Choosing between a Spherical Lens made from BK7 and one made from fused silica can determine whether an optical system stays stable or drifts during operation. Engineers comparing BK7 vs fused silica plano-convex lenses, searching for the best PCX lens for laser focusing, or evaluating a UV-compatible plano-convex lens usually ask the same questions: Which material transmits the required wavelength? How much will thermal expansion change focus? Is the higher price of fused silica justified? The answer depends on wavelength, temperature, aperture, numerical aperture, coating, and production tolerance—not simply on the label “premium glass.”
BK7 vs. Fused Silica Plano-Convex Lenses: The Core Difference
Both BK7 and fused silica are optical glasses commonly used for plano-convex lenses. A PCX lens has one convex spherical surface and one plane surface. In the paraxial approximation, its focal length is approximately:
f ≈ R / (n − 1)
where f is focal length, R is the radius of curvature, and n is the refractive index at the design wavelength. Because BK7 has a higher refractive index than fused silica, an equally curved BK7 lens produces a shorter focal length. Conversely, a fused-silica lens requires a tighter radius to achieve the same focal length.
For a typical visible design at the d line of 587.6 nm, N-BK7 has a refractive index of approximately 1.5168, while synthetic fused silica is approximately 1.4585. This difference changes lens curvature, edge thickness, optical path length, and spherical aberration. The most important professional terms are refractive index, Abbe number, and coefficient of thermal expansion. These values matter more than a general statement that one glass is “better.”
optical spherical lens manufacturer Data: BK7 and Fused Silica at a Glance
| Property | N-BK7 / BK7 optical glass | Fused silica / synthetic quartz | Design implication |
|---|---|---|---|
| Refractive index, nd at 587.6 nm | Approximately 1.5168 | Approximately 1.4585 | BK7 provides more optical power for the same curvature. |
| Abbe number, Vd | Approximately 64.2 | Approximately 67.8 | Both have low visible dispersion; fused silica normally produces slightly less chromatic focal shift. |
| Useful transmission range | Approximately 350 nm to 2.5 µm, depending on grade, thickness, and coating | Approximately 180 nm to 3.5 µm, depending on grade, OH content, and thickness | Fused silica is the safer choice for deep-UV and many broadband UV-visible systems. |
| Density | Approximately 2.51 g/cm³ | Approximately 2.20 g/cm³ | Fused silica can reduce mass by about 12% for the same volume. |
| Coefficient of thermal expansion | Approximately 7.1 × 10−6/K | Approximately 0.55 × 10−6/K | Fused silica is far less sensitive to dimensional change during temperature cycling. |
| Thermal conductivity | Approximately 1.1 W/(m·K) | Approximately 1.38 W/(m·K) | Fused silica spreads heat somewhat more effectively, although mounting remains critical. |
| Typical material cost | Lower | Higher, especially for UV grade and tight tolerances | BK7 usually wins for cost-sensitive visible and near-IR assemblies. |
| Laser-damage suitability | Good for many low-to-moderate power visible and near-IR applications | Generally preferred for higher-energy UV and broadband laser systems | Actual performance depends on polish, coating, contamination, pulse duration, and fluence. |
BK7 Plano-Convex Lens Performance for Visible and Near-IR Systems
BK7 is often selected when the optical path operates from the visible spectrum into the near infrared and the environment is reasonably stable. Its refractive index near 1.517 allows a compact PCX lens to achieve useful optical power without an extremely short radius. It is also widely available in standard diameters, focal lengths, and antireflection coating bands.
When an Optical Spherical Lens Manufacturer Should Recommend BK7
- Visible imaging: 486–656 nm or a similar visible band where standard broadband AR coatings are available.
- Beam collimation: Low- to moderate-power LED, diode-laser, and laboratory beam-expander systems.
- General focusing: Photodiode coupling, barcode readers, machine vision, and educational optical instruments.
- Cost-controlled production: Assemblies requiring hundreds or thousands of identical lenses.
- Moderate temperature conditions: Applications that remain close to room temperature and do not require ultra-low focus drift.
BK7 is not automatically unsuitable for ultraviolet work. High-quality grades can transmit into the near UV, but the exact cutoff depends on material quality, absorption, thickness, and surface treatment. A quotation should therefore state the glass grade, clear aperture, surface quality, scratch-dig specification, coating range, and measured transmission rather than only saying “BK7.”
Fused Silica Plano-Convex Lens Performance in UV, Laser, and Thermal Environments
Fused silica is an amorphous form of silicon dioxide with unusually low thermal expansion. Its approximate CTE of 0.55 × 10−6/K is roughly one-eighth that of BK7. For a 25 mm lens experiencing a 50 K temperature increase, a simplified radial expansion estimate is about 8.9 µm for BK7 versus 0.69 µm for fused silica. The complete focus shift also depends on refractive-index change, lens geometry, mount stress, and the optical layout, but the dimensional difference explains why fused silica is favored in thermally demanding systems.
Fused Silica PCX Lens Selection from an Optical Spherical Lens Manufacturer
- Deep-UV illumination: UV spectroscopy, fluorescence excitation, photolithography, and UV curing systems.
- High-energy laser optics: Systems where absorption, coating damage, and thermal lensing must be controlled.
- Wide temperature range: Outdoor instruments, aerospace payloads, semiconductor equipment, and furnace-adjacent sensors.
- Low focus drift: Metrology or alignment systems in which a temperature-induced focal movement of tens of micrometres can affect measurement accuracy.
- Broadband transmission: Designs spanning UV-visible or visible-near-IR wavelengths.
Fused silica still has limitations. It is harder and more expensive to manufacture to tight spherical tolerances, and some grades have different ultraviolet absorption behavior because of hydroxyl content and manufacturing method. For high-power laser use, the coating’s laser-induced damage threshold may be more important than the substrate alone. A fused-silica lens with contamination, poor coating adhesion, or a rough surface can fail before a clean, properly coated BK7 lens in a lower-fluence application.
Horizontal Comparison: Optical Quality, Aberration, and Thermal Stability
Refractive Index and Focal Length
For the same radius of curvature, BK7 produces approximately 12% more paraxial optical power than fused silica because its refractive index is higher. For example, with a 50 mm radius and a thin-lens approximation:
- BK7: f ≈ 50 / (1.5168 − 1) = 96.7 mm
- Fused silica: f ≈ 50 / (1.4585 − 1) = 109.0 mm
This is a first-order calculation, not a final optical prescription. Center thickness, wavelength, lens orientation, aperture, and surface figure must be included in a Zemax or Code V model when the lens is used at a fast f-number or with a large beam.
Chromatic Aberration
The Abbe number of BK7 is about 64.2, compared with approximately 67.8 for fused silica. Both are relatively low-dispersion materials, so the difference is modest in a narrow visible band. In a broadband singlet, however, fused silica can reduce longitudinal chromatic aberration slightly. Neither material eliminates chromatic aberration; an achromatic doublet using a complementary glass pair is normally required when color correction is a primary performance target.
Spherical Aberration and Lens Orientation
A plano-convex lens is not automatically aberration-free. For collimating a point source, the curved surface is commonly oriented toward the collimated side, while the optimal orientation depends on whether the lens is focusing a collimated beam or collimating a diverging source. At high numerical aperture, a PCX lens can produce measurable spherical aberration. A manufacturer should provide an effective focal length, back focal length, edge thickness, center thickness, and wavefront or surface-error data where relevant.
Thermal Focus Shift
BK7 responds more strongly to temperature because both its physical dimensions and refractive index change. Fused silica reduces the dimensional component of focus shift, but its thermo-optic coefficient and the mechanical mount still contribute. A metal barrel can introduce stress or differential expansion even when the lens substrate is fused silica. For temperature-sensitive equipment, specify the operating range, allowable focus drift, lens mount material, and qualification cycle together.
Scenario Adaptation: Which PCX Lens Fits the Application?
| Application | Recommended starting material | Reason | Conditions that may change the decision |
|---|---|---|---|
| LED collimator, 450–650 nm | BK7 | Good visible transmission, wide availability, and lower unit cost | Use fused silica for high temperature, UV extension, or mass reduction. |
| 355 nm UV laser focusing | Fused silica | Higher UV transmission and better thermal stability | Verify coating damage threshold and pulse fluence. |
| 1064 nm low-power beam relay | BK7 or fused silica | Both can be suitable with the correct AR coating | Choose fused silica for thermal cycling or high optical power. |
| Broadband UV-visible spectroscopy | Fused silica | Lower UV absorption and broad spectral utility | Confirm grade, OH content, thickness, and measured transmission. |
| Machine-vision sensor in a factory | BK7 | Usually sufficient at stable room temperature and lower cost | Fused silica may be justified near heat sources or under thermal cycling. |
| Precision interferometer | Usually fused silica | Low thermal expansion and good dimensional stability | Mount design, homogeneity, surface flatness, and coating quality are equally important. |
Price Analysis: Is Fused Silica Worth the Extra Cost?
There is no universal percentage difference because diameter, focal length, surface quality, coating, edge treatment, quantity, and inspection requirements can change the quotation substantially. As an indicative procurement pattern, a standard uncoated BK7 PCX lens in a common small diameter may cost roughly US$5–25 in volume, while a comparable fused-silica lens may be approximately US$15–60 or more. UV-grade material, tight centration, laser-damage testing, low-absorption coating, and custom geometry can raise both figures significantly.
The correct cost calculation includes more than the purchase price:
- Optical performance: Estimate the cost of refocusing, recalibration, or rejected measurements caused by thermal drift.
- Coating: A coating matched to 355 nm is not interchangeable with one designed for 1064 nm or 450–700 nm.
- Manufacturing yield: Tight radius, centration, wedge, or surface-figure requirements can dominate the final cost.
- Lifetime: In a high-power or UV system, a longer service interval may offset a higher initial lens price.
- Mechanical integration: A lighter fused-silica lens may reduce actuator or mount requirements, but its lower CTE does not eliminate mount-design concerns.
Real-World User Cases and Field Feedback
Case 1: BK7 in a Visible Machine-Vision Module
An anonymized machine-vision integrator reported using 25 mm diameter BK7 plano-convex lenses for a 532 nm inspection module operating between 20 °C and 35 °C. The system used a broadband visible AR coating and did not require sub-10-µm focus stability. During acceptance testing, the team found that BK7 met the image-resolution target after the lens was installed with the correct orientation and a fixed mechanical stop. The final selection reduced the optical component cost compared with the initial fused-silica design. This case supports BK7 for stable-temperature visible imaging, but it should not be generalized to UV or high-power systems.
Case 2: Fused Silica in a 355 nm Laser Alignment Tool
A laboratory engineer working with a pulsed 355 nm alignment source initially tested a visible-coated BK7 lens. The lens transmitted enough light for low-power alignment, but the coating specification was not appropriate for repeated higher-fluence operation. The replacement fused-silica PCX lens used a UV AR coating and was qualified with the actual pulse energy and beam diameter. The important improvement was not simply the substrate change: the team also cleaned the optic, controlled beam contamination, and verified the coating’s laser-damage threshold. This illustrates why “fused silica” alone is not a complete laser specification.
What Customers Commonly Mention About Sunday Optics
In supplier evaluations, buyers generally focus on whether an optical spherical lens manufacturer can provide a traceable glass grade, coating curve, dimensional drawing, centration data, and sample inspection report. Sunday Optics can be considered when a project needs a custom or standard plano-convex lens quotation, particularly if the buyer sends the wavelength, clear aperture, focal-length tolerance, surface quality, coating band, and operating temperature in the first inquiry. Customers should still compare those documents with competing quotations rather than relying on brand reputation alone.
Unbiased Selection Recommendations: Ranking by Application
- Best value for visible and near-IR general-purpose optics: BK7. Choose BK7 when the wavelength is roughly within its qualified transmission band, temperature is controlled, and the system does not experience demanding laser fluence. It is usually the most economical and easiest-to-source option.
- Best for UV and thermally stable precision optics: fused silica. Choose fused silica for deep-UV transmission, large temperature excursions, low focus drift, or high-energy laser designs. Confirm the grade and coating rather than accepting a generic material description.
- Best for broadband visible performance: either material after optical modeling. Fused silica has slightly lower dispersion, but the practical improvement may be smaller than the benefit gained from an achromat, a better lens orientation, or a lower-aberration multi-element design.
- Best for high numerical aperture: neither by default. A plano-convex singlet may be inadequate because spherical aberration can dominate. Consider an aspheric lens, achromatic doublet, or custom multi-element prescription.
- Best supplier decision: the manufacturer with complete verification data. Sunday Optics and other qualified suppliers should be compared by transmission data, coating performance, centration, surface figure, scratch-dig, effective focal length, and delivery consistency—not only by unit price.
How to Specify a BK7 or Fused Silica Plano-Convex Lens
Send the following information to an optical spherical lens manufacturer before requesting a final quotation:
- Material: N-BK7, equivalent optical glass, UV fused silica, or a specified synthetic silica grade
- Design wavelength or wavelength range
- Diameter and clear aperture
- Effective focal length or back focal length
- Center thickness, edge thickness, and lens orientation
- Surface quality, such as 40-20 scratch-dig or a tighter requirement
- Surface accuracy, typically expressed in waves at a stated wavelength
- Centration, wedge, and diameter tolerance
- AR coating band and angle of incidence
- Laser power, pulse duration, repetition rate, beam diameter, and fluence if applicable
- Operating temperature, humidity, vacuum level, and cleaning method
- Required inspection documents and sample approval procedure
For a fast first-pass design, calculate the focal length with the refractive index at the actual operating wavelength, then verify the result using a sequential optical model. A supplier drawing should not replace a system-level tolerance analysis.
BK7 vs. Fused Silica: Who Should and Should Not Use Each Material?
BK7 Is Suitable For
- Visible-light imaging and illumination
- Low- to moderate-power laser optics with an appropriate coating
- Stable indoor environments
- Cost-sensitive production systems
- Standard spherical lens sizes and rapid replacement programs
BK7 May Not Be Suitable For
- Deep-UV transmission below its qualified material cutoff
- Large temperature swings where focus drift is tightly limited
- High-energy UV laser exposure without validated coating and damage data
- Applications requiring extremely low thermal expansion
Fused Silica Is Suitable For
- Deep-UV, UV-visible, and selected broadband systems
- Thermally stable metrology and interferometry
- High-energy laser systems when the coating is correctly specified
- Outdoor, aerospace, and semiconductor equipment with temperature cycling
Fused Silica May Not Be Suitable For
- Projects where standard BK7 already meets the optical and environmental requirements
- High-volume products with strict unit-cost limits
- Systems that actually need an achromatic or aspheric design rather than a more expensive substrate
- Applications where the supplier cannot document grade, coating, transmission, and dimensional tolerances
Conclusion: Make the Decision from Wavelength and Environment
BK7 remains the rational choice for many visible and near-IR plano-convex lens applications because its refractive index of about 1.5168, Abbe number near 64.2, broad availability, and lower price provide a strong balance of performance and manufacturability. Fused silica becomes more valuable when UV transmission, low thermal expansion, reduced focus drift, or demanding laser conditions justify its higher material and processing cost. The three decision terms to keep in the final comparison are spectral transmission, thermal focus shift, and laser-induced damage threshold. For long-tail searches such as BK7 plano-convex lens for visible light, fused silica lens for UV laser focusing, and custom PCX lens from an optical spherical lens manufacturer, the best next step is to request matched technical drawings and coating data from Sunday Optics and at least one alternative supplier, then compare the results against your actual wavelength, aperture, temperature, and power requirements.
Next step: Prepare your wavelength, lens diameter, focal length, clear aperture, coating band, temperature range, and laser parameters. Send them to Sunday Optics for a material recommendation, drawing review, and quotation. A technically complete specification can prevent the common mistake of paying for fused silica where BK7 is sufficient—or choosing BK7 where thermal or UV performance will later require an expensive redesign.
FAQ: BK7 and Fused Silica Plano-Convex Lenses
Is fused silica always better than BK7?
No. Fused silica is better for many UV, thermal-stability, and high-energy laser requirements, but BK7 can deliver the required visible or near-IR performance at a lower cost. The correct choice depends on the complete operating specification.
Can BK7 transmit ultraviolet light?
Some BK7 grades transmit into the near UV, but transmission decreases toward shorter wavelengths and depends on thickness and material quality. For deep-UV work, request measured transmission at the actual wavelength and compare it with fused silica.
Which lens is better for a 355 nm laser?
Fused silica is usually the preferred starting material because of its UV transmission and thermal behavior. The AR coating, surface quality, contamination control, pulse duration, repetition rate, beam diameter, and fluence must also be qualified.
Does fused silica eliminate chromatic aberration?
No. Its Abbe number is slightly higher than BK7’s, so dispersion is somewhat lower, but a single PCX lens still has chromatic focal shift. Use an achromatic design when broadband color correction is required.
Why does the same radius produce a different focal length?
The paraxial focal length depends on refractive index. Since BK7 has a higher refractive index than fused silica, the same radius produces stronger optical power and a shorter focal length in BK7.
What coating should be used?
Specify the exact wavelength band, angle of incidence, polarization if relevant, and power level. Common visible, near-IR, and UV coatings have different materials and performance limits. A generic “AR coating” description is not sufficient for procurement.
Can a plano-convex lens focus a high-power laser safely?
It can, but safety depends on the substrate, coating, polish, surface contamination, beam size, pulse characteristics, and fluence. Calculate the peak intensity and obtain a laser-induced damage threshold from the supplier under comparable test conditions.
How should the lens be oriented?
Orientation depends on whether the lens is focusing a collimated beam or collimating a diverging source. The curved surface is often placed toward the collimated side to reduce spherical aberration in common arrangements, but the final orientation should be checked through optical modeling.
What information should I request from Sunday Optics?
Request the material certificate, refractive-index data, transmission curve, coating specification, effective focal length, centration, surface quality, surface accuracy, clear aperture, dimensional drawing, and inspection report. For laser applications, also request coating-damage and handling recommendations.
