Handling and Cleaning Procedures for Optical Components

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Post on Thursday 18 December, 2014
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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 ok 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

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

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.

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.

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, isopropyl alcohol, and TravelSAFE Precision Optical Cleaner. 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 solvent like acetone, methanol, or another quick drying solvent like TravelSAFE’s Precision Optical Cleaner 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.

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.

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.