Quartz Tube Transmittance: Thickness, Surface, and Key Factors
In practical use, the light transmittance of quartz tubes depends not only on the material itself, but also on wall thickness, surface condition, and manufacturing processes. These factors are often underestimated, especially in ultraviolet and infrared applications, leading to selection errors.
This article focuses on quartz tube transmittance from an engineering perspective, summarizing how wall thickness, surface polishing, and material factors influence transmittance across different wavelength ranges, and providing practical guidance for design and selection.
Table of Contents
Relationship Between Quartz Tube Wall Thickness and Transmittance
The relationship between the wall thickness of quartz tubes and their transmittance is a highly fundamental question in optical properties.
Overall, for quartz tubes made of the same material, an increase in wall thickness leads to a decrease in transmittance. However, this decrease is not a simple linear relationship. The specific manner and extent of the change depend on several factors.
Core Principle: Lambert–Beer Law
This relationship is mainly described by the classical Lambert–Beer Law.
The law states that when light travels through a homogeneous medium, its attenuation is proportional to the incident intensity, the medium’s thickness, and the medium’s intrinsic properties.
Its mathematical expression is:
Where:
- T is the transmittance, a dimensionless value between 0 and 1 (or 0% to 100%).
- I₀ is the incident light intensity.
- I is the transmitted light intensity.
- d is the optical path length through the medium (i.e., the wall thickness of the quartz tube).
- α(λ) is the absorption coefficient of the medium, which is a function of wavelength.
From this formula, two key conclusions can be drawn:
1. Exponential attenuation relationship
Transmittance T decreases exponentially with thickness d. This means that as wall thickness increases, the rate of decline becomes increasingly rapid, rather than decreasing proportionally.
Example: Suppose the absorption coefficient α at a certain wavelength is constant.
When thickness increases from 1 mm to 2 mm, transmittance may drop from 90% to 81%.
When thickness increases from 2 mm to 3 mm, it drops further to 72.9%.
For each equal thickness increment, the loss becomes more significant.
2. Absorption coefficient α is the key factor
The coefficient α(λ) determines the “speed” of attenuation.
A larger α means stronger absorption at that wavelength, causing wall thickness to have a much more pronounced effect.
Behavior at Different Wavelengths
The absorption coefficient α(λ) of quartz depends strongly on wavelength.
Thus, the influence of wall thickness varies significantly across spectral regions.
1. In the “transparency window” (e.g., visible region)
High-purity quartz exhibits very weak intrinsic absorption in this range; α is extremely small.
Even when wall thickness increases, the exponential term remains close to 1, so the transmittance decrease is minimal. For example, increasing thickness from 2 mm to 4 mm might only reduce transmittance from 92% to 91%.
In this region, the main factors affecting transmittance may be surface scattering and reflection losses, rather than bulk absorption.
2. Near absorption peaks (e.g., OH absorption at 2.7 μm or metallic impurity bands)
In these specific wavelengths, the absorption coefficient α becomes very large.
Even a small increase in thickness can cause transmittance to drop drastically, and light may be unable to penetrate thicker tube walls at all.
Example: Near the 2.7 μm OH absorption peak, for high-OH quartz, a thickness of just 0.5 mm may be enough to absorb most of the IR light. Further thickness increases become meaningless.
3. Near the UV cutoff edge
Quartz transmittance naturally declines as wavelength approaches the ultraviolet cutoff (~200 nm).
In this region, α increases significantly.
Wall thickness has a major impact here.
A thinner-wall quartz tube allows a shorter UV cutoff wavelength and higher deep-UV transmittance.
This is critical for DUV applications.
A simplified way to understand how wavelength and quartz tube wall thickness affect transmittance:
- In the UV cutoff region, thick-wall tubes have much lower transmittance than thin-wall tubes.
- In the visible region, the transmittance difference between thin and thick walls is very small.
- At the IR OH absorption peak, the transmittance of thick-wall and high-OH quartz drops sharply.
Practical Trade-offs in Applications
When selecting the wall thickness of quartz tubes, the following must be considered:
- Mechanical strength and pressure resistance
Thicker walls provide greater mechanical strength and can withstand higher internal/external pressure differences and impact. - Thermal capacity and thermal response
Thicker walls have greater thermal mass, requiring longer heating and cooling times and resulting in slower thermal response. - Transmittance requirements
After meeting mechanical and thermal requirements, if the application demands high transmittance—especially in UV or specific IR wavelengths—choosing thinner walls is preferable.
Conclusion
Wall thickness and transmittance follow an exponential attenuation relationship, but the degree of this effect is highly dependent on wavelength, as well as the material purity and OH content of the quartz tube.
In strong absorption bands, the influence of wall thickness is particularly dramatic.
The Relationship Between Quartz Tube Surface Polishing and Transmittance
This is a very practical and important topic. The surface polishing quality of a quartz tube is closely related to its transmittance, and its impact can even be as significant as the material purity itself.
Overall: The polishing quality directly determines the reflection and scattering losses of light, thereby significantly affecting the actual transmittance of the quartz tube. A precisely polished surface can maximize effective transmittance.
Below is a detailed analysis from the perspectives of principles, impact factors, and quantification.
Core Principle: Fresnel Reflection and Scattering
When light passes through any interface between media of different refractive indices (such as from air to quartz), both reflection and refraction occur. Transmittance loss mainly comes from two aspects:
Fresnel Reflection Loss
Even for an ideally smooth surface, due to the refractive index difference between air and quartz (air n ≈ 1, quartz n ≈ 1.46), reflection occurs at the interface.
For light incident perpendicularly, the reflectance R is calculated by:
R = [(n₂ – n₁) / (n₂ + n₁)]².
Substituting the values, the reflectance at a single surface is approximately:
[(1.46 – 1) / (1.46 + 1)]² ≈ 3.5%.
A quartz tube has two interfaces (air–quartz–air), so in ideal conditions, the theoretical reflection loss is about 7%.
This means that even if the material itself is perfectly transparent (100% transmission), the theoretical maximum transmittance under non-vacuum conditions is only about 93%.
Scattering Loss (The Core Issue Polishing Addresses)
If the surface is rough and uneven, light will undergo diffuse scattering rather than regular mirror reflection.
These scattered rays deviate from the original optical path and eventually leave the optical system, causing additional energy loss.
The rougher the surface, the greater the scattering loss, and the lower the effective transmittance.
How Polishing Affects Transmittance
The main purpose of polishing is to eliminate surface roughness, turning the surface from “frosted” to “mirror-like,” thus reducing scattering and increasing the proportion of mirror reflection.
Surface Condition and Its Optical Impact
| Surface Condition | Optical Effect | Impact on Transmittance |
|---|---|---|
| Rough surface (unpolished or raw tube) | Produces significant scattering | Low transmittance; losses include reflection, material absorption, and severe scattering. Light spots become blurred and may show “haze.” |
| Precision-polished surface (optical mirror) | Mirror reflection dominates; minimal scattering | Highest transmittance. Total transmittance approaches the theoretical limit (93% minus material absorption). Imaging is clear and energy is concentrated. |
Simple Analogy
A rough surface is like frosted glass—light becomes soft and dim due to heavy scattering.
A polished surface is like a mirror or window glass—light passes through directly, bright and clear.
Quantitative Comparison: Polishing Grades and Transmittance
Different polishing techniques produce widely varying results:
Fire Polishing
- Process: Heating with a flame to melt the surface slightly so that surface tension smooths it.
- Effect: Removes sharp edges and large defects; better than raw tubes but still has waviness and micro-roughness. Scattering remains relatively high.
Suitable for applications with moderate optical requirements, such as lamp housings.
Mechanical Polishing
- Process: Abrasives and polishing wheels mechanically refine the surface.
- Effect: Achieves good smoothness and significantly reduces scattering. Common for industrial quartz tubes and provides high transmittance.
Optical (Precision) Polishing
- Process: Ultra-fine abrasives and strict process control to achieve atomic-level surface smoothness.
- Effect: Extremely low roughness (nanometer scale); scattering is negligible.
Transmittance reaches over 99% of the theoretical maximum.
Used in lasers, optical windows, lenses, and other high-end optical systems.
Benefits of Polishing Beyond Transmittance
Improved Mechanical Strength
Polishing removes micro-cracks and defects, reducing stress concentration and significantly improving the tube’s mechanical (especially bending) strength.
Better Contamination Resistance
A smooth surface is less prone to adsorbing contaminants, easier to clean, and more resistant to chemical adhesion or corrosion.
Higher Laser Damage Threshold
In high-power laser systems, rough surfaces generate heat spots and local damage.
Optical-grade polished surfaces can withstand much higher laser power.
Conclusion
The surface polishing quality of quartz tubes is one of the decisive factors for achieving high transmittance.
Even with the highest-purity synthetic quartz, poor surface quality results in very low actual transmittance.
Therefore, selection must balance application needs and cost:
- For lighting, viewing windows, ordinary heating tubes: fire polishing or mechanical polishing is sufficient.
- For UV curing, sterilization, precision sensing: high-quality mechanical polishing is required to ensure UV efficiency.
- For laser optics, spectroscopy, imaging systems: optical-grade precision polishing is essential; otherwise, the performance of the quartz material cannot be fully realized.
Factors Affecting the Transmittance of Quartz Tubes
The transmittance of quartz tubes—especially in the ultraviolet region—is a core performance indicator and is closely related to several key factors.
Overall, the transmittance of a quartz tube depends mainly on material purity, hydroxyl content, physical structure, and the environment and conditions of use.
Below is a detailed breakdown:
1. Material Purity (Key Factor)
This is the most critical factor affecting quartz tube transmittance, particularly UV transmittance.
Metal Impurity Ions
Ordinary glass contains large amounts of sodium, calcium, iron, and other metal ions, which strongly absorb ultraviolet light and result in extremely low UV transmittance.
Quartz glass (silicon dioxide, SiO₂), however, is naturally transparent to UV light.
Transition Metal Elements
Elements such as iron (Fe), chromium (Cr), titanium (Ti), and copper (Cu)—even at ppm levels—produce strong absorption bands in the UV and visible regions, significantly reducing transmittance.
Therefore, high-purity quartz tubes must be made from high-purity natural crystal or through chemical synthesis (e.g., SiCl₄ vapor deposition) to drastically reduce impurity content.
Purity Levels
- Standard quartz tubes: Higher impurity content, weaker UV transmittance
- Premium quartz tubes: Low impurity content (typically below tens of ppm), good UV transmittance
- High-purity or synthetic quartz tubes: Extremely low impurity content (< a few ppm, even ppb), highest UV transmittance, ideal for UV lamps and precision optics
2. Hydroxyl Content (-OH Group Content)
Hydroxyl groups are structural defects in quartz glass, existing in the form of Si-OH.
Their presence has a decisive influence on infrared transmittance.
Infrared Absorption
Hydroxyl groups show strong absorption peaks near 2.7 μm (2700 nm) and 1.38 μm (1380 nm).
Higher hydroxyl content results in lower IR transmittance in these bands.
Sources
- Flame-fused quartz: Introduces large amounts of hydroxyl due to hydrogen-oxygen flames (typically >150 ppm)
- Electric-fused or vacuum-fused quartz: Extremely low hydroxyl content (<5 ppm), referred to as “low-OH” or “no-OH” quartz, with excellent IR performance
- Synthetic quartz: Hydroxyl content can be precisely controlled
Selection Criteria
- For UV or visible applications (e.g., sterilization, fluorescence detection): focus on metal impurity purity; hydroxyl levels matter less
- For IR applications (heating, spectroscopy): choose low-OH quartz
3. Bubbles, Inclusions, and Uniformity (Physical Defects)
These are macroscopic defects introduced during manufacturing.
Bubbles and Stones
These cause direct light scattering, reducing overall transmittance and potentially creating spots or uneven heating.
High-quality quartz should be free of bubbles and inclusions.
Striae and Density Non-uniformity
Poor melting processes may cause uneven density and refractive index, leading to light scattering and wavefront distortion—particularly detrimental for imaging systems.
4. Surface Quality
Light transmission occurs across two interfaces (air-quartz, quartz-air).
Surface Smoothness
Rough surfaces increase scattering and reduce effective transmittance.
Optical-grade quartz tubes require precision polishing.
Cleanliness
Contamination (dust, fingerprints, residues) absorbs or scatters light.
Maintaining surface cleanliness is essential.
5. Wall Thickness
According to Lambert–Beer Law, absorption is proportional to the thickness of the medium.
Effect of Thickness
For the same material:
- Thicker walls → higher absorption → lower transmittance
Especially true in wavelength regions sensitive to impurities or hydroxyl absorption.
Quartz tubes should therefore be as thin as possible while still meeting strength requirements.
6. Operating Environment and Aging
Quartz tube transmittance changes over time.
Solarization (Irradiation-Induced Darkening)
Under long-term UV irradiation (especially short-wave UV), structural defects are activated to form color centers, reducing UV and visible transmittance (notably 240–300 nm).
High-purity or synthetic quartz has stronger resistance to solarization.
High Temperature
Long-term high-temperature use may trigger impurity migration or devitrification (formation of white cristobalite crystals), which scatter light and severely reduce transmittance.
Contamination
Chemical exposure may cause corrosion or contamination layers inside the tube, forming absorbing films.
Summary and Application Selection Guide
| Application Scenario | Key Factors | Recommended Quartz Type |
|---|---|---|
| UV lamps (sterilization, curing) | Ultra-high UV purity, anti-solarization | Synthetic quartz or ultra-high-purity quartz |
| Infrared heating | Very low hydroxyl content | Low-OH quartz (electric-fused) |
| Optical windows, lenses | High purity, low OH, high uniformity, no bubbles | Optical-grade synthetic quartz |
| Semiconductor diffusion/oxidation | High purity, thermal stability (anti-devitrify) | High-purity quartz |
| General viewing windows, lighting | Cost | Standard or premium quartz |
Therefore, selecting a quartz tube requires considering your specific wavelength requirements, operating temperature, and environment, with focus on purity, hydroxyl content, and physical defects.
Discuss Your Application Requirements
If your application requires a specific quartz tube transmittance, selecting the right material and structure is not always simple.
We supply quartz tubes for defined optical and application requirements. You can submit your application details through the form, and we will review suitable options.
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