How to store optical prisms that have been idle for a long time to prevent performance degradation?

As a high-precision optical component, the surface smoothness, coating integrity, and angular accuracy of an optical prism directly determine its optical performance. When idle for a long time, improper storage environment (such as high humidity, high temperature, dust pollution) or lack of protective measures can easily lead to surface oxidation corrosion, coating layer detachment, angle deviation, and even performance degradation, affecting subsequent use. Therefore, it is necessary to establish a three-dimensional storage system that combines environmental control, physical protection, and regular maintenance. Differentiated solutions should be developed based on the characteristics of prisms made of different materials (glass, crystal, plastic) to preserve their original optical performance to the greatest extent possible. Below, we will provide a detailed analysis of scientific storage strategies for optical prisms that have been idle for a long time from four dimensions: storage environment requirements, packaging protection methods, regular maintenance points, and special material adaptation.

1、 Strictly control storage environment: avoid the three core risks of temperature, humidity, light, and pollution

The performance degradation of optical prisms is directly related to environmental factors - high humidity leads to surface mold and coating corrosion; High temperature causes material thermal deformation; Strong light accelerates coating aging; Dust pollution scratches the surface. Long term idle storage requires the establishment of a "basic environment" that meets the requirements, with core control of the following three indicators:

（1） Temperature and humidity control: stable range is key

Prisms of different materials have varying degrees of tolerance to temperature and humidity, but overall they need to follow the principle of "low temperature and low humidity, small fluctuations". The specific parameters are as follows:

Temperature control: The ideal storage temperature is 15-25 ℃, with a temperature fluctuation range of ≤± 2 ℃/24 hours. High temperature can cause: optical plastic prisms (such as PMMA) to soften and deform (PMMA softening temperature is about 105 ℃, and long-term exceeding 80 ℃ is prone to dimensional deviation); Crystal prisms (such as calcium fluoride) experience thermal expansion, which affects angular accuracy (the thermal expansion coefficient of calcium fluoride is about 18.8 × 10 ^ -6/℃, and a temperature fluctuation of 5 ℃ can cause an angle deviation of 0.01 °); The adhesion between the coating layer (such as anti reflective film) and the substrate decreases, accelerating the detachment.

Control method: Choose a storage cabinet with constant temperature function (accuracy ± 1 ℃), avoid placing the cabinet next to the heating system, near windows (direct sunlight causing local high temperatures), or at the air conditioning outlet (sudden temperature changes); If it is a regular warehouse, temperature sensors need to be installed for real-time monitoring. Industrial air conditioning should be used for cooling in summer, and direct heating should be avoided in winter.

Humidity control: The relative humidity should be controlled between 30% and 50%, with a maximum of 60%. High humidity can cause condensation of water vapor on the surface of optical glass prisms, forming a water film that can lead to long-term corrosion of the glass (generating sodium silicate and leaving "white mist" like marks that cannot be removed by cleaning); Crystal prisms (such as lithium niobate) absorb moisture, resulting in a decrease in transmittance (transmittance in the 2500nm infrared band can decrease by 10% -15%); Metal coating layers (such as aluminum film, silver film) oxidize and rust, with spots appearing.

Control method: Place a color changing silicone desiccant in the storage cabinet (500g per cubic meter of space, observe regularly, and replace it promptly after color change), or install a small dehumidifier (humidity control accuracy ± 5%); Place a small package of desiccant (such as 3g/package of montmorillonite desiccant) separately in the prism packaging and seal the packaging; Avoid opening the storage cabinet during the rainy season. If it needs to be taken out or placed, first let your hands or tools adapt to the temperature inside the cabinet (avoid bringing in external humid and hot air).

（2） Light control: isolate strong light and ultraviolet rays

Long term exposure to light, especially ultraviolet radiation, can accelerate the degradation of prism performance. Specific hazards include: aging and yellowing of optical plastic prisms (such as PC), and a decrease in visible light transmittance (long-term exposure to sunlight can cause a 3% -5% annual decrease in PC transmittance); The organic adhesive in the coating layer (such as multi-layer anti reflective film) decomposes, causing the coating to peel off; Some crystal prisms (such as quartz) exhibit "photochromism", which affects optical uniformity.

Control requirements: The storage environment should be completely dark, and direct sunlight or strong light (such as LED lights) should be avoided. Only low-power infrared lights (without ultraviolet components) should be used for illumination during retrieval and placement.

Control method: The storage cabinet is made of opaque materials (such as cold-rolled steel plate spray painted cabinet), and the lighting inside the cabinet is equipped with 12V low-voltage infrared lamps; Install blackout curtains (such as UV resistant sunshade cloth) on warehouse windows to prevent natural light from entering; Prism packaging uses opaque black anti-static bags to further isolate light.

（3） Pollution control: dust prevention, chemical corrosion prevention

Dust and chemical pollutants are the "invisible killers" of prism surfaces, with specific hazards: dust particles (such as dust and fibers) falling on the prism surface, and long-term accumulation can scratch the coating layer (especially soft coatings such as magnesium fluoride film, with a Mohs hardness of only 4); Chemical pollutants (such as volatile acidic and alkaline gases in the laboratory, formaldehyde in the warehouse) can corrode the prism substrate and coating layer (such as hydrochloric acid gas can cause corrosion pits on the surface of glass prisms, and nitric acid gas accelerates silver film oxidation).

Control method:

Dust prevention: The storage cabinet needs to have good sealing (the cabinet door should use rubber sealing strips with a gap of ≤ 0.1mm), and the dust inside the cabinet should be cleaned regularly with compressed air (with a filtration accuracy of 0.3 μ m); When taking and placing prisms, it is necessary to operate them on a clean workbench (Class 1000), and the operator should wear dust-free gloves and masks to avoid contaminating the prisms with grease and dandruff on their hands;

Chemical corrosion prevention: It is prohibited to store prisms in the same warehouse with volatile pollutants such as alcohol, acetone, paint, adhesives, etc; The warehouse should be kept away from pollution sources such as laboratories and paint spraying workshops, and the concentration of harmful gases in the air should be regularly tested (such as formaldehyde concentration ≤ 0.1mg/m ³); If there is slight chemical pollution in the storage environment, activated carbon adsorbent can be placed in the cabinet (replaced once a month).

2、 Scientific packaging protection: physical isolation is the core

Even in an ideal environment, direct exposure of prisms may still cause damage due to collisions and friction. Long term idle needs to adopt a "multi-layer packaging" strategy to achieve triple protection of "buffering+sealing+isolation". The specific steps are as follows:

（1） Inner packaging: directly protects the surface of the prism

The inner packaging should be soft, clean, and non corrosive, avoiding direct friction with the prism surface. Specific plan:

Single prism packaging:

Clean and dry the prism (it needs to be cleaned according to the specifications first to ensure that the surface is free of dust and fingerprints), wrap it with 2-3 layers of acid free dust-free paper (such as 70g/m ² pure wood pulp dust-free paper), and avoid pulling hard when wrapping to prevent paper fibers from scratching the coating layer;

After wrapping, put it into a customized foam slot (EVA foam with density ≥ 30kg/m ³, process the slot according to the prism size, and the gap between the slot and the prism is ≤ 0.5mm) to ensure that the prism does not shake in the slot (shaking will lead to long-term friction, affecting angle accuracy);

A layer of transparent anti-static polyethylene bag (thickness ≥ 0.08mm, surface resistance 10 ^ 8-10 ^ 11 Ω) shall be set outside the foam slot, the bag mouth shall be sealed (heat sealer or sealing tape shall be used to ensure complete isolation of air and moisture), and 1-2 small bags of desiccant (such as silica gel desiccant, which shall be wrapped with air permeable paper to avoid direct contact with the prism) shall be placed inside the bag.

Special prism treatment:

Coated prisms (such as anti reflective film and reflective film prisms): A "corrosion warning card" should be placed inside the packaging to avoid contact with other metal components (metal ions may migrate to the coating layer, causing local corrosion);

Irregular prism (such as roof prism and pyramid prism): its edges and corners are easy to collapse, so it is necessary to wrap additional foam protective sleeves at the corners (thickness ≥ 5mm);

Plastic prisms (such as PMMA): Avoid using packaging materials containing plasticizers (such as some PVC bags, plasticizers may migrate to the plastic surface, causing the surface to become sticky), and prioritize using polyethylene or polypropylene materials for packaging.

（2） Intermediate packaging: cushioning and classification

The middle layer packaging mainly serves as a buffer to prevent the impact of the outer layer from being transmitted to the prism, while achieving classification management:

Buffer box selection: ABS plastic material buffer box (thickness ≥ 2mm, impact strength ≥ 20kJ/m ²) is used. The box is divided into independent grids according to the prism size, and each grid is filled with an inner packaged prism;

Buffer filling: the remaining space in the grid is filled with dust-free foam blocks (density 20kg/m ³) to ensure that the prism does not shake; Stick a layer of sponge (thickness ≥ 3mm) on the inside of the box cover to avoid damaging the prism when covering the box;

Classification identification: Label the outside of the buffer box, indicating the prism model, material, size, coating type, storage date, next inspection time, and other information (the label should be written with an acid free waterproof pen to avoid ink penetration and contamination of the packaging).

（3） Outer packaging: Overall protection and storage

The outer packaging should be sturdy, moisture-proof, and suitable for long-term stacking and storage. Specific plan:

Storage box selection: Use five layer corrugated cardboard boxes (with a burst strength of ≥ 1800kPa and edge compression strength of ≥ 5000N/m), or cold-rolled steel plate storage boxes (with a thickness of ≥ 1.2mm and surface spray rust prevention);

Protection inside the box: A layer of EPE pearl cotton (thickness ≥ 10mm) is laid at the bottom of the box, and the buffer boxes are neatly stacked, with a stacking height of no more than 3 layers (to avoid deformation of the lower buffer boxes under pressure, which may affect the internal prisms); The surrounding of the box is filled with foam plates (thickness ≥ 50mm) to prevent the buffer box from sliding in the box;

Sealing and labeling: The cardboard box is packed with high-strength strapping (width ≥ 15mm) in a cross shape, and the box mouth is sealed with waterproof tape; Label the outside of the box with warning signs such as "optical prism", "fragile item", "moisture-proof", "light avoidance", as well as the storage box number, number and type of prisms inside.

3、 Regular maintenance: dynamic monitoring to prevent hidden dangers

Long term idleness is not just about "storing it", it requires regular inspection and maintenance, timely detection and handling of potential problems, to avoid performance degradation and expansion. Specific maintenance plan:

（1） Short term storage (3-6 months): Basic check

Environmental monitoring: Check whether the temperature and humidity data are within the normal range through the temperature and humidity recorder in the storage cabinet every week. If there is any abnormality (such as humidity exceeding 60%), the desiccant should be replaced or the dehumidification equipment adjusted in a timely manner;

Appearance inspection: Open the outer storage box every month to check if the buffer box is intact and if there are any signs of moisture or damage; No need to open the inner packaging, observe whether the prism has obvious displacement or discoloration (such as plastic prism yellowing) through a transparent anti-static bag;

Desiccant replacement: Replace the desiccant in the buffer box every 2 months, and check the sealing condition of the inner packaging. If the sealing bag is found to be damaged, it needs to be resealed and the desiccant needs to be replenished.

（2） Mid term storage (6-12 months): in-depth inspection

Sampling inspection: Randomly select 10% of prisms (at least 3) from the storage box every 6 months, open the inner packaging on a clean workbench, and gently wipe the surface of the prisms with a small amount of anhydrous ethanol (purity ≥ 99.7%) dipped in a dust-free cloth to check for mold spots, corrosion marks, or coating peeling;

Optical performance testing: Conduct transmittance testing (visible light band 400-700nm, transmittance deviation should be ≤ 2%) and angle accuracy testing (such as 90 ° angle deviation of right angle prisms should be ≤ 0.001 °) on the sampled prisms. If the performance indicators exceed the allowable range, a comprehensive inspection of the prisms in the same batch is required;

Packaging renewal: if the packaging materials (such as dust-free paper and foam) are aging and damaged, new packaging materials shall be replaced and resealed.

（3） Long term storage (over 1 year): comprehensive maintenance

Full inspection: Conduct a full inspection of all prisms every 12 months, open the inner packaging one by one, clean the surface of the prisms, and check the surface smoothness (observed with a 60x magnifying glass, no obvious scratches or spots) and the integrity of the coating layer (no peeling or flaking);

Performance retest: Conduct optical performance testing on all prisms, including transmittance, refractive index, and angle deviation, to ensure compliance with the original technical parameters; For crystal prisms (such as calcium fluoride), infrared band transmittance (such as 10 μ m band transmittance ≥ 85%) needs to be tested, and for plastic prisms, yellowing index (yellowing index ≤ 1.5) needs to be tested;

Environmental disinfection: Empty the storage cabinet and use a UV lamp (wavelength 254nm) to irradiate the inside of the cabinet for 30 minutes for disinfection (avoid direct UV irradiation of the prism), then re lay the desiccant and buffer material, and return the qualified prism to the storage cabinet in its original packaging.

4、 Special material adaptation: differentiated storage solution

The characteristics of prisms made of different materials vary greatly, and it is necessary to adjust the storage strategy accordingly based on universal storage principles to avoid performance degradation caused by a one size fits all approach

（1） Crystal prism: focus on moisture absorption and corrosion prevention

Calcium fluoride (CaF ₂) prism: easily deliquescent, requiring a "double-layer sealing+strong desiccant" solution - the inner layer is sealed with an aluminum-plastic composite bag (with better barrier properties than polyethylene bags), and 5g/bag of high moisture absorbing desiccant (such as calcium chloride desiccant, wrapped in breathable film to avoid direct contact with the prism) is placed inside the bag; The storage temperature should be controlled at 18-22 ℃, the humidity should be ≤ 40%, and the desiccant should be replaced every 3 months;

Lithium niobate (LiNbO) prism: susceptible to chemical corrosion, needs to be stored separately in a corrosion-resistant polytetrafluoroethylene container (avoid contact with metal or glass containers), and an activated carbon adsorbent should be placed inside the container to prevent corrosion by acidic and alkaline gases in the air;

Quartz crystal prism: although it has high stability, it needs to avoid collision with other hard objects for long-term storage (the Morse hardness of quartz is 7, and the collision is prone to edge collapse). A thin layer of silicone pad (thickness ≥ 2mm) needs to be laid in the foam card slot to increase buffer.

（2） Optical plastic prism: focus on preventing high temperature and aging

PMMA (acrylic) prism: It is prone to softening at high temperatures for a long time, and the storage temperature should be ≤ 25 ℃, avoiding close proximity to any heat source; Anti aging requires the placement of antioxidants (such as 0.1g/pack of hindered phenolic antioxidants) inside the packaging, while avoiding direct contact with rubber products (such as rubber seals) (plasticizers in rubber will migrate to the PMMA surface, causing surface cracking);

PC (polycarbonate) prism: Although it has strong impact resistance, it is prone to aging under long-term light exposure. It needs to be double shielded from light with a "black opaque bag+aluminum foil packaging", and no visible light is allowed in the storage cabinet; Check the yellowing condition every 6 months, and replace the prism with a new one if the yellowing index exceeds 2.0.

（3） Coated Prism: Focus on preventing coating detachment and scratching

Antireflective film prism (such as MgF ₂ antireflective film): the coating layer is relatively soft (Mohs hardness 4), and extremely soft dust-free cloth (such as ultra-fine fiber dust-free cloth, fiber diameter ≤ 0.5 μ m) should be used for packaging to avoid direct friction with foam and paper; Avoid stacking too high (no more than 2 layers) during storage to prevent damage to the coating layer caused by heavy pressure;

Metal coated prisms (such as aluminum film, silver film): prone to oxidation, rust proof paper (such as gas-phase rust proof paper) should be placed inside the packaging, and a nitrogen generator should be placed in the storage cabinet to maintain a nitrogen concentration of ≥ 95% (an inert gas environment can effectively prevent metal oxidation); Check the coating layer every 3 months. If spots appear, use a specialized coating repair agent (such as silver film repair solution) for treatment.