How should long-term used optical prisms be cleaned and maintained?

Optical prisms, as the core components of optical systems, achieve light path control and imaging through refraction, reflection, dispersion, and other effects. Their surface cleanliness and structural integrity directly determine the optical performance. Over long-term use, the prism surface is prone to adhering to dirt, oil, fingerprints, and other contaminants. If not properly maintained, it can not only reduce light transmittance and generate stray light but also lead to scratching of the prism surface and detachment of coatings due to corrosion from contaminants or improper cleaning operations, seriously affecting the imaging quality and service life of the optical system. This article will elaborate on the cleaning and maintenance methods for long-term use of optical prisms from four dimensions: preparation before cleaning, classification and cleaning of contaminants, daily maintenance, and maintenance for special scenarios, providing professional guidance for users of optical equipment.

1. Core preparation before cleaning and maintenance: Avoiding "operational risks"

Optical prism materials (such as K9 glass, quartz glass) and surface coatings (such as anti-reflective coatings, reflective coatings) are extremely precise. Adequate preparation is necessary before cleaning to avoid secondary damage caused by improper tools, unclean environment, or blind operation.

(1) Environmental preparation: Creating a "dust-free cleaning space"

Cleaning should be carried out on a Class 100 or Class 1000 clean bench. If conditions are limited, an enclosed, dry, dust-free indoor environment (such as an optical laboratory) can be chosen. Close the doors and windows 1 hour before cleaning and turn on the air purifier to reduce the concentration of particulate matter in the air (the diameter of particulate matter should be controlled below 0.5μm). It is strictly prohibited to clean in outdoor areas, workshops, or other environments with high dust content, to avoid dust reattaching to the prism surface or hard particles scratching the coating during the cleaning process.

Meanwhile, the environmental temperature and humidity should be controlled within a suitable range: temperature between 20℃ and 25℃ (to avoid thermal stress on the prism caused by sudden temperature changes), and relative humidity between 40% and 60% (excessive humidity can easily lead to condensation on the prism surface, while low humidity can easily cause static electricity to attract dust). During cleaning, it is necessary to turn off air conditioning vents, fans, and other equipment that may generate airflow, to prevent airflow from carrying dust and polluting the prism.

(II) Tool preparation: Select "dedicated cleaning tools"

Cleaning tools must possess the characteristics of "softness, scratch-free, and residue-free". It is strictly prohibited to use ordinary paper towels, cloths, cotton swabs, and other tools that are prone to generating fibers or scratches. Core tools include:

Cleaning air blower: Use a professional optical air blower made of silicone material (such as a lens air blower) to ensure stable airflow and purity, which is used to blow off surface dust;

Dust-free cloth: Choose a microfiber dust-free cloth (fiber diameter ≤ 0.1μm, weight ≥ 80g/㎡), cut into small pieces of 5cm×5cm for use, and avoid folding large areas of dust-free cloth to prevent wrinkles from scratching the prism;

Cleaning solution: Select a dedicated optical cleaning solution based on the type of stain. For routine cleaning, a mixture of isopropyl alcohol and deionized water (volume ratio 1:1) can be used. Isopropyl alcohol can quickly dissolve oil stains, while deionized water prevents residual impurities. If the stain is stubborn fingerprints or grease, 0.5% neutral surfactant (such as a dedicated optical cleaning agent) can be added. It is strictly prohibited to use strong corrosive solvents such as alcohol and acetone to avoid damaging the prism coating;

Tweezers and Gloves: Wear powder-free nitrile gloves (to avoid direct fingerprint contact with the prism) and use titanium alloy or plastic tweezers (to avoid scratching the prism edges with metal tweezers) to grasp the prism. The tweezers should be wrapped in silicone sleeves.

(III) Prism inspection: Clarify "stains and damage conditions"

Before cleaning, it is necessary to inspect the prism surface using a high-power magnifying glass (magnification of 10-20 times) or an optical testing instrument (such as an interferometer). First, observe the type (floating dust, oil stain, fingerprint, mold spot) and location of the stains. Next, confirm whether the prism has any damages such as scratches, coating peeling, or edge damage. If there are deep scratches (scratch width > 0.1mm) or large-area coating peeling, it is necessary to first assess whether professional repair is required before cleaning, to avoid aggravating the damage during the cleaning process.

II. Cleaning by type: "Precision treatment plan" for different stains

The stains on the surface of optical prisms that have been used for a long time mainly fall into three categories: dust, oil/fingerprints, and mold. Different stains have different causes and adhesion strengths, requiring differentiated cleaning methods to ensure thorough cleaning without damaging the prism.

(1) Floating dust cleaning: "Blow first, then wipe", to avoid scratching by hard particles

Dust is a common stain, mainly caused by the adhesion of dust particles in the air. If wiped directly, hard particles are prone to scratching the surface of the prism. The correct cleaning steps are as follows:

Air blowing dust removal: Hold the cleaning air blower at a distance of 10-15cm from the prism surface, slowly blow air from the center of the prism towards the edge, repeat 3-5 times, and remove loose dust on the surface; if the prism has grooves or edges, adjust the angle of the air blower (at a 45° angle to the edge) to ensure that dust in the grooves is removed, avoiding excessive concentration of airflow that could cause dust particles to impact the prism surface;

Light wiping with a lint-free cloth: If air blowing fails to remove stubborn dust (such as dust adsorbed by static electricity), take a clean lint-free cloth, dip it in a small amount of cleaning solution (just wet it without dripping), fold the lint-free cloth into two layers, and use the "spiral wiping method" to wipe from the center of the prism to the edge. The wiping force should be gentle (pressure ≤50g/cm²) to avoid scratching the coating with the lint-free cloth fibers due to hard pressing. After wiping, immediately use another dry lint-free cloth to dry the cleaning solution in the same way to prevent water marks caused by residual cleaning solution.

(II) Oil stain/fingerprint cleaning: "dissolution + wiping" to avoid solvent residue

Oil stains and fingerprints primarily originate from the operator's hand contact and the evaporation of grease inside the optical system (such as lens lubricant). These types of stains possess a certain viscosity and require dissolution with cleaning solution before wiping. The cleaning steps are as follows:

Localized dissolution: Dip a lint-free cloth into an appropriate amount of cleaning solution containing a neutral surfactant, gently press it onto the oil stain or fingerprint area for 3-5 seconds, allowing the cleaning solution to fully dissolve the stain. Avoid directly wiping as this may cause the stain to spread;

Layered wiping: First, use a damp, lint-free cloth to wipe the stain in the same direction (such as horizontally). After each wipe, replace the clean area of the lint-free cloth to avoid re-adhesion of dissolved stains. If the stain remains unremoved, repeat the dissolving-wiping steps, but no more than 3 times, to prevent the cleaning solution from repeatedly contacting the prism coating;

Residual cleaning: After removing the stains, use a lint-free cloth dampened with pure deionized water to wipe the surface of the prism to remove any residual cleaning solution and surfactant. Then, use a dry lint-free cloth to dry the surface, ensuring that there are no water marks or residues.

(III) Mold Spot Cleaning: "Handle with Care" to Prevent Mold Spreading

Optical prisms that are exposed to high humidity environments for extended periods (such as outdoor optical equipment and unsealed optical systems) are prone to developing mold spots. Molds secrete acidic substances that can corrode the prism glass and coatings. If not treated promptly, the mold spots can rapidly spread, leading to damage to the prism. The cleaning steps are as follows:

Mold inhibition: First, transfer the prism to a dry environment (relative humidity < 40%) to prevent further mold proliferation. Soak a lint-free cloth in a cleaning solution containing 0.1% mold inhibitor (such as optical-specific mold inhibitor solution), and gently wipe the moldy area to inhibit mold activity;

Mildew removal: If the mildew is relatively shallow (only attached to the coating surface), it can be dissolved and wiped off using a cleaning solution according to the oil stain cleaning method; if the mildew has corroded the glass (with pits or white spots appearing on the surface), it is necessary to stop self-cleaning and send it to a professional optical repair institution for treatment, to avoid blind wiping that may expand the corroded area;

Subsequent protection: After cleaning, the prism needs to undergo anti-mildew treatment. This can be achieved by spraying an ultra-thin optical anti-mildew coating (such as a silica nano-coating) on its surface, or by placing silica gel desiccant in the storage environment to prevent mold from growing again.

III. Key points of daily maintenance: full-cycle protection from "use to storage"

In addition to regular cleaning, scientific maintenance during daily use and storage can effectively reduce the occurrence of prism stains and extend its service life. The core points include three aspects: usage specifications, storage protection, and regular inspection.

(1) Usage guidelines: Reduce "human-induced damage and contamination"

Operational precautions: When installing or disassembling prisms, it is necessary to wear powder-free nitrile gloves and goggles. Direct contact with the prism surface by hand is strictly prohibited. When using tweezers to handle prisms, grasp the edge of the prism (not the optical working surface) to avoid scratching the optical surface due to contact with the tweezers;

Avoid overuse: Optical prisms, when operated under strong light conditions (such as laser or intense illumination) for extended periods, are prone to coating degradation or glass annealing. It is necessary to control the usage time based on the prism's operational parameters (such as maximum power tolerance and operating temperature) to prevent overload operation. For instance, if the prism in a laser device is exposed to laser radiation exceeding its rated power for an extended period, regular inspections should be conducted to check for any burn marks on the coating, and damaged prisms should be replaced promptly;

System sealing: If the prism is installed inside an optical device, it is necessary to ensure that the device housing is well sealed. An air filter should be installed at the air inlet of the device to prevent external dust and moisture from entering and contaminating the prism. Regularly check the desiccant (such as molecular sieve) inside the device. If the desiccant absorbs moisture and changes color (such as blue silica gel turning pink), it needs to be replaced promptly.

(II) Storage protection: Creating a "stable and safe storage environment"

Prisms that are not used for a long time should be stored separately to avoid collision or contamination with other optical components

Packaging protection: Place the prism into a dedicated optical element packaging box (with soft sponge or suede lining), and place silica gel desiccant inside the box (with a weight of desiccant per box ≥ 10% of the prism weight). Mark the prism model, specifications, and storage date on the outside of the packaging box. It is strictly prohibited to store the prism directly in a plastic bag to avoid dust attraction due to static electricity generated by the plastic bag or moisture inside the bag causing the prism to become damp;

Storage conditions: The storage environment must meet the requirements of "constant temperature and humidity, dust-free and shielded from light". The temperature should be controlled between 18℃ and 22℃, with a relative humidity of 35% to 50%. Direct sunlight or strong light exposure should be avoided (a shading board can be installed inside the storage cabinet). The storage cabinet should be kept away from heat sources (such as heating systems and air conditioning vents) and water sources (such as humidifiers and sinks) to prevent sudden temperature changes or excessive humidity that could damage the prism;

Avoid Stacking: When storing, it is strictly prohibited to stack prism packaging boxes too high (stacking height ≤ 3 layers) to prevent deformation of lower boxes due to pressure, which could lead to prism collisions. Prisms of different materials or coatings should be stored separately to avoid mutual contamination caused by material differences (for example, long-term contact between metal-coated prisms and glass prisms may lead to electrochemical corrosion).

(III) Regular inspection: timely detection of "potential problems"

Establish a regular inspection system for prisms, and determine the inspection cycle based on the frequency of use: prisms used daily (such as those in optical instruments) should be inspected every 1-2 months, while prisms stored for long periods should be inspected every 3-6 months. The inspection contents include:

Surface inspection: Observe the prism surface with a high-powered magnifying glass to check for any stains, scratches, or coating peeling. If minor stains are found, promptly follow the cleaning method;

Performance testing: Utilize optical testing equipment (such as transmittance testers, interferometers) to assess performance parameters like transmittance and angular accuracy of prisms. If the transmittance decreases by more than 5% (compared to the initial value), or if the angular accuracy deviation exceeds the allowable range (such as a deviation of more than 30" for a right-angle prism at a 90° angle), analyze the cause and proceed with repairs;

Environmental inspection: Inspect the temperature, humidity, and cleanliness of the environment where the prism is used and stored. If the environmental parameters exceed the suitable range, make timely adjustments (such as turning on a dehumidifier or replacing an air filter) to prevent environmental factors from causing continuous damage to the prism.

IV. Special scenario maintenance: "Enhanced maintenance plan" for extreme environments

If optical prisms are used in extreme environments (such as outdoor, high temperature, high humidity, and corrosive environments) for a long time, conventional cleaning and maintenance methods are difficult to meet the needs. Targeted and enhanced maintenance measures need to be taken to ensure stable performance of the prisms.

(1) Outdoor environment: focus on protection against "dust, rain, and ultraviolet rays"

Prisms used outdoors (such as those in surveying instruments and astronomical telescopes) are susceptible to dust, rainwater, and ultraviolet rays. When maintaining them, it is necessary to:

Installing protective devices: Install a transparent protective cover (such as a quartz glass protective cover) on the exterior of the prism. The surface of the protective cover needs to be sprayed with an anti-ultraviolet coating to reduce the aging effect of ultraviolet rays on the prism coating. The protective cover needs to be cleaned regularly (once a week) to avoid stains on the surface of the protective cover affecting the optical path;

Clean promptly after rain: After use in rainy weather, immediately blow off the rainwater on the prism surface with a clean air blower, then wipe it clean with a lint-free cloth dampened with deionized water, and dry it with a dry lint-free cloth to prevent residual impurities (such as acid rain components) from corroding the prism;

Regular anti-corrosion treatment: If there are corrosive gases in the outdoor environment (such as salt spray at the seaside, acidic gases in industrial areas), it is necessary to apply a thin layer of anti-corrosion grease (such as silicone-based anti-corrosion grease) on the edges of the prism (not on the optical working surface) every month to prevent corrosion of the metal components at the edges (such as the prism bracket) from contaminating the prism.

(II) High temperature environment: prevent "thermal stress and coating aging"

Prisms used in high-temperature environments, such as those in industrial furnace monitoring equipment and high-temperature sensors, require special attention regarding their thermal stability:

Temperature reduction protection: Install cooling devices (such as water cooling jackets or air cooling systems) around the prism to control its operating temperature within its tolerance range (for example, the high tolerance temperature of a quartz prism is 1200℃, so it is necessary to ensure that the operating temperature does not exceed 1000℃);

Material selection and adaptation: Prisms that are used in high-temperature environments for extended periods of time require the selection of high-temperature resistant materials (such as quartz glass or sapphire) to prevent softening and deformation of ordinary K9 glass at high temperatures. Additionally, high-temperature resistant coatings (such as metal oxide coatings) should be chosen to avoid decomposition of organic coatings at high temperatures;

Regular replacement: High temperature environments will accelerate the aging of prisms. Even with cooling measures taken, it is necessary to shorten the inspection and replacement cycle (such as inspecting every 3-6 months and replacing annually) to ensure that the prism performance meets the usage requirements.

V. Conclusion: Scientific cleaning and maintenance are the "guardians" of optical prism performance

The cleaning and maintenance of optical prisms that have been used for a long time is not simply a matter of "wiping and dust removal". It involves systematic work that takes into account material characteristics, stain types, and usage scenarios - from preparing the environment and tools before cleaning, to precise cleaning by type, to full-cycle maintenance during daily use and storage. Each step must follow scientific norms to remove stains while avoiding damage to the prism and maintaining its stable optical performance.

For users of optical equipment, it is essential to fully understand the importance of cleaning and maintenance, abandon the concept of "emphasizing use and neglecting maintenance", establish a comprehensive cleaning and maintenance system, and develop personalized maintenance plans based on the actual usage of prisms. Only by integrating scientific cleaning and maintenance into the full lifecycle management of optical prisms can their optical performance be maximized, providing reliable guarantees for the stable operation of optical systems.