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All Optical Tutorials on Thorlabs.comOptics CleaningHandling and Cleaning Procedures for Optical ComponentsThe 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. HandlingBy 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. StorageNever 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. InspectionIn 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 ProceduresAlways 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 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” (15 cm) 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 MethodsIf 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 Drop and Drag Method 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 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 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
Important Notes
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. | |||||||||||||||||
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