Is the decrease in transmittance of UV quartz lenses after long-term use caused by surface contamination or material aging?

UV quartz lenses are widely used in fields such as UV curing, spectral analysis, photolithography, sterilization, and disinfection due to their excellent UV transmittance, high temperature resistance, and chemical stability. However, the decrease in transmittance after long-term use is a common problem, with the core causes concentrated in two dimensions: surface pollution and material aging. There are significant differences in the causes, impact characteristics, and response methods between the two, and precise identification through scientific analysis is necessary to address the issue of transmittance attenuation and ensure the accuracy and efficiency of equipment operation.

1、 Surface pollution: the "immediate and reversible" cause of decreased transmittance

Surface contamination is a common cause of decreased transmittance of ultraviolet quartz lenses, which refers to the adhesion of foreign objects on the surface of the lens due to improper environment, process, or operation, hindering the penetration of ultraviolet light. Its core feature is that pollution only acts on the surface of the lens, and the transmittance can be restored through professional cleaning, which is a reversible fault.

（1） The main types and causes of surface pollution

Organic pollutant adhesion

Organic pollution is a common type in UV application scenarios. For example, in UV curing equipment, volatile organic compounds (VOCs) such as resins and inks will partially decompose or polymerize under UV radiation, forming a viscous film that adheres to the surface of the lens; If spectral analysis equipment is exposed to laboratory air for a long time, dust, oil stains (such as residue from operator's finger contact), and organic aerosols in the air will gradually deposit. This type of pollutant has a strong absorption effect on ultraviolet light (especially in the UVC band, 200-280nm), and the transmittance decreases linearly with increasing adhesion thickness.

Inorganic pollutant deposition

The main sources are dust in the environment (such as quartz sand, metal oxide particles), and salt carried by condensed water (such as salt in the air attached to the lens surface with condensed water in high humidity environments). Although inorganic pollutants have weak absorption of ultraviolet light, they can cause light scattering - when ultraviolet light shines on particles on the surface of the lens, some of the light will deviate from the original propagation direction and cannot reach the target area, resulting in a decrease in transmittance. Especially in high-precision equipment such as photolithography lenses, even micrometer sized particles can significantly affect transmittance.

Chemical corrosion products

If there are corrosive gases (such as ozone, chlorine, sulfur dioxide) or liquids (such as acid mist, alkali mist) in the environment where the lens is used, it will undergo chemical reactions with the surface of quartz glass. For example, ozone under ultraviolet light will oxidize trace impurities (such as aluminum and iron oxides) on the surface of quartz glass, forming brown corrosion spots; Acid mist reacts with silicon oxygen bonds in quartz to form soluble silicon compounds, resulting in surface roughness. These corrosion products not only directly absorb ultraviolet light, but also intensify the adhesion of subsequent pollutants, forming a vicious cycle of "pollution corrosion re pollution".

（2） Identification characteristics of surface pollution

To determine whether the decrease in transmittance is caused by surface contamination, the following visual and detection methods can be used for identification:

Appearance observation: Use a high-power magnifying glass (100-200 times) to observe the surface of the lens. If obvious particles, spots, oil films, or water stains are found, and their positions coincide with the area of decreased transmittance (which can be determined by scanning with a transmittance meter), it is highly likely to be surface contamination. For example, oil film usually presents irregular rainbow colors, while dust particles are small black or white dots.

Cleaning test: Use a professional cleaning process (such as gently wiping the lens surface with a dust-free cloth dipped in analytical pure alcohol, or using a UV lens specific cleaning agent) to treat the lens surface, and then test the transmittance again after cleaning. If the transmittance significantly increases (by more than 10%), it can be confirmed that it is caused by surface pollution; If there is no significant change in transmittance after cleaning, surface contamination factors can be ruled out.

Time correlation: The decrease in transmittance caused by surface pollution usually has a "gradual" or "sudden" nature - gradual is often due to long-term accumulation (such as a monthly transmittance decrease of 1% -2%), sudden is often due to a single pollution event (such as finger contact with the lens during equipment maintenance, resulting in a short-term transmittance decrease of more than 5%), and the rate of decrease is positively correlated with the degree of pollution in the usage environment (such as in dusty workshops, transmittance decreases faster).

2、 Material aging: a long-term and irreversible cause of decreased transmittance

Material aging refers to the irreversible changes in the physical and chemical properties of the substrate (quartz glass) or surface coating of ultraviolet quartz lenses during long-term use, resulting in a decrease in transmittance. Unlike surface pollution, the impact of material aging on the interior or coating of the lens cannot be restored through cleaning and can only be resolved by replacing the lens, which is an irreversible fault.

（1） The main types and causes of material aging

Structural degradation of quartz substrate

The main component of quartz glass is silicon dioxide (SiO ₂), and its excellent UV transmittance depends on its regular amorphous structure. However, long-term exposure to ultraviolet radiation (especially short wavelength ultraviolet light such as UVC) can cause "photo induced structural changes": the energy of ultraviolet light can break the Si-O-Si bond in quartz glass, forming defects such as oxygen vacancies and silicon radicals. These defects will become absorption centers for ultraviolet light - when ultraviolet light passes through, the defects will absorb photon energy, resulting in a decrease in transmittance. This aging phenomenon is called "UV induced darkening", which is particularly evident in the UVC wavelength range of 200-250nm.

In addition, high temperature environments can accelerate structural degradation. If the lens works at high temperatures (such as over 300 ℃) for a long time (such as the focusing lens of a UV curing machine), the atomic thermal motion of quartz glass will intensify, leading to the transformation from amorphous structure to microcrystalline structure (i.e. "crystallization"). The absorption of ultraviolet light in the microcrystalline region is much higher than that in the amorphous region, resulting in a continuous decrease in transmittance, and once the crystallization process occurs, it cannot be reversed.

Aging and Failure of Surface Coating

To improve the UV transmittance, most UV quartz lenses are coated with anti reflective films (such as MgF ₂, Al ₂ O3 coatings) or anti reflective films. But the coating layer will age due to the following reasons during long-term use:

UV radiation degradation: Coating materials (such as organic coating agents) will undergo molecular chain breakage under UV light irradiation, resulting in coating layer detachment, cracking, or decreased transparency; Even for inorganic coatings, long-term ultraviolet radiation can lead to a decrease in the adhesion between the coating layer and the substrate, resulting in the phenomenon of "delamination".

Thermal aging: High temperatures can cause changes in the crystal structure of the coating layer (such as grain growth), or internal stresses due to differences in thermal expansion coefficients between the coating layer and the substrate, leading to cracking of the coating layer. For example, if MgF ₂ coating is used for a long time above 200 ℃, grain agglomeration may occur, resulting in a decrease in UV transmittance.

Environmental erosion: The chemical stability of the coating layer is usually lower than that of the quartz substrate, and it is more prone to oxidation and hydrolysis in high humidity and corrosive environments. For example, Al ₂ O ∝ coating absorbs moisture in high humidity environments, generating aluminum hydroxide, which causes the coating layer to become white and the light transmittance to decrease.

Photoactivation of internal impurities

Quartz glass inevitably contains trace impurities such as metal ions such as iron, chromium, nickel, titanium, or hydroxyl groups (- OH). Under long-term ultraviolet radiation, these impurities will be "activated": metal ions will transform from low valence states to high valence states (such as Fe ² ⁺ to Fe ³ ⁺), and high valence metal ions have a stronger ability to absorb ultraviolet light; Hydroxyl groups will decompose into hydrogen and oxygen atoms under the action of ultraviolet light, and hydrogen atoms will combine with silicon oxygen bonds to form new absorption centers. These activated impurities significantly increase the absorption of ultraviolet light by quartz glass, resulting in a decrease in transmittance. For example, quartz glass with a high content of hydroxyl groups (hydroxyl content>10ppm) will experience a rapid decrease in transmittance in the UVC band over time, as the absorption centers generated by hydroxyl decomposition strongly absorb ultraviolet light below 250nm.

（2） Identification characteristics of material aging

The decrease in transmittance caused by material aging can be distinguished from surface contamination by the following characteristics:

Wavelength selectivity of decreased transmittance: The decrease in transmittance caused by material aging (especially substrate photoinduced darkening) usually has a "wavelength dependence" - the impact on short wavelength ultraviolet light (UVC) is much greater than that on long wavelength ultraviolet light (UVB, UVA). For example, the transmittance of UVC band (254nm) may decrease by 20% after aging, while the transmittance of UVA band (365nm) only decreases by 5%; The decrease in transmittance caused by surface pollution usually has a more uniform impact on various wavelengths of ultraviolet light (unless it is a targeted pollutant that absorbs UVC more strongly, such as organic oil films, but this situation is rare).

No improvement after cleaning: As mentioned earlier, no matter what professional cleaning methods are used, the decrease in transmittance caused by material aging will not be significantly improved because aging occurs inside the lens or coating layer, and cleaning cannot repair internal structural defects.

The correlation between time and environment: The decrease in transmittance caused by material aging is "cumulative" and "irreversible", and is positively correlated with UV radiation dose (radiation intensity x usage time) and temperature - the longer the usage time, the stronger the UV radiation, and the higher the environmental temperature, the more severe the decrease in transmittance. For example, using a UVC sterilization lamp lens for 8 hours a day usually results in a 15% -20% decrease in transmittance after 1-2 years due to material aging, and the rate of decrease will accelerate with increasing service life.

3、 The synergistic effect and response strategies of surface pollution and material aging

In practical use, surface pollution and material aging do not exist independently, but often promote each other, exacerbating the decrease in transmittance. For example, pollutants on the surface of the lens will absorb ultraviolet light, causing local temperature rise and accelerating the thermal aging of the substrate; The surface roughness or defects caused by material aging will increase the adhesion area of pollutants, making them more difficult to clean. Therefore, it is necessary to adopt a strategy of "prevention first, precise governance" based on the characteristics of both.

Preliminary testing: Use a transmittance meter to measure the transmittance of the lens in different ultraviolet bands (UVC, UVB, UVA), and record the magnitude of the decrease and wavelength distribution characteristics.

Appearance and Cleaning Test: Observe the appearance through a high-power magnifying glass, then perform professional cleaning, and retest the transmittance after cleaning to determine if it is surface contamination.

Aging confirmation: If there is no improvement in transmittance after cleaning, and the decrease in transmittance is wavelength selective (with a more significant decrease in UVC), combined with usage time and environment (such as long-term high temperature, strong UV), it can be confirmed that the material is aging.

Solution: If there is surface pollution, strengthen cleaning and environmental control. If the material is aging, evaluate whether it needs to be replaced (for equipment with high precision requirements, it needs to be replaced in a timely manner; for equipment with low precision requirements, it can be temporarily used, but the detection cycle needs to be shortened).

4、 Conclusion

After long-term use, the transmittance of ultraviolet quartz lenses decreases. It is necessary to first use the three-step method of "appearance observation - cleaning test - wavelength analysis" to accurately distinguish whether it is caused by surface pollution or material aging: surface pollution is reversible, and the transmittance can be restored through professional cleaning; Material aging is irreversible and needs to be slowed down through selection optimization, environmental control, or timely replacement of lenses. In practical operation and maintenance, it is necessary to pay attention to the synergistic effect of the two. We should not only do a good job in daily cleaning and environmental management to reduce pollution adhesion, but also choose suitable lens materials according to the usage scenario, control ultraviolet radiation and temperature, delay aging from the source, maximize the service life of the lens, and ensure the stable operation of the equipment.