Aug. 19, 2026
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Choosing a Spherical Lens for UV, visible, or infrared work is not simply a matter of selecting the clearest glass. The correct choice depends on the wavelength range, optical transmission, refractive index, absorption coefficient, operating temperature, coating, and beam power. Engineers searching for an optical spherical lens manufacturer, a UV visible IR spherical lens selection guide, or a custom spherical lens for spectroscopy usually need to solve one practical problem: how to transmit the required light without introducing unacceptable absorption, focusing error, fluorescence, or thermal damage.
A lens that performs well at 532 nm may become nearly useless at 2.5 µm. A material that transmits 10.6 µm may absorb strongly in the ultraviolet. This guide explains how to match lens material to UV, visible, and IR wavelengths, how to specify coatings and tolerances, and how to avoid common purchasing mistakes. The key terms used throughout are wavelength range, optical transmission, anti-reflection coating, refractive index, Abbe number, and absorption coefficient.
Many optical problems appear to be design problems but are actually material-selection problems. A detector may receive too little energy, a laser focus may shift after several minutes, or an image may show unexpected color fringes. These symptoms often come from selecting a lens by diameter and focal length while ignoring its spectral transmission.
The practical question is not “Which lens is the clearest?” It is “Which material and coating provide adequate transmission, optical power, environmental stability, and image quality at the actual operating wavelength and temperature?”
| Material | Typical useful range | Advantages | Limitations |
|---|---|---|---|
| UV fused silica | Approximately 180 nm to 2.2 µm, depending on grade | Low UV absorption, good thermal stability, low fluorescence, high laser-damage resistance | Lower refractive index than many optical glasses; relatively high cost for tight tolerances |
| Calcium fluoride (CaF2) | Approximately 180 nm to 8–9 µm | Broad UV-to-IR transmission, low dispersion, useful for achromatic designs | Soft, brittle, moisture and thermal-shock sensitivity; requires careful handling |
| Magnesium fluoride (MgF2) | Approximately 115 nm to 7 µm in suitable grades | Low refractive index and strong UV capability | Difficult to machine into some geometries; performance depends strongly on crystal quality and orientation |
| UV-grade sapphire | Approximately 200 nm to 5.5 µm | High hardness, high thermal conductivity, strong mechanical durability | Birefringence, higher index, and more difficult polishing |
For wavelengths around 248 nm or 266 nm, UV fused silica is often the first material considered because it combines low absorption with good resistance to excimer and solid-state laser radiation. For a broadband UV application extending toward the visible or near-IR, calcium fluoride can be attractive because its transmission window is broad and its dispersion is relatively low.
Do not assume that “fused silica” and “quartz” are interchangeable in every quotation. Fused silica is an amorphous material, while natural quartz is crystalline and birefringent. Ask the optical spherical lens manufacturer to state the exact material grade, spectral transmission data, and whether the component is intended for laser or imaging use.
| Material | Typical visible use | Important properties |
|---|---|---|
| N-BK7 or equivalent borosilicate crown glass | General imaging, collimation, beam expansion, laboratory optics | Refractive index near 1.517 at the helium d line; Abbe number near 64; good visible transmission and cost balance |
| Fused silica | Broadband visible systems, high-power lasers, UV-visible instruments | Low thermal expansion and broad transmission; refractive index near 1.46 at 589 nm |
| SF or flint glass | High optical power and achromatic combinations | Higher refractive index and lower Abbe number; may have greater internal absorption and density |
| Optical-grade sapphire | High-temperature or abrasion-resistant visible systems | High hardness and thermal conductivity, but birefringent and more difficult to process |
For a single-color visible laser, such as 405, 488, 532, 633, or 650 nm, N-BK7 is often sufficient when the power level is moderate and the environment is controlled. For broadband imaging from blue to red, the material’s dispersion matters. A spherical lens focuses different wavelengths at different distances because the refractive index changes with wavelength. This longitudinal chromatic aberration can be reduced with an achromatic doublet rather than a single spherical element.
| Material | Typical transmission region | Common applications |
|---|---|---|
| Fused silica | Visible through approximately 2.2 µm | 800–1,550 nm communications, spectroscopy, high-power laser optics |
| N-BK7 and similar glasses | Visible into the near-IR, often approximately 2 µm depending on grade | Low-cost beam handling and imaging near 850 or 1,064 nm |
| CaF2 | UV through approximately 8–9 µm | Broadband spectroscopy and low-dispersion IR optics |
| Silicon | Approximately 1.2–7 µm | Thermal imaging and mid-IR beam focusing |
| Germanium | Approximately 2–14 µm | Long-wave IR imaging and thermal systems |
Silicon is opaque in much of the visible spectrum but useful in the infrared. Germanium provides high refractive index and strong long-wave IR performance, but its transmission declines toward the shorter IR wavelengths and its temperature-dependent refractive index can be significant. For systems near 1,064 nm, fused silica, N-BK7, or specialized IR glass may be suitable. For 1,550 nm, all three may be considered, but the coating and detector response must also be checked.
| Material | Typical useful range | Design considerations |
|---|---|---|
| Germanium | Approximately 2–14 µm | High index, compact optical power, strong thermal sensitivity; often used with DLC or AR coatings |
| Silicon | Approximately 1.2–7 µm | Lower density than germanium and good thermal conductivity; not suitable for long-wave IR beyond its transmission limit |
| Zinc selenide (ZnSe) | Approximately 0.5–20 µm | Broad transmission and common use at 10.6 µm; relatively soft and susceptible to surface damage |
| Zinc sulfide (ZnS) | Approximately 0.4–14 µm, depending on grade | Good broadband performance and stronger mechanical properties than some IR materials |
| CaF2 | Approximately 0.18–8–9 µm | Useful for shorter and mid-IR wavelengths, but not normally the first choice at 10.6 µm |
At 10.6 µm, zinc selenide is a common option because its transmission window covers the CO2 laser wavelength. Germanium can also be used, but its high refractive index and temperature sensitivity require careful thermal analysis. At 3–5 µm, silicon, germanium, ZnSe, ZnS, and CaF2 may all be possible, so the selection should include detector band, temperature, power density, and coating durability.
Write the optical requirement as a range rather than a single number. For example:
Also record average power, peak power, pulse duration, repetition rate, beam diameter, and estimated power density. A lens that works at 1 W continuous-wave may fail under a 1 mJ nanosecond pulse if the coating has a lower laser-induced damage threshold.
Ask for a transmission curve or material data at the actual thickness. Bulk transmission is influenced by absorption coefficient and path length. In a simplified form, internal transmission follows the Beer–Lambert relationship:
T = e−αL
where T is internal transmission, α is the absorption coefficient, and L is the optical path length. Surface reflection is a separate loss. An uncoated surface with refractive index near 1.5 reflects approximately 4% at normal incidence, so two surfaces can lose roughly 8% before bulk absorption is considered. Higher-index IR materials can reflect substantially more without an anti-reflection coating.
The refractive index determines how much curvature is needed for a target focal length. A higher-index material can produce the same optical power with a flatter surface, but it may also increase Fresnel reflection and chromatic sensitivity.
The Abbe number is a useful visible-spectrum dispersion indicator. N-BK7 has an Abbe number of approximately 64, while many flint glasses have values below 40. A lower Abbe number means stronger dispersion. For a broad visible band, combine materials with different dispersion rather than expecting one spherical lens to eliminate chromatic focus shift.
For IR systems, use the refractive index at the operating wavelength, not the value listed at 589 nm. Germanium, silicon, and other IR materials can have large index changes across the band.
Coating selection must follow the wavelength and angle of incidence:
| Application | Typical coating direction | Questions to ask the supplier |
|---|---|---|
| UV laser | Dedicated 248, 266, 355, or 365 nm coating | What is the measured reflectance at the laser line? What is the pulse damage threshold? |
| Visible broadband | 400–700 nm or 450–700 nm broadband AR | Is the quoted reflectance average, maximum, or single-wavelength? |
| Near-IR | 800–1,100 nm, 1,064 nm, 1,310 nm, or 1,550 nm coating | Does the coating remain stable at the intended humidity and temperature? |
| Mid-IR | 3–5 µm, 8–12 µm, or a wavelength-specific IR coating | Is the coating compatible with the substrate and cleaning process? |
A coating optimized for 532 nm should not be assumed to work at 1,064 nm. In addition, a coating designed for normal incidence may have higher reflectance when the beam arrives at 30° or 45°. Polarization can also change coating performance, especially at oblique incidence.
Temperature can change focal position through thermal expansion and the thermo-optic coefficient, dn/dT. This is especially important for germanium, where the refractive index and focal length can shift as the lens warms.
Consider the following environmental conditions before ordering:
CaF2 and other soft or brittle materials may need protective mounting. ZnSe surfaces require careful handling because scratches and contamination can reduce transmission and increase local laser heating.
A complete request to an optical spherical lens manufacturer should include:
Over-specifying every tolerance can increase cost and lead time. For a detector collimator, surface quality may be less critical than transmission and centering. For a high-resolution imaging assembly, surface irregularity and chromatic correction may dominate.
In a supplier application review, a university spectroscopy team reported that its 365 nm fluorescence signal was approximately 30–40% lower than expected after replacing a damaged lens with a general-purpose visible glass element. The lens diameter and focal length were correct, so the replacement initially appeared compatible.
The team measured the optical path with a calibrated photodiode and compared the original and replacement components. The replacement glass transmitted adequately in the green portion of the spectrum but showed a significant drop near 365 nm. The coating was also specified for 450–700 nm rather than for ultraviolet light. The practical failure was therefore caused by both substrate absorption and coating mismatch.
The team changed to a UV-grade fused silica spherical lens with a 355–400 nm AR coating. After alignment and detector normalization, the measured signal increased by approximately 35% compared with the incorrect replacement. The result was not caused by a stronger lamp; it came from matching the substrate and coating to the excitation band.
The lesson is simple: verify measured transmission at the actual wavelength and lens thickness. A catalog description such as “visible and near-IR compatible” does not prove suitability at 365 nm, 405 nm, or any other boundary wavelength.
A manufacturing integrator needed to focus a 10.6 µm beam through a compact optical head. An initial design used a visible fused-silica lens because the lens was easy to source and had the required focal length. During testing, the beam power after the lens was substantially reduced, and the lens mount became warmer during extended operation.
The design was revised to use a ZnSe spherical lens with a coating specified for 10.6 µm. The supplier also provided a coating reflectance target, clear-aperture data, and a damage-threshold recommendation. The revised lens transmitted the CO2 laser wavelength and reduced the thermal load in the optical head. The team still had to control dust and avoid contact with the optical surface, because contamination can create localized absorption even when the material is spectrally suitable.
This case illustrates why wavelength matching must be combined with power-density analysis. A correct substrate does not automatically make an optic safe for high-power use; coating quality, cleanliness, beam diameter, and mounting stress remain important.
Prepare a one-page optical requirement sheet containing:
For demanding systems, request an inspection report, coating witness sample, interferometric data, and transmission measurements taken using the same material thickness or an equivalent test coupon.
Record the minimum and maximum wavelength. If the system uses a laser, include the nominal line and linewidth. If it uses a broadband source, include the full band and identify whether every wavelength has equal importance.
Remove materials that absorb strongly in the required band. For example, do not begin a 10.6 µm design with BK7, and do not use ordinary visible glass for a 248 nm excimer beam without measured data.
Build a comparison table showing transmission range, refractive index, dispersion, thermal expansion, hardness, chemical resistance, and cost. This prevents the design from being driven by one property alone.
Estimate uncoated Fresnel losses using the refractive index. For normal incidence, the reflectance of one air-to-material surface can be approximated by:
R = ((n − 1) / (n + 1))2
For a material with n = 1.5, the result is approximately 4% per surface. For a high-index IR material with n near 4, the uncoated reflection can approach 36% per surface. This is why IR coatings are not an optional specification in many systems.
State the central wavelength, acceptable bandwidth, polarization, and incidence angle. If the beam enters at 45°, tell the coating supplier before production rather than after the first test.
For broadband visible imaging, calculate chromatic focal shift. For high-power UV or IR systems, estimate absorption heating and focal-length change. If the lens operates near a temperature boundary, request thermo-optic data or test the assembled lens at operating temperature.
Check whether the lens diameter, edge thickness, chamfer, mount clearance, and retaining force are suitable. Brittle materials such as CaF2 can fracture under excessive clamping. High-index materials may require a smaller radius or different holder geometry than a glass lens with the same focal length.
Inspect the packaging before opening. Under suitable lighting, check for scratches, chips, coating marks, fingerprints, and edge damage. Measure the focal behavior with the actual source and detector, not only with a visible alignment laser. Keep the certificate and measurement record with the component.
Problem: The lens fits mechanically but transmits poorly at the required wavelength.
Solution: Add wavelength, substrate grade, coating band, and measured transmission to the purchase specification.
Problem: A broadband visible coating is used in the UV or mid-IR.
Solution: Ask for the numerical wavelength interval and reflectance curve. “Broadband” should never replace a defined range such as 400–700 nm or 3–5 µm.
Problem: The substrate transmits the wavelength, but the uncoated surfaces reflect too much energy.
Solution: Specify an AR coating designed for the exact wavelength, angle, and polarization. Confirm whether the reflectance value is per surface or for the complete optic.
Problem: A visible glass lens is used at 355 or 365 nm, producing lower signal and possible fluorescence.
Solution: Use UV fused silica, CaF2, or another verified UV material. Request fluorescence and transmission data when the application is fluorescence imaging or spectroscopy.
Problem: The beam is correctly focused during initial alignment but drifts after the laser or thermal source warms up.
Solution: Evaluate thermal expansion, dn/dT, mount stress, and absorbed power. Germanium systems in particular may require active focus compensation or thermal stabilization.
Problem: The coating becomes cloudy, stained, or scratched.
Solution: Follow the supplier’s cleaning instructions. Use clean gloves, filtered air, approved lens tissue, and a solvent compatible with the coating. Never rub a dusty IR surface dry.
Problem: Transmission falls when the lens is installed at an oblique angle.
Solution: Include the actual angle of incidence and polarization in the coating design. Recheck the clear aperture because oblique beams can clip the edge of a small lens.
When contacting Sunday Optics or another optical spherical lens manufacturer, provide the complete technical information instead of only writing “please quote a 25 mm lens.” A useful request can read as follows:
Please quote a 25 mm diameter spherical lens for 1,550 nm operation. Required effective focal length: 50 mm. Material candidates: fused silica or IR-grade glass. Coating: 1,530–1,570 nm AR, normal incidence, reflectance below 0.5% per surface. Surface quality: 40-20 or better. Centering: 3 arc minutes maximum. Operating temperature: 10–40°C. Continuous-wave power: 2 W, beam diameter 3 mm. Please provide material certificate, coating data, focal-length tolerance, and measured transmission.
This format allows the supplier to identify unsuitable materials before production, recommend a more stable substrate, and price the actual inspection and coating requirements.
Match the lens material to the complete optical condition, not simply to the nominal wavelength:
The best custom spherical lens for spectroscopy is the one whose substrate, coating, geometry, and tolerances are all matched to the measurement. When comparing an optical spherical lens manufacturer, request traceable material and coating data rather than relying on general marketing terms. For a reliable UV visible IR spherical lens selection, verify the wavelength range, optical transmission, anti-reflection coating, refractive index, Abbe number, and absorption coefficient before approving the order.
UV fused silica is a common choice from approximately 180 nm into the near-IR because it offers low UV absorption and good thermal stability. CaF2 is also useful when broad UV-to-IR transmission and low dispersion are required. The final choice depends on wavelength, power, mechanical requirements, and coating.
Yes, many BK7-grade lenses can be used at 1,064 nm, particularly for moderate-power applications. However, verify the exact glass grade, internal transmission, coating, beam power, and thermal conditions. Do not extend this assumption automatically to 1,550 nm or 2 µm.
ZnSe is widely used because its transmission range covers 10.6 µm. Germanium is another option, especially where high refractive index and compact optical power are useful, but its thermal sensitivity must be considered.
Fused silica is suitable for visible and near-IR wavelengths and can be used to approximately 2.2 µm depending on grade and thickness. It is not normally suitable for long-wave IR applications such as 8–12 µm thermal imaging.
Transmission through the bulk material does not eliminate surface reflection. High-index materials can lose a large fraction of incident light at uncoated surfaces. A wavelength-specific AR coating can reduce this loss and improve system throughput.
A spherical lens can be economical and effective for collimation, focusing, and simple imaging. An aspheric lens may reduce spherical aberration when a high numerical aperture or small focused spot is required. The material decision remains important for both designs.
Request a material certificate, coating transmission or reflectance curve, dimensional inspection report, surface-quality data, radius or effective-focal-length measurement, centering data, and laser-damage information when applicable.
Start with visual inspection under clean conditions. Then measure transmission or system throughput at the working wavelength, verify focal position, check beam quality or image performance, and repeat the test at the intended operating temperature and power level.
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