Handling and Cleaning Optical Components Application Note

Optical components require special methods to be followed to maximize their performance and lifetime. Daily use of optics can cause them to be dirty due to dust, water, and skin oils. This dirt increases scattering off the optical surface and absorbs radiation which in turn will create hot spots on the surface leading to permanent damage. This guide talks about common handling and cleaning methods that are useful for optical components. As optical components vary in size, material, etc. we must use the right method to handle and clean each type of component.

Handling

Proper handling methods during cleaning can decrease the frequency of handling the optic to maximize its lifetime. Ensure that the unpacking procedure of the optical components is done in clean and temperature-controlled surrounding. Avoid handling the components with bare hands as the oils from your skin may damage the optical surface quality permanently. Alternatively, you may wear gloves when handling the optics, and for smaller optical components; use optical tweezers. When holding the optical components, it is best to hold the components along the non-optical surfaces.

For holographic or ruled gratings, you should never touch them with your bare hands or any optical handling instruments. Their first surface is unprotected with metallic mirrors and pellicle beam splitters. These optical components are very sensitive and any form of physical contact will damage the components. Crystals are temperature sensitive and can break if exposed to thermal shock. Hence, you should always allow the package to come into thermal equilibrium before unpacking. Crystals are also much softer, unlike conventional optics, and thus, require more careful handling when cleaning.

Storage

As optics are easily scratched, contaminated, and hygroscopic, proper storage is vital for preserving the optical components. Optics should always be wrapped in lens tissue and stored inside an optical storage box designed for optics. Never put optics on hard surfaces as it will cause the optic surface to be contaminated. Optical storage boxes should be kept in a temperature-controlled environment with low humidity and contaminant.

Inspection

Optical components should be inspected before and after cleaning and usage. If necessary, utilize a magnification device to aid in the inspection as most of the contaminations and defects are small. In addition, you can shine a bright line onto the optical surface which will increase the intensity of the specular reflections from the contaminations and defects.

When inspecting an optical component whose surface is reflectively coated, the component should be held parallel to the inspector’s line of sight. Looking parallelly across the surface will allow you to see the contaminations and not reflections. When inspecting an optical component whose surface is polished, the component should be held perpendicular to the inspector’s line of sight which will enable them to look through the optics.

If there is a surface defect on a clean optical surface, utilizing a scratch-dig paddle allows the categorization of the size of the defect by comparing the size of the calibrated defect on the scratch-dig paddle to the size of the defect on the optical surface. If the size of the defect is more than the manufacturer’s scratch-dig specification, it may be required to replace the optic which will allow one to achieve the desired performance.

Cleaning Procedures

If available, we should always read the manufacturer’s recommended cleaning and handling procedures. Before cleaning an optical component, inspect the component to determine the type or severity of the contaminants. It is best not to skip this inspection as the cleaning process involves solvents and physical contact with the optical surface. If the involvement of solvents and physical contact occurs too frequently, it may damage the optical surface. For optics with multiple contaminants, the order in which they are removed is important as the optical surface may be damaged by one contaminant when removing another.

Blowing Off the Surface of an Optic

A canister of inert dusting gas or a blower bulb can be used to blow off dust or any other loose contaminants. Avoid using your mouth to blow on the surface of the component as droplets of saliva will be deposited onto the optical surface.

When using the inert dusting gas, do not shake the can before or during its usage. Hold the can upright and roughly 6” from the optical surface throughout the procedure to start the flow of gas with the spray pointed away from the optical component. This will help to ensure that the inert gas propellant does not get deposited on the optical surface. Wave the spray over the optical surface at a grazing angle to its surface. When spraying over a large surface area, trace a figure of eight pattern over it.

This method of cleaning can be applied to almost all types of optics but for certain optics (holographic/rules gratings, calcite polarizers, etc.) which can be damaged by physical contact, this is the only method for them to be properly cleaned.

Alternate Cleaning Procedures

There are alternate cleaning procedures if blowing off the surface of the optic is not sufficient. Optics should always be cleaned using clean wipes and optical-grade solvents to avoid damage from contaminants. The wipes should always be moist with an approved solvent and never be used dry. Some acceptable wipes are pure cotton, lens tissue, and cotton-tipped applicators. Common solvents used are acetone, methanol, and isopropyl alcohol. Solvents should be used with care as most of them are toxic, flammable, or both.

Cleaning procedures using clean wipes and optical grade solvents:

Washing the Optic

Fingerprints and large dust particles on optical surfaces can be removed by immersing the optics in a mild solution of distilled water or optical soap if approved by the manufacturer. However, the optic should not remain immersed in the liquid any longer than required to remove the contaminants. After immersing the optic, rinse it in clear distilled water. Depending on the type of optic component, the drop and drag or lens tissue procedures can be used to apply a quick-drying solvent (acetone/methanol) to the optic to increase the speed of drying. Do not pool any cleaning solutions on its surface while the optic dries as it often leaves streaks on the surface.

Drag and Drop Method

The drag and drop method can be applied to cleaning flat optical surfaces that are elevated above their surrounding surfaces. First, start by examining the optic to locate the contaminants. Locating the contaminants before starting this cleaning method allows us to lift them from the surface as soon as possible rather than dragging them across the optical surface during the process.

After inspecting, place the optic in a stable manner that keeps the optic stationary. Use a clean sheet of lens tissue to hold the optic above while pulling it across the optic surface. Ensure to do this in a manner that does not push down on the surface. Subsequently, drip about 1 to 2 drops of an approved quick-drying solvent on the lens tissue while holding it above the optic.

The weight of the solvent will result in the lens tissue meeting the optical surface. Drag the lens tissue slowly and steadily across the optic surface until it is off the surface ensuring that the lens tissue does not leave the surface during the process. The right amount of solvent will ensure that the lens tissue is damp throughout the entire process yet does not leave any visible trace of solvent on the optical surface.

Examine the optic and repeat the step if needed but use each sheet of lens tissue only once each time. This is a preferred cleaning method as the lens tissue is only in minimal contact with the optical surface. This method can remove small particles and oil from the optical surface. Repeated treatments can be used to remove heavy concentrations of contaminants.

Lens Tissue with Forceps or Applicator Method

This method is commonly used for mounted or curved surface optics that require cleaning with a solvent. First, start by examining the optic to locate the contaminants. Plan a wiping path that will not cause the drag of any contaminants over the optical surface. If the lens tissue is used, the tissue must be folded in such a way that a portion of tissue that meets the optic is not touched. Hold the folded lens tissue with forceps in a way that a smooth wipe over the optical surface can be conducted. Next, apply a few drops of solvent to the lens tissue to make it damp (not dripping). The lens tissue should be able to wipe across the optical surface in a smooth motion.

When wiping, continuously rotate the lens tissue slowly. After each wipe, inspect the optic for any leftover contaminants or streaks and repeat the procedure with a new lens tissue if required. If streaks form at the edge of the lens tissue, use a larger applicator or apply a continuous wiping path that removes the wiped interface on the optical surface.

Cleaning with Webril Wipes

Webril wipes are soft cotton wipes that are recommended for cleaning most optics. They absorb solvent effectively, do not dry out as quickly compared to the lens tissue, and do not fall apart quickly. The surrounding edges of the Webril wipes may leave some lint hence always use a folded edge when cleaning. During the entirety of this process, always wear gloves or finger cots.

For smaller optics, roll the wipe into a cone shape with the folded edges at the point, moisten the tip, and use the point as the wiping area. For bigger optics, cut the wipe into 3 parts that are roughly 2.6”x4”. Fold the wipe lengthwise so that it measures 1.3”x4” and then make a fold roughly 1” from the end. Moisten the last folded edge with solvent and use the edge to clean the optical surface. Utilizing a pump bottle to release the solvent will allow it to be easier to hold the optic in one hand while moistening the wipe with the other.

Pick up the optic in one hand and wipe the optic lightly and slowly across the entire optical surface to avoid streaking. The amount of solvent, pressure applied to the wipe, and speed of the wipe may have to be adjusted to avoid those streaks from appearing. Wiping duration may also vary with the solvent used. For example, acetone requires you to wipe faster than alcohol as acetone dries faster.

Optical Handling and Cleaning Tools

1. Gloves

Gloves are crucial when handling any optical component. The material of the gloves should be either cotton or powder-free latex.

2. Tweezers

Optical or vacuum tweezers are commonly used to handle smaller optics. Optical tweezers are made to hold small hard objects securely. The uniquely designed optical tweezers’ tips are made of a material that reduces the risk of scratching the optical component. Meanwhile, vacuum tweezers utilize a suction cup to hold the optical component. The tips are usually of a few variants that are specialized to
hold certain shapes and sizes of optical components.

3. Webril Wipes

Webril Wipes are made from pure cotton. These wipes can be used to clean optics or any surfaces. The edges, however, leave lint particles. This can simply be avoided by folding the wipes and using the folded edge to clean the optical component.

4. Lens Tissue

Lens tissues are used to handle and clean optics as it provides a soft surface that can safely touch various types of optical surfaces. Lens tissues are often used to wrap optical components.

5. Optical Storage Boxes

These boxes have foam or molded plastic inserts. The inserts ensure that the optical components are stationary in the box and that the optical surfaces will not touch the hard surfaces.

6. Magnifiers

Small optics can be examined using magnifiers. Magnifiers are useful in checking the cleanliness and integrity of the optical surfaces which will determine the cleaning procedure.

7. Scratch-Dig Paddle

Most optics have specific scratch-dig tolerance that categorizes the optical surface’s optical quality. This paddle has a series of calibrated optical defects that will determine how deep or thick a scratch is.

8. Inert Dusting Gas

This gas is excellent for cleaning dust and other contaminants that have not adhered to the optical surfaces. An alternate source would be a bulb blower. Inert dusting gas can provide a continuous stream of pressurized gas with which contaminants can be blown away from the optical surface. As the gas is released from the pressurized canister, it is often cooler than the surrounding temperature and cools the optical surface. The stream of pressurized gas could create droplets of the propellant that may cause it to be deposited on the optical surface.

9. Forceps

Forceps are small, lockable clamps that are frequently used to hold lens tissues during certain cleaning procedures. Since forceps can easily scratch an optical surface, it is important to make sure they never touch the optical surface.

10. Cotton-Tipped Applicators

Cotton-tipped applicators are 6” wooden sticks with cotton fibers on one end. They are used as an applicator for different cleaning agents. These are useful for cleaning small optical surfaces. For larger optical surfaces, it is harder to get a streak-free finish with these applicators. Cotton-tipped applicators are not to be confused with Q-tips or other drugstore cosmetic applicators. These applicators are made from nonabrasive materials and fibers. They are optical-graded instruments that will not leave contaminants on optical surfaces.

11. Optical Cleaning Solvents

Common optical cleaning solvents include acetone, distilled water, methanol, or propanol. It is vital to use only optical-grade solvents as contaminants may reside in the non-optical-graded solvents. This could result in contaminants being deposited onto the optical surface during the cleaning procedure. It is standard practice to only use manufacturer-approved solvents on optical surfaces to avoid further damage to them.

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