Cleaning Procedures for Infrared Optics

Preparation

NOTE!:

The following description for to clean IR optics does not apply to water soluble crystals, highly hydroscopic materials, or certain other specialized materials:

Some examples are:
KBr
NaCl
KCl
CsI
KRS-5

Simply put, these optics are cleaned as part of the polishing process, and would be damaged by further cleaning attempts. They will be severely damaged or degraded by water, or by rubbing with acetone or alcohol cleaning products.
At best you can lightly brush them in order to remove dust or lint. When they arrive at your facility, they should be clean. Take great care in keeping them clean.
Clean foam mat or other suitable cushioning surface. Finger cots or gloves ( latex or cotton). Lens tissue or Tex-wipe, Squeeze bulb, or dry filtered air or Nitrogen. De-ionized water, Reagent grade Methanol, Acetone or Isopropyl alcohol.

Handling Optics

Before handling or touching optics, it is very important to understand basic physical characteristics of the material. Infrared optics can be brittle or soft and can be easily damaged while handing or transporting. Even if it seems negligible, slight damage can propagate into a crack or ruin the performance of optics in a system. Oils from fingerprints can easily damage or etch the surface of an optic and often the damage cannot be removed. Therefore you should not handle optics with your bare hands. Even if you wear finger cots or gloves avoid touching the surface of the optics. Be certain that your gloves are clean and free of contamination before cleaning. Allways hold optics on the edges or non optical surface.

Cleaning of Coated Optics

Start by blowing of the surface of the optic to remove dust or particulates which may scratch the surface.
The type of contamination on the surface will determine what solvent will be most effective for removal. De-ionized water may be suitable for common dirt or marks. Organic solvents may be needed to remove finger prints, epoxies or resins from mounting. Use a lens tissue or Tex-Wipe and thoroughly moisten with the appropriate cleaner. Gently wipe surface in a linear motion slowly pulling wipe off opposite edge. If this is not effective, wipe surface in a circular motion slowly pulling wipe towards the edge
and then off
If this is still not effective contact ISP for further information.

Inspection or Viewing of Optics

These are common inspection rules regarding physical evaluation of surface quality and inspection methods, which most optics companies follow. According to para. 4.5.3.1 MIL-O-675, optical component shall be inspected using 15 watt cool white fluorescent light tubes and the viewing distance shall be 45.7cm or less. Also it indicates how to examine optics at a appropriate angle.
For more information about coating surface evaluation, please check the following MIL-specifications:
  • MIL-C-675 - Coating of glass optical elements
  • MIL-M-13508 - Mirror, front surface aluminized: for optical elements
  • MIL-C-14806 - Coating, reflection reducing, for instrument cover glasses and lighting wedges
  • MIL-C-48497 - Coating, single or multilayer, interference, durability requirements
  • MIL-F-48616 - Filter (coatings), infrared interference: general specification