The delicate nature of optical components requires special procedures to be followed in order to maximize their performance and lifetime. Through everyday use, optics can come into contact with contaminants such as dust, water, and skin oils. These contaminants increase scatter off the optical surface and absorb incident radiation, which can create hot spots on the optical surface, potentially leading to permanent damage. Optical components with coatings are particularly susceptible to such damage.
This guide outlines the common handling and cleaning procedures applicable to various optical components. Due to variations in materials, size, and delicacy of optical components, it is crucial to use the correct handling and cleaning methods. What works for one type of optical component may damage another. Therefore, we recommend reading this guide in its entirety before cleaning an optic.
Handling
Proper handling techniques minimize the need for frequent cleaning and help extend the lifetime of optical components. Always unpack or open optics in a clean, temperature-controlled environment. Never handle optics with bare hands, as skin oils can permanently damage the optical surface quality. Instead, wear gloves; alternatively, for smaller optical components, optical or vacuum tweezers may be useful. If possible, handle optics only by non-optical surfaces, such as the ground edges of the optic.
Important Handling Considerations
The optical surface of holographic gratings, ruled gratings, first surface unprotected metallic mirrors, and pellicle beamsplitters should never be touched by hands or optical handling instruments as they are extremely sensitive to physical contact.
Many crystals, such as calcite polarizers, beam displacers, lithium niobate wafers, and EO modulators, are temperature-sensitive and can crack if exposed to thermal shock. Always allow the package and contents to reach thermal equilibrium before opening. These crystals are also softer than conventional optics and require extra care when cleaning.
Storage
Never place optics on hard surfaces, as contaminants on either the optic or the surface can cause scratches. Most optics should be wrapped in lens tissue and stored inside an optics storage box designed for their protection. The storage environment should have low humidity, low contaminants, and stable temperatures. Proper storage prevents scratches, contamination, and damage to optical coatings, some of which are hygroscopic.
Inspection
Optical components should be inspected before use, as well as before and after cleaning. Since contaminants and surface defects are often microscopic, using a magnification device can be helpful. Additionally, shining a bright light onto the optical surface enhances specular reflections from surface contaminants and defects.
Inspection Techniques
When inspecting a reflectively coated surface, hold the optic nearly parallel to your line of sight to view contamination rather than reflections.
Polished surfaces, such as lenses, should be held perpendicular to your line of sight for easier examination.
For assessing surface defects, a scratch-dig paddle can be used to compare the size of observed defects with calibrated defect samples. If a defect exceeds the manufacturer’s scratch-dig specification, replacing the optic may be necessary to maintain optimal performance.
Before cleaning, inspect the optic to determine the type and severity of contaminants. Cleaning often involves solvents and physical contact with the optical surface, so repeated cleaning should be avoided to minimize potential damage.
For optics with multiple contaminants, the removal order matters. For example, if an optic has both oil and dust, wiping off the oil first may drag dust particles across the surface, causing scratches.
Blowing Off the Surface
Dust and loose contaminants should be removed using a canister of inert dusting gas or a blower bulb before employing any other cleaning method. Avoid using your mouth to blow on the surface, as saliva droplets can deposit on the optic.
Caution
The thin Nitrocellulose membrane on pellicle beamsplitters is extremely fragile and can be broken by the force of air. Maintain a safe distance when using canned air.