmacy have no glue and wood sticks, and are very inexpensive. Sometimes you can get the excellent ones that are individually wrapped like chopsticks in 
sterile paper packages.

Tip two, flush the swab with solvent once before cleaning, then "fling" the excess solvent off. Then rewet the swab with fresh solvent. This cleans the cotton before it cleans the optic.

Tip three, never use a eye-dropper to dispense your solvent, the rubber in the eye dropper will mess up your nice pure solvent. Instead, pour the solvent onto the swab, or get a lab grade pump dispenser, designed for the task. Spectra Physics used to think this was so important that they used to ship the dispenser with ion lasers.

It is, as Karl stated, important to not back contaminate your solvents.

And I agree, it is best to design your projectors to keep smoke and dust off the optics.  Once a optic has micro-scratches, you never get a "clean" beam off it.  Diffraction from dirty optics looks like crap.  If you cannot do that, at least pressurize them with HEPA filtered air. A aquarium pump with a good filter can do wonders for keeping set of galvo mirrors clean. AR coated windows are inexpensive. 

Steve

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 Why to not use drug store IPA on your optics:

Rubbing Alcohol and all preparations under the classification of Rubbing Alcohols are manufactured in accordance with the requirements of the U.S. Treasury Department, Bureau of Alcohol, Tobacco, and Firearms,Formula 23-H (8parts by volume of acetone, 1.5parts by volume of methyl isobutyl ketone, and 100parts by volume of ethyl alcohol) being used. It contains not less than 68.5 percent and not more than 71.5 percent by volume of dehydrated alcohol,the remainder consisting of water and the denaturants, with or without color additives,and perfume oils. Rubbing Alcohol contains, in each 100mL, not less than 355mg of sucrose octacetate or not less than 1.40mg of denatonium benzoate. The preparation may be colored with one or more color additives, listed by the FDA for use in drugs. A suitable stabilizer may be added. 

mixedgas

Bottom-line: 'Rubbing Alcohol: Ideal for cleaning buttocks prior to injections, Not-ideal for cleaning laser optics / mirrors / dichros.'

gringoloco
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