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As a key category of edge filters (alongside shortpass filters), they find applications in microscopy, industrial inspection, and environmental monitoring—where isolation of longer wavelengths (e.g., near-infrared, mid-infrared) is critical to avoiding interference from shorter wavelengths (e.g., UV, visible). Our longpass filters are manufactured using advanced thin-film coating technology (ion-beam sputtering, IBS) to achieve steep transition edges (<10nm between 10% and 90% transmission), high transmission efficiency (>95% beyond cut-on), and deep blocking of shorter wavelengths (OD 4–6). Unlike conventional longpass filters that suffer from wavelength drift due to temperature changes, our filters maintain <0.5nm cut-on shift over -20°C to +60°C, ensuring consistent performance in diverse environments. With substrate options including UV fused silica (for broad spectral coverage) and specialty IR glasses (for mid-infrared applications), they cater to needs from biomedical imaging to industrial process control.
Cut-on Wavelengths: Available from 240nm to 7300nm, with 50% transmission points (cut-on) at standard values like 420nm (visible/near-infrared separation), 550nm (green/red separation), 630nm (visible/NIR separation), and 1550nm (short-wave IR/mid-wave IR separation). Custom cut-on wavelengths (e.g., 850nm for night vision, 3000nm for chemical detection) are available to fit specialized applications .
Broad Transmission Range: Ensures >95% transmission beyond the cut-on wavelength (e.g., 430–1100nm for 420nm cut-on models, 1560–7300nm for 1550nm cut-on models). This high transmission is achieved through optimized thin-film designs (40–80 layers of HfO₂/SiO₂ for visible/NIR, Ge/ZnS for mid-IR) that minimize reflection and absorption in the passband .
Deep Blocking: Attenuates shorter wavelengths (e.g., 200–410nm for 420nm cut-on filters) to minimize background noise, with OD 4–6 blocking (OD 4 = 99.99% blocking, OD 6 = 99.9999% blocking). For example, a 630nm cut-on filter blocks 400–620nm visible light with OD 5, making it ideal for NIR imaging systems where visible light scatter would degrade image quality .
Anti-Reflection Coatings: Rear-surface AR coatings (e.g., MgF₂ for visible, Al₂O₃ for IR) reduce backscatter to <0.5% per surface, improving overall throughput and reducing ghosting in imaging applications. Front-surface coatings are optimized for longpass performance, ensuring steep transition edges and deep blocking .
Surface Quality: Manufactured to 20-10 or 10-5 scratch-dig standards (per MIL-PRF-13830B) for optimal imaging clarity. A 10-5 surface (10 scratch width, 5 scratch density) reduces light scatter in high-resolution microscopy, ensuring sharp images of biological samples (e.g., cell nuclei stained with NIR dyes) .
Dimensional Options: Standard 25.4mm diameter (1 inch) with ±0.1mm tolerance to fit standard optical mounts, while custom sizes (12.5–100mm diameter, 20×20mm square) accommodate specialized systems (e.g., large-format IR cameras, compact microscopes). Thickness options (1–5mm) balance mechanical stability and weight—1mm thin filters for portable devices, 5mm thick filters for high-power laser systems .
Parallelism: Maintains <3 arcsec tolerance (per ISO 10110-5) to prevent beam distortion in precision systems, such as laser interferometers used for optical component metrology. Poor parallelism (>5 arcsec) can cause beam deviation, leading to measurement errors in spectroscopy or misalignment in laser systems .
Fluorescence Microscopy: Blocks excitation light (e.g., UV/blue) while transmitting longer-wavelength emission signals. For example, a 510nm cut-on filter blocks 488nm excitation light (used to excite GFP) while transmitting 510–550nm GFP emission light, eliminating excitation light glare and improving signal-to-noise ratio by >20x .
Biosensing: Isolates near-infrared (NIR) signals for deep-tissue imaging applications. In optical coherence tomography (OCT) for ophthalmology, a 850nm cut-on filter transmits 850–1000nm NIR light (which penetrates 2–3mm into retinal tissue) while blocking visible light, enabling high-resolution imaging of the retina’s outer layers .
Heat Control: Removes short-wavelength heat from optical systems in oil & gas infrared monitoring. For example, IR cameras used to detect gas leaks (e.g., methane) use 1550nm longpass filters to block 400–1540nm visible/short-wave IR light (which carries heat from sunlight or industrial equipment), preventing thermal saturation of the detector and ensuring accurate gas concentration measurements .
Laser Systems: Separates harmonic wavelengths in Nd:YAG lasers (e.g., transmitting 1064nm fundamental wavelength while blocking 532nm second harmonic). This is critical in laser welding, where 532nm light would cause uneven heating of metal surfaces, leading to inconsistent weld quality .
Night Vision: Enhances NIR sensitivity in security cameras and military goggles. A 700nm cut-on filter blocks 400–690nm visible light (which is absent in low-light conditions) while transmitting 700–900nm NIR light (emitted by night-vision illuminators), extending detection range by 2–3x compared to unfiltered cameras .
Astronomy: Filters out atmospheric UV/visible light to isolate infrared stellar emissions. Ground-based telescopes use 1200nm longpass filters to block 400–1190nm light (which is scattered by Earth’s atmosphere) while transmitting 1200–2500nm IR light, enabling observation of cool stars and distant galaxies that emit primarily in the IR .
Q: How is the cut-on wavelength defined?
A: The cut-on wavelength is the point where transmission reaches 50% of the peak value (T50), with steep transitions (typically <10nm) between the blocked (shorter wavelengths, <10% transmission) and transmitted (longer wavelengths, >90% transmission) regions. For example, a 630nm cut-on filter has <10% transmission at 625nm, 50% at 630nm, and >90% at 635nm. This steep transition ensures precise spectral separation, critical for applications like laser harmonic separation .
Q: Can longpass filters be combined with other filters?
A: Yes, pairing longpass filters with shortpass filters creates custom bandpass filters for specific wavelength ranges— a common practice in spectroscopy and imaging. For example, combining a 550nm longpass filter (transmits >550nm) with a 700nm shortpass filter (transmits <700nm) creates a 550–700nm bandpass filter, ideal for detecting red fluorophores (e.g., Cy5) in fluorescence microscopy. We also offer integrated filter stacks (e.g., longpass + AR coating) to simplify system integration and reduce alignment errors .
Q: What substrate materials are used?
A: We use UV fused silica (UVFS) for broad spectral coverage (240–1200nm) due to its high UV transmission and low autofluorescence—ideal for UV/visible/NIR applications (e.g., fluorescence microscopy, laser spectroscopy). For mid-infrared applications (1200–7300nm), we use specialty materials like germanium (Ge, transmits 2000–16000nm), zinc selenide (ZnSe, 600–16000nm), and calcium fluoride (CaF₂, 180–8000nm). These substrates are selected for their high IR transmission and mechanical stability in harsh environments .
Q: Are these filters suitable for high-power lasers?
A: Standard models work with moderate laser power (up to 1W/cm² CW at 532nm) for applications like laser imaging. For high-energy systems (e.g., pulsed lasers with >1J/cm² energy density, CW lasers with >10W/cm² power density), inquire about our hard-coated, high-damage-threshold variants. These use thicker substrates (3–5mm) and enhanced coatings (e.g., TiO₂/SiO₂) to achieve LIDT up to 5J/cm² @ 1064nm, 10ns pulses, preventing coating degradation or substrate damage. We also offer water-cooled mounts for extreme high-power applications (e.g., 100kW laser cutting) .
