What is the transmittance of ultraviolet quartz lens?

As the core component of ultraviolet optical systems, the transmittance of ultraviolet quartz lenses is a key indicator for measuring performance, which directly affects the efficiency and accuracy of optical equipment. So, what is the transmittance of ultraviolet quartz lenses? This issue requires a comprehensive analysis of multiple factors such as material properties, wavelength range, and processing technology.

1、 Basic transmittance range: correlation between material and wavelength

The transmittance of ultraviolet quartz lenses first depends on the inherent characteristics of the substrate - quartz glass. Quartz glass (mainly composed of silicon dioxide) has a much lower ability to absorb ultraviolet light than ordinary optical glass, which is the fundamental reason why it serves as the core material of ultraviolet lenses.

In the ultraviolet band (usually divided into UVC: 100-280nm, UVB: 280-320nm, UVA: 320-400nm), the transmittance of high-quality ultraviolet quartz lenses is as follows:

UVA band (320-400nm): The transmittance can reach over 90%, and some high-precision products can even approach 95%. This is because the ultraviolet energy in this band is relatively low, and quartz glass absorbs it weakly. Moreover, reflection losses can be further reduced through processes such as coating.

UVB band (280-320nm): The transmittance is usually between 85% and 90%. As the wavelength shortens and ultraviolet energy increases, trace impurities (such as hydroxyl groups and metal oxides) in quartz glass will absorb light to a certain extent, resulting in a slight decrease in transmittance.

UVC band (100-280nm): The transmittance of this band varies greatly, with the transmittance at the short wavelength end (such as below 180nm) possibly dropping below 70%, while the transmittance at the long wavelength end (such as 250-280nm) can be maintained at 80% -85%. Among them, around 185nm is one of the absorption peaks of quartz glass, and there will be a significant decrease in transmittance.

In contrast, ordinary optical glass has significantly lower transmittance in the ultraviolet band, for example, ordinary silicate glass is almost completely opaque below 300nm, highlighting the irreplaceability of quartz materials in the ultraviolet field.

2、 Key factors affecting transmittance

The transmittance of ultraviolet quartz lenses is not a fixed value, but is influenced by various factors, with the following types of factors being particularly critical:

1. Material purity

The purity of quartz glass directly determines the lower limit of transmittance. High purity quartz glass (such as synthetic quartz) contains more than 99.99% of silicon dioxide, almost free of hydroxyl (OH ⁻) and metal impurities, and its light transmittance in deep ultraviolet wave band (such as below 200nm) is 10% -15% higher than that of ordinary fused silica. For example, the light transmittance of synthetic quartz with hydroxyl content less than 5ppm can exceed 80% at 200nm, while that of ordinary fused silica may only be about 60%.

2. Lens thickness

According to Lambert Beer's law, the transmittance of light decreases with increasing material thickness. In the ultraviolet band, this pattern is more pronounced. Taking the 250nm wavelength as an example, the transmittance of a 1mm thick high-purity quartz lens is about 88%, while the transmittance of a 5mm thick lens of the same type may decrease to around 75%. Therefore, ultraviolet quartz lenses are usually designed with thin structures, especially in the deep ultraviolet band (<250nm), and the thickness is often controlled within 1-3mm.

3. Surface processing accuracy

The flatness and smoothness of the lens surface directly affect the reflection loss of light. If there are scratches, dents, or high roughness on the surface (such as Ra>5nm), it will lead to increased scattering and a 5% -10% decrease in transmittance. By precision polishing (such as magneto rheological polishing) to control the surface roughness at Ra<1nm, scattering loss can be minimized to the greatest extent, laying the foundation for high transmittance.

4. Anti reflective film layer

The light reflectivity in the ultraviolet band is relatively high (the natural light reflectivity at the air quartz interface is about 8%), and the transmittance can be significantly improved by coating the ultraviolet anti reflective film. For example, at a wavelength of 300nm, the transmittance of uncoated quartz lenses is about 85% (considering double-sided reflection loss), while after coating with a single-layer magnesium fluoride anti reflective film, the transmittance can be increased to over 93%; If multi-layer broadband anti reflective film is used, the transmittance can be stabilized at over 90% in the 200-400nm wavelength range.

3、 Transparency requirements for different application scenarios

The transmittance of ultraviolet quartz lenses needs to be designed according to specific application scenarios, and it is not necessarily better to have a higher transmittance (balancing cost and performance):

UV sterilization equipment (mainly using 254nm wavelength): The requirement for transmittance is usually above 80%. At this time, the cost and weather resistance of the lens should be taken into account, and ordinary fused silica plus a single layer antireflective film should be used to ensure sterilization efficiency while controlling the cost.

Lithography equipment (deep ultraviolet band, such as 193nm): The transmittance needs to reach over 90%, and the wavelength stability is required to be extremely high. This type of lens must use high-purity synthetic quartz and be paired with multiple layers of precision anti reflective films to meet the high-precision requirements of photolithography patterns.

UV spectrometer (covering the 200-400nm wavelength band): It is necessary to maintain high transmittance in a wide frequency band (usually requiring 85% or more), so broadband anti reflective film technology is needed to ensure that light signals of different wavelengths can be effectively transmitted, ensuring the accuracy of spectral measurement.

UV curing equipment (365nm, 395nm, etc.): The transmittance should reach 85% or above, with a focus on the stability of transmittance after long-term use. The lens needs to have anti UV aging ability to avoid a decrease in transmittance due to film peeling or material degradation.

4、 Testing and Verification of Transmittance

To ensure that the transmittance of ultraviolet quartz lenses meets the standard, professional equipment testing is required:

Testing instrument: UV visible spectrophotometer (such as Shimadzu UV-2600, Perkin Elmer Lambda 950), which needs to cover the wavelength range of 190-400nm with an accuracy of ± 0.1%.

Testing method: Using the dual beam method with air as a reference, measure the transmittance of the lens at different wavelengths, deduct reflection and scattering losses, and obtain the actual transmittance data.

Environmental requirements: Testing must be conducted in a clean room (Class 1000 or above) to avoid dust adhesion affecting measurement accuracy; At the same time, it is necessary to control the ambient temperature (25 ± 2 ℃) to reduce the impact of temperature changes on the refractive index of quartz.

5、 Summary

The transmittance of ultraviolet quartz lenses is not a fixed value, but fluctuates within the range of 70% -95%, depending on factors such as wavelength, material purity, thickness, and processing technology. In the UVA band (320-400nm), the transmittance of high-quality products can reach 90% -95%; In the UVB band (280-320nm), the transmittance is mostly 85% -90%; In the UVC band (100-280nm), the transmittance gradually decreases with wavelength shortening, and the deep ultraviolet end (<250nm) is usually 70% -85%.

In practical applications, it is necessary to select lenses with appropriate transmittance according to equipment requirements (such as sterilization, photolithography, spectral analysis, etc.), and achieve a balance between cost and reliability while meeting performance requirements through material optimization, precision machining, and coating technology. With the deepening application of ultraviolet technology in medical, semiconductor, environmental protection and other fields, the requirements for the transmittance of ultraviolet quartz lenses will continue to increase, promoting continuous innovation in material processes and coating technologies.