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Optics Cleaning

Handling and Cleaning Procedures for Optical Components

The delicate nature of optical components requires that special procedures be followed in order to maximize their performance and lifetime. Through everyday use, optics can come in 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, resulting in permanent damage. Optical components with coatings are particularly susceptible to this sort of damage.

The content of this guide covers common handling and cleaning procedures that are applicable to many optical components. Due to variation in materials, size, delicacy, etc. of optical components, it is important that the correct handling and cleaning methods are used. What is okay for one type of optical component will destroy another type of optical component. Because of this, we recommend that the guide be read in its entirety before cleaning an optic. If the type or category of optic is not specifically mentioned in the guide, please contact the optical component manufacturer for handling and cleaning instructions.

Handling

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By practicing proper handling techniques, you will decrease the necessity to clean your optics and thus maximize their lifetime. 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, it may be helpful to use optical or vacuum tweezers. Independent of the method used to hold the optic, if at all possible, only hold the optic along non-optical surfaces, such as the ground edges of the optic.

Important: The optical surface of holographic gratings, ruled gratings, first surface unprotected metallic mirrors, and pellicle beamsplitters (this is not an exhaustive list) should never be touched by hands or optical handling instruments. They are extremely sensitive, and any physical contact will cause damage.

Caution: Most crystals (e.g., calcite polarizers, beam displacers, lithium niobate wafers, and EO modulators) are temperature sensitive and can crack if exposed to thermal shock. Therefore, it is important to always allow the package and contents to come to thermal equilibrium prior to opening. These crystals are also much softer than conventional optics, and thus, need to be handled more carefully when cleaning.

Storage

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Never place optics on hard surfaces because any contaminant on the optic or the surface will be ground in. Instead, most optics should be wrapped in lens tissue and then stored inside an optic storage box designed for the optic. Typically, the box should be kept in a low humidity, low contaminant, and temperature-controlled environment. Optics are easily scratched or contaminated, and some optical coatings are hygroscopic, so proper storage is important for preserving the optical component.

Inspection

In general, optics should be inspected prior to use and before and after cleaning. It is often necessary to use a magnification device when inspecting an optical component due to the small size of most contaminants and surface defects. Even with a magnification device, it is sometimes useful to shine a bright light onto the optical surface in order to increase the intensity of the specular reflections from surface contaminants and defects.

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When inspecting a reflectively coated surface, the optic should be held nearly parallel to your line of sight. By looking across the surface rather than directly at it, you will see contamination and not reflections. Polished surfaces such as lenses should be held perpendicular to your line of sight so that you can look through the optic.

If a surface defect is located on a clean optical surface, a scratch-dig paddle can be used to categorize the size of the defect by comparing the size of the calibrated defects on the scratch-dig paddle to the size of the defects on the surface of the optic. If the size of the defect on the surface exceeds the manufacturer’s scratch-dig specification, it may be necessary to replace the optic in order to achieve the desired performance.

Cleaning Procedures

Always read the manufacturer’s recommended cleaning and handling procedures if available. Since cleaning an optic almost invariably involves handling it, please make sure to follow the proper handling procedures at all times when using the cleaning guideline discussed below. Optics can be permanently damaged if cleaned or handled incorrectly.

Before cleaning an optic, take time to inspect the optic in order to determine the type and severity of the contaminants. This inspection step should not be skipped because the process of cleaning the optic often involves solvents and physical contact with the optical surface, which can result in damage to the optical surface if repeated too frequently.

For optics with multiple contaminants, the order with which they are removed can be important so that the optical surface is not damaged by one contaminant while removing a separate contaminant. For instance, if an optic is contaminated with oil and dust, it is possible that wiping the oil off first will scratch the optical surface as the dust is drug along the surface by the wipe.

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Blowing Off the Surface of an Optic
Dust and other loose contaminants usually should be blown off before any other cleaning technique is employed. A canister of inert dusting gas or a blower bulb is needed for this method. Do not use your mouth to blow on the surface because it is likely that droplets of saliva will be deposited on the optical surface.

If you are using inert dusting gas, hold the can upright before and throughout the procedure. Do not shake the can prior to or during use. Also, start the flow of gas with the nozzle pointed away from the optic. These steps help prevent the deposition of the inert gas propellant on the optical surface. If using canned gas, hold the can roughly 6” from the optic and use short blasts. Wave the nozzle of the inert gas can over the optic with the nozzle at a grazing angle to the optical surface. For large surfaces, trace a figure-eight pattern over the optical surface.

This cleaning method can be used on almost all types of optics. However, for some optics such as holographic gratings, ruled gratings, unprotected metallic mirrors, calcite polarizers, and pellicle beamsplitters, which can be damaged by physical contact, this is the only approved method for cleaning. Due to the non-contact and solvent-free nature of this cleaning method, it should be used as a first step in cleaning almost all optics.

Caution: The 2 μm thick Nitrocellulose membrane on pellicle beamsplitters is extremely fragile and easily broken by the force of air on the surface. If using canned air with these optics, ensure that the bottle is sufficiently far away so as not to break the membrane.

Caution: The polished escape face on calcite polarizers is very delicate and can be damaged by blowing air too directly at the surface.

Alternative Cleaning Methods

If blowing off the surface of the optic is not sufficient, the following are other acceptable cleaning methods and materials. When cleaning an optic, always use clean wipes and optical grade solvents to prevent damage from contaminants. Wipes should always be moist with an acceptable solvent and never used dry. Acceptable wipes (in order of softness) are pure cotton (such as Webril Wipes or Cotton Balls), lens tissue, and cotton-tipped applicators.

Typical solvents employed during cleaning are acetone, methanol, and isopropyl alcohol (isopropanol). Use all solvents with caution since most are poisonous, flammable, or both. Read product data sheets and MSDS sheets carefully before using any solvents.

Washing the Optic
If approved by the manufacturer, fingerprints and large dust particles can be removed by immersing the optic in a mild solution of distilled water and optical soap. The optic should not remain immersed any longer than necessary to remove the contaminants. Afterwards, rinse the optic in clean distilled water. Depending on the optic, the Drop and Drag or Lens Tissue (applicator) methods can be used to apply a quick-drying solvent like acetone or methanol to the optic to accelerate drying. Avoid pooling of any cleaning solutions as they dry because that tends to leave streaks on the optical surface.

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Drop and Drag Method
The Drop and Drag Method can be used for cleaning flat optical surfaces that are elevated above any surrounding surfaces. First, inspect the optic to determine the location of the contaminants. This allows you to plan your drag so that the contaminant is lifted from the surface of the optic as soon as possible instead of being dragged across the surface of the optic. After inspection, place or hold the optic so that a weak lateral force on the surface will not cause the optic to move. Take a fresh, clean sheet of lens tissue and hold it above (not in contact) the optic so that as you pull the lens tissue it will be drawn across the optical surface. Next place one or two drops of an approved quick drying solvent on the lens tissue being held above the optic. The weight of the solvent will cause the lens tissue to come into contact with the optical surface. Slowly but steadily drag the damp lens tissue across the optic being careful not to lift the lens tissue off of the surface. Continue dragging the lens tissue until it is off of the optical surface.

The correct amount of solvent will keep the lens tissue damp for the entire drag but not leave any visible trace of solvent on the optical surface after the drag is finished. Inspect the optic and repeat if necessary, but only use each sheet of lens tissue once. This cleaning method is preferred by many since the lens tissue is only in light contact with the optical surface. This method can be used successfully to remove small adhered particles and oils from an optical surface. Heavy concentrations of contaminants often require repeated treatments.

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Lens Tissue with Forceps or Applicator Method
This method is often used with mounted or curved surface optics that require cleaning with a solvent. Inspect the optic to locate the sources of contamination. Plan a wiping path that will not result in dragging any large contaminants over more of the optical surface than is necessary. If lens tissue is used, it is important to fold the tissue in such a way that the portion of tissue that comes into contact with the optic is not touched. Clamp the folded lens tissue with forceps in such a way that a smooth wipe over the optical surface can be executed. Next, apply a couple of drops of solvent to the lens tissue. The tissue should be damp, but not dripping. If too much solvent was added, safely shake the excess solvent from the lens tissue. The lens tissue should now be wiped over the optical surface in a smooth motion.

During the wipe, continuously, but slowly, rotate the lens tissue. This will continuously change the portion in contact with the optical surface, which will rotate upward and away from the surface any accumulated contaminants. After the wipe, inspect the optic for any remaining contaminants or streaks and repeat the cleaning procedure if necessary with a new lens tissue. Streaks tend to form if too much solvent was on the lens tissue or on the optical surface where the edge of the tissue was wiped. If streaks are forming at the edge of the lens tissue, choose a larger applicator or plan a continuous wiping path that eliminates the wiped interface on the optical surface. If a spiral or snaking wipe path is used, it might be necessary to use a slower drying solvent so that the optical surface doesn’t dry until the wipe is complete.

Cleaning with Webril Wipes
Webril Wipes are soft, pure-cotton wipers that are highly recommended for cleaning most optics. They hold solvent well, do not dry out as fast as lens tissue or cotton-tipped applicators, and do not fall apart quickly like some other wipes. The outside edges of these wipes may leave some lint, so always use a folded edge when cleaning.

For smaller optics, roll the Webril wipe into a cone with the folded edge at the point, moisten the tip with a solvent and use the point as the wiping area. For larger optics, first cut the wipe into three pieces that are approximately 2.6” x 4”. Fold the wipe length wise so that it now measures 1.3” x 4”, and then make a fold approximately 1” from the end. Moisten the final folded edge with solvent and use that edge to wipe/clean the optical surface. Using a pump bottle to dispense the solvent will make it easier to hold the optic in one hand while moistening the wipe with the other.

During cleaning, wear gloves or finger cots. Pick up the optic in one hand and then wipe the Webril wipe lightly, continuously, and slowly across the entire surface of the optic so as to avoid streaking. You may need to adjust the amount of solvent, pressure applied to the wipe, and/or speed of the wipe to avoid streaks. Wiping times will also vary with solvent. For instance, if using acetone, you would need to wipe slightly faster than if you used alcohol since acetone dries faster.

Optic Handling and Cleaning Tools

1. Gloves: Gloves are important when handling almost any unmounted optical component. Typically, gloves for handling optics are either cotton or powder-free latex.
2. Tweezers: Optical Tweezers and Vacuum Tweezers are commonly used to handle smaller optics. Optical tweezers are designed to hold small hard objects without slipping and with increased tactile feel. In addition, the specially designed tips of optical tweezers are made from a material (like carbon resin) that reduces the risk of scratching the optical component. Vacuum Tweezers use a suction cup to hold an optic. They usually have a variety of tips that are specialized to hold certain shapes and sizes of optical components. In addition, since a vacuum is being used to hold the optic, many users find it easier to handle the optic because they do not need to be concerned about maintaining the proper pressure on the optic as is necessary with conventional optical tweezers.
3. Webril Wipes: Fabricated from pure cotton, these wipes can be used to clean optics or any other surface. Although the edges may leave lint particles behind, this can be avoided if they are folded and the folded edge is used to clean the optic. These wipes can also be used as a soft surface to lay optics on.
4. Lens Tissue: Lens tissue is used to handle and clean optics since it offers a soft non-abrasive surface that can safely come into contact with many types of optical surfaces. Lens tissue is also often used to wrap optical components before placing the optic in a storage box.
5. Optic Storage Boxes: Optic Storage Boxes typically have foam or molded plastic inserts. These inserts ensure that the optic will not move around in the box and that the optical surfaces do not come in to contact with a hard surface. Most optics should be wrapped in lens tissue prior to storage in an optic storage box. For smaller optics, it is usually easier to fold lens tissue over the optic rather than wrapping it.
6. Magnifiers: Magnifiers and loupes allow the user to examine smaller optics closely. They are useful in determining the cleanliness and integrity of optical surfaces, which aids in determining the proper cleaning procedure. Alternatively, if the optic appears damaged under a magnifier, the optic may need to be replaced.
7. Scratch-Dig Paddle: Most optics have manufacturer’s specified Scratch-Dig tolerance, which categorizes the optical quality of a surface. A Scratch-Dig Paddle has a series of calibrated optical defects which aid in determining how thick or deep a scratch is. By comparing the defects on the scratch-dig paddle to an optic, it is possible to determine if the optic meets the manufacturer’s specifications. If the specifications are not met, the optic may need to be returned or replaced.
8. Inert Dusting Gas: Pressurized Inert Dusting Gas is great for cleaning off dust and other contaminants that are not adhered to the optical surfaces. An alternative source of air is a bulb blower. Pressurized inert dusting gas can provide a sustained stream of pressurized gas with which loose contaminants can be blown from the optical surface. However, because the gas is released from a pressurized canister it is often cooler than the surrounding environment and as a result can cool the surface of the optic. In addition, the stream of pressurized gas may contain droplets of the canister propellant that can be deposited on the surface of the optic. A blower bulb avoids the temperature and propellant issues but the air blown onto the optic surface will contain contaminants that may or may not stick to the surface. Blowing off a surface is the only acceptable cleaning method for some types of optics that have surfaces that cannot be touched.
9. Forceps: Forceps are a small, lockable clamp that is often used to hold lens tissue when it is used in some cleaning procedures. Since forceps can easily scratch an optical surface, it is important to make sure forceps never come in contact with the optic.
10. Cotton-Tipped Applicators (Some users may decide to avoid forceps and use Cotton-Tipped Applicators as an alternative. These are typically 6″ (15 cm) wooden sticks with cotton on one end to be used as an applicator for various cleaning agents. These applicators are especially useful for cleaning small optical surfaces like connectorized fiber tips. For larger surfaces it can be difficult to get a streak free finish. Note: cotton-tipped applicators are NOT Q-Tips or other drugstore applicators. These are optical grade applicators that will not deposit contaminants on an optic and the applicator material (frequently cotton) is free from abrasive fibers found in drugstore applicators.
11. Optical Cleaning Solvents: There are a variety of optical cleaning solvents including distilled water, acetone, methanol, and propanol. Sometimes a mild soap is added to the distilled water. Newer solvents like the precision optical cleaner, fiber preparation fluid, and fiber cleaner are also available. It is important that only optical grade solvents be used because if contaminants exist in the solvent then those contaminants might be deposited on the optical surface during the cleaning process. It is critical that only manufacturer approved solvents be used on the optic because of potential damage to the optic. For instance, many optical adhesives are solvent in acetone, so the use of acetone to clean such optics can cause permanent damage.

Important Notes

• Always make sure optics are cool before cleaning.
• Be sure to follow the outlined handling techniques at all times.
• Cemented optics should not be immersed.

Be cautious when working with cleaning agents. Some may be poisonous or flammable, so read labels carefully before handling them. Through proper handling and cleaning of your optics, you will maximize their lifetime. Please contact our Technical Support team if you have questions regarding handling and cleaning optics.