When purchasing JGS fused silica plates, wafers, windows, lenses or custom quartz components, buyers often request only a material grade such as JGS1, JGS2 or JGS3.
This is not always sufficient.
Two products sold under the same general JGS designation may differ in hydroxyl content, internal inclusions, refractive-index homogeneity, striae, stress birefringence and production traceability. These differences may have little effect on a general-purpose viewing window but can be critical in UV optics, infrared systems, laser equipment, semiconductor processing and precision metrology.
A reliable material certificate should therefore identify not only the nominal JGS grade but also the tested properties, measurement methods, acceptance criteria and relationship between the finished component and its original fused-silica blank.
This guide explains how buyers should review OH content, inclusion grade, optical homogeneity and traceability information before approving JGS fused-silica material.
1. What Do JGS1, JGS2 and JGS3 Mean?
JGS1, JGS2 and JGS3 are commonly used commercial classifications for different types of optical quartz glass and fused silica. They are generally associated with different manufacturing processes, spectral ranges and hydroxyl concentrations.
One published JGS material datasheet gives the following typical comparison. These values should be treated as supplier-specific reference data rather than universal acceptance limits.
| Property | JGS1 | JGS2 | JGS3 |
|---|---|---|---|
| General material type | Synthetic fused silica | Optical-grade fused quartz | Low-OH fused silica or fused quartz |
| Typical production method | Synthetic CVD or flame hydrolysis | Oxy-hydrogen melting | Electrical melting |
| Typical transmission range | 0.17–2.10 μm | 0.26–2.10 μm | 0.185–3.50 μm |
| Typical OH content | Around 1,200 ppm | Around 150 ppm | Around 5 ppm |
| Typical impurity content | Around 5 ppm | Around 20–40 ppm | Around 40–50 ppm |
| Typical birefringence constant | 2–4 nm/cm | 4–6 nm/cm | 4–10 nm/cm |
| Common application direction | UV and laser optics | Visible optics and high-temperature windows | Near-IR and broader-spectrum applications |
These figures are useful for preliminary material selection, but they do not prove that every JGS1 blank contains exactly 1,200 ppm OH or that every JGS3 blank contains exactly 5 ppm.
For comparison, Corning specifies an OH content of 800–1,000 ppm for some HPFS synthetic fused-silica wafer and ArF-grade products. This demonstrates why buyers should request the supplier’s actual material specification rather than assuming that every UV-grade synthetic fused silica has one fixed OH value.
JGS Classification Is Not a Complete Certificate
The grade name generally indicates a material family or intended spectral application. It does not automatically define:
- Exact OH concentration
- Maximum bubble diameter
- Total inclusion area
- Refractive-index homogeneity
- Striae class
- Stress birefringence
- Metallic impurity content
- Measurement wavelength
- Inspection aperture
- Blank orientation
- Batch identity
- Country or factory of origin
For critical optical applications, the purchase specification should list each required property separately.
2. OH Content in Fused Silica
OH content refers to hydroxyl groups incorporated into the silica-glass structure. It is normally reported as parts per million by weight.
Hydroxyl concentration is strongly influenced by the raw material and manufacturing process. Synthetic fused silica manufactured through flame-based processes may contain relatively high OH concentrations, while specially dried or electrically fused materials can have much lower OH levels. Heraeus notes that OH groups produce pronounced absorption bands in high-purity fused silica, including bands near 945, 1,245, 1,385, 2,210 and approximately 2,700–2,800 nm.
Why OH Content Matters
OH content should be evaluated against the operating wavelength rather than judged as simply “high” or “low.”
High-OH synthetic fused silica can provide excellent deep-UV performance when metallic contamination and other absorbing impurities are well controlled. However, OH-related absorption can reduce transmission in specific near-infrared and infrared wavelength regions. Low-OH materials are therefore commonly selected when transmission through these OH absorption bands is important.
A buyer should therefore provide:
- Operating wavelength
- Required internal transmittance
- Component thickness
- Optical path length
- Maximum acceptable absorption
- Laser type and power, when applicable
- Whether the requirement applies to typical or guaranteed performance
A low OH result alone does not guarantee good UV transmission. Metallic impurities, defects, manufacturing route and irradiation behavior must also be considered.
How OH Content Is Measured
Fourier-transform infrared spectroscopy is commonly used to determine OH concentration in fused silica by measuring absorption associated with hydroxyl groups. Published research has used FTIR transmission spectra to calculate OH concentrations in fused-silica samples with different thicknesses and OH levels.
A useful OH test report should identify:
- Measurement method
- Instrument type
- Sample thickness
- Measurement wavelength or wavenumber range
- Absorption band used for calculation
- Baseline method
- Conversion or extinction coefficient
- Result in ppm by weight
- Detection limit
- Whether the result is measured, calculated or typical
- Whether the finished part or parent blank was tested
Without these details, two laboratories may report different OH values for the same material because of differences in sample thickness, calibration and calculation method.
Typical Value vs Guaranteed Maximum
A certificate may contain statements such as:
- Typical OH content: 1,000 ppm
- OH content range: 800–1,000 ppm
- Maximum OH content: 10 ppm
- Actual measured OH content: 6.4 ppm
These statements are not equivalent.
A typical value describes normal production but is not necessarily an acceptance limit.
A specified range or maximum is a contractual material requirement.
An actual measured result provides lot-specific evidence, provided that the tested sample can be traced to the supplied component.
For demanding applications, the purchase order should state whether an actual batch result is required.
3. Bubble and Inclusion Certification
Bubbles and inclusions are internal material imperfections that can scatter light, distort a transmitted beam or create local optical defects.
The current ISO 10110-18 standard covers drawing indications for stress birefringence, bubbles and inclusions, homogeneity and striae in optical materials. It replaced the earlier separate ISO 10110 sections for bubbles, inclusions and material inhomogeneity.
What Is an Inclusion Grade?
An inclusion specification should describe more than whether the glass appears clear to the unaided eye.
For example, Corning’s HPFS quality system defines inclusion classes using:
- The total cross-sectional area of inclusions in a defined volume of glass
- The diameter of the largest individual inclusion
In one HPFS classification, inclusion Class 0 limits total inclusion cross-section to no more than 0.03 mm² per 100 cm³ of glass and limits the largest specified inclusion to 0.10 mm. Larger class numbers permit progressively greater total inclusion area and maximum individual size.
This illustrates an important purchasing principle: the buyer should define both quantity or cumulative area and maximum individual defect size.
Information Required for Inclusion Inspection
A complete inclusion requirement should define:
- Clear aperture
- Inspection volume
- Maximum individual bubble or inclusion size
- Maximum number of defects
- Total permitted cross-sectional area
- Minimum defect size included in the count
- Inspection illumination
- Magnification
- Inspection direction
- Treatment of elongated defects
- Treatment of clusters
- Acceptance of defects outside the clear aperture
A statement such as “bubble-free” may be interpreted differently under normal room lighting and high-intensity inspection.
Corning’s 2025 fused-silica wafer datasheet, for example, describes inclusions as none detected under high-intensity illumination. This wording connects the result to a defined inspection condition rather than claiming that no microscopic defect could exist at any detection level.
Bubble, Inclusion and Surface Defect Are Different
The certificate should distinguish internal material defects from processing defects.
- Bubble: A gas-filled internal cavity.
- Inclusion: Foreign material or a localized internal imperfection.
- Pit: Local material loss from a polished surface.
- Chip: Material loss at an edge.
- Scratch: A linear surface defect.
- Dig: A localized surface defect under optical surface-quality classification.
- Striae: Refractive-index variation with a streak-like structure.
A material inclusion certificate does not automatically certify polished-surface scratch-dig quality.
4. Refractive-Index Homogeneity
Optical homogeneity describes how uniformly the refractive index is distributed through the usable material.
A fused-silica blank may appear visually clear while still containing small refractive-index variations. These variations can distort the transmitted wavefront and reduce the performance of lenses, windows, prisms, beam homogenizers and laser optics.
Corning defines index homogeneity as the maximum relative refractive-index variation measured over the clear aperture. Its published HPFS documentation states that homogeneity is certified by interferometry at 632.8 nm and reported as an average through the thickness of the component.
Example Homogeneity Grades
One Corning fused-silica quality classification includes the following homogeneity levels:
| Homogeneity grade | Maximum relative index variation |
|---|---|
| AA | ≤0.5 ppm |
| A | ≤1 ppm |
| B | ≤1.5 ppm |
| C | ≤2 ppm |
| D | ≤3 ppm |
| E | ≤4 ppm |
| F | ≤5 ppm |
| G | Not specified |
These grades are manufacturer-specific and should not automatically be applied to every JGS supplier. However, they show how homogeneity can be converted from a vague statement such as “highly uniform” into a measurable acceptance criterion.
What a Homogeneity Certificate Should State
The certificate should identify:
- Maximum refractive-index variation
- Result in ppm or another defined unit
- Measurement wavelength
- Clear-aperture diameter
- Measurement direction
- Optical-use axis
- Component thickness
- Test temperature
- Interferometer or measurement method
- Whether piston, tilt or power terms were removed
- Whether the finished part or parent blank was tested
- Orientation of the finished component within the parent blank
These details matter because the measured result can depend on the test direction and data-processing method.
Thin Components and Parent-Blank Certification
Direct homogeneity measurement may be difficult for thin windows or wafers. Corning states that its minimum thickness for direct index-homogeneity verification is 20 mm; thinner parts may be certified using their parent material.
When parent-blank certification is used, traceability becomes essential. The certificate should connect the finished component to:
- Parent boule or ingot
- Blank number
- Cut position
- Orientation
- Production batch
- Homogeneity map, when applicable
A certificate for an unrelated blank from the same material grade is not sufficient.
5. Homogeneity, Striae and Birefringence Are Not the Same
These three properties are frequently confused.
Homogeneity
Homogeneity is the overall variation in refractive index across the specified aperture or material volume.
Striae
Striae are localized streaks or layers of refractive-index variation caused by non-uniformities in the glass-production process.
ISO 10110-18 includes tolerancing conventions for both homogeneity and striae. Corning also reports certain HPFS grades as meeting ISO 10110-4 Class 5 for striae, although ISO 10110-4 has since been replaced by ISO 10110-18.
Stress Birefringence
Stress birefringence is optical anisotropy caused by residual or applied stress. It is commonly reported as retardation per unit optical path, such as nm/cm.
A material can have good refractive-index homogeneity but still exhibit unacceptable stress birefringence after cutting, grinding, polishing, welding or unsuitable annealing.
For polarization-sensitive, laser or interferometric applications, all three requirements may need to be specified separately.
6. Transmission Certification
A JGS certificate should not rely only on a broad statement such as “UV grade” or “IR grade.”
The buyer should define:
- Test wavelength range
- Component thickness
- Internal or external transmittance
- Surface condition
- Coated or uncoated status
- Incidence angle
- Measurement resolution
- Minimum transmission at critical wavelengths
- Whether Fresnel surface losses are included
ISO 15368:2021 provides rules for measuring regular spectral transmittance and reflectance of plane-parallel coated or uncoated optical components without optical power.
Internal transmission removes the effect of surface reflection and is useful for evaluating the glass material itself. External transmission includes reflection losses from the component surfaces and therefore depends on refractive index, surface quality and coatings.
A certificate should clearly state which value is reported.
7. Material Traceability
Traceability connects a finished fused-silica product to its raw material, manufacturing batch and inspection records.
Corning states that it provides a certificate of compliance with each sale of its HPFS fused-silica materials. Its wafer documentation also lists laser serialization as an available option, illustrating how physical part identification can be linked to manufacturing records.
Recommended Traceability Chain
A complete traceability chain may include:
Raw material or precursor → production batch → boule or ingot → cut blank → processed component → final inspection → shipment
Each stage should retain an identifier that can be traced backward.
Information to Include on the Certificate
A JGS fused-silica certificate should ideally include:
- Supplier name
- Manufacturing location
- Material designation
- JGS grade
- Supplier material code
- Purchase order number
- Customer part number
- Drawing number and revision
- Lot or batch number
- Boule, ingot or blank number
- Finished-part serial number
- Production date
- Quantity
- Component dimensions
- Tested properties
- Actual test results
- Specification limits
- Test methods
- Test equipment identification
- Calibration status
- Inspector or approver
- Certificate issue date
- Authorized signature
- Statement of conformity
- Approved deviations, when applicable
For wafers or large optical blanks, the certificate may also identify cut orientation and position within the parent material.
8. Certificate of Conformity vs Material Test Report
These documents serve different purposes.
Certificate of Conformity
A certificate of conformity generally states that the supplied product meets the purchase order, drawing and agreed specifications.
It may not contain all measured values.
Material Test Report
A material test report provides actual or batch-level results for properties such as:
- OH concentration
- Inclusion class
- Homogeneity
- Striae
- Birefringence
- Transmission
- Chemical impurity level
- Density or other material properties
Inspection Report
An inspection report normally records the dimensions and processed quality of the finished component, such as:
- Diameter
- Thickness
- Flatness
- Parallelism
- Surface roughness
- Scratch-dig
- Edge quality
- Hole dimensions
- Coating performance
For critical projects, buyers may require all three documents.
9. Recommended JGS Certificate Review Checklist
Before approving a fused-silica shipment, verify the following:
| Review item | Key question |
|---|---|
| Material grade | Is JGS1, JGS2 or JGS3 clearly identified? |
| Supplier specification | Is the applicable material datasheet or revision listed? |
| OH content | Is it typical, guaranteed or actually measured? |
| OH method | Are FTIR method and sample thickness identified? |
| Inclusions | Are maximum size and total quantity or area defined? |
| Inspection condition | Are illumination and detection limits stated? |
| Homogeneity | Is a numerical refractive-index variation reported? |
| Measurement wavelength | Is the interferometer wavelength stated? |
| Clear aperture | Does the tested aperture match the usable optical area? |
| Striae | Is the applicable class or acceptance criterion listed? |
| Birefringence | Is residual stress within the required limit? |
| Transmission | Are wavelength, thickness and internal/external conditions clear? |
| Traceability | Can the finished component be linked to the tested blank? |
| Lot identification | Does the certificate match the packaging label? |
| Actual results | Are measured values supplied where required? |
| Approval | Is the certificate signed or electronically authorized? |
10. Example RFQ Specification
A practical RFQ note may be written as follows:
Material: JGS1 synthetic fused silica or approved equivalent.
OH content: Supplier shall state the guaranteed range and provide an actual batch result measured by FTIR.
Inclusions: Maximum individual inclusion and cumulative inclusion limits shall be stated. Inspection shall cover the full specified clear aperture.
Homogeneity: Maximum relative refractive-index variation shall be specified in ppm. The measurement wavelength, aperture, thickness and measurement direction shall be included in the report.
Striae: Material shall comply with the agreed ISO 10110-18 indication or customer-defined acceptance standard.
Stress birefringence: Maximum permitted retardation shall be stated in nm/cm.
Transmission: Internal transmission shall meet the specified minimum at the operating wavelengths and component thickness.
Traceability: Each finished component or production lot shall be traceable to the parent blank and material batch.
Documentation: Certificate of conformity, material test report and final dimensional inspection report shall be provided with shipment.
11. Common Certification Problems
Listing Only “JGS1”
This does not confirm actual OH content, inclusion quality or homogeneity.
Providing Only a Generic Datasheet
A generic datasheet describes a material family but does not prove that the supplied lot was tested.
Reporting Typical Values as Actual Results
A typical OH value should not be presented as a measured batch result.
Omitting the Measurement Thickness
Transmission and OH calculations can depend on sample thickness and optical path length.
Using “Bubble-Free” Without a Detection Limit
The certificate should identify the inspection conditions and minimum counted defect size.
Certifying the Parent Blank Without Traceability
Parent-material data are useful only when the supplier can demonstrate that the finished parts came from that blank.
Confusing Homogeneity with Surface Flatness
Homogeneity describes refractive-index uniformity inside the material. Flatness describes the geometrical form of a processed surface.
Ignoring Orientation
Homogeneity and striae performance can vary with optical direction. The certified measurement direction should match the use direction whenever this is critical.
Conclusion
A JGS1, JGS2 or JGS3 designation is useful for preliminary fused-silica selection, but it should not be treated as a complete optical-material specification.
A reliable certificate should clearly define:
- OH content
- Measurement method
- Bubble and inclusion acceptance
- Refractive-index homogeneity
- Clear aperture
- Measurement direction
- Striae
- Stress birefringence
- Spectral transmission
- Parent-blank identity
- Batch and component traceability
- Actual test results
- Applicable acceptance limits
For general industrial components, a supplier certificate of conformity may be sufficient. For UV optics, IR systems, laser components, semiconductor equipment and precision optical assemblies, buyers should request numerical material data and traceable lot-specific documentation.
The most effective purchasing approach is to specify every critical optical property in the RFQ rather than relying solely on the JGS grade name.
Frequently Asked Questions
Is JGS1 always better than JGS2 or JGS3?
No. The correct grade depends on operating wavelength, absorption requirements, impurity limits, homogeneity and cost. JGS1 is commonly selected for UV applications, while lower-OH materials may be more suitable for particular infrared wavelengths.
Does low OH content mean higher material purity?
Not necessarily. OH content is only one property. A low-OH electrically fused material may contain more metallic impurities than a high-OH synthetic fused silica.
Can OH content be checked on the finished component?
It can be measured when the component geometry, thickness and surface condition permit reliable spectroscopic testing. In other cases, the supplier may certify the parent blank.
What is the difference between an inclusion and striae?
An inclusion is a localized internal defect or foreign feature. Striae are streak-like variations in refractive index.
Is visible inspection sufficient for optical inclusions?
It may be sufficient for general industrial components, but precision optics normally require controlled illumination, defined detection limits and numerical acceptance criteria.
Should every component have an individual serial number?
Individual serialization is useful for high-value optical parts and critical assemblies. Batch-level traceability may be sufficient for repetitive lower-risk components.
Can a supplier use an equivalent material instead of JGS1?
Yes, provided the buyer approves the equivalent material and its certified OH content, transmission, inclusion level, homogeneity, striae and other critical properties meet the application requirements.
