Longpass Filters for Optical and Laser Systems

Did You Know?

Optical filters selectively transmit light in a particular range of wavelengths. When designed to pass long wavelengths only, they are referred to as longpass filters. Longpass filters are designed to attenuate shorter wavelengths and transmit longer wavelengths over the active range of the target spectrum. Because they isolate specific wavelengths, longpass filters are desirable in systems where they can be used to trim exess wavelengths or stray light to reduce unwanted near band emissions. Longpass filters are useful in a variety of applications and can be used in combination with shortpass filters for custom bandpass filtering.

Related Resources

Frequently Asked Questions

A longpass filter is an optical filter that transmits wavelengths longer than a specified cutoff wavelength while blocking or attenuating shorter wavelengths. It typically uses either absorption glass or multilayer thin-film interference coatings to create a sharp spectral transition between the blocking and transmitting regions. The key functional parameter is the cutoff wavelength, usually defined at 50% transmission.

Overview of Longpass Filters

Longpass Filters are optical components that transmit wavelengths longer than a specified cutoff while attenuating shorter wavelengths. They are commonly implemented in imaging systems, fluorescence setups, Raman spectroscopy, machine vision, and laser-based instruments where controlled spectral separation is required. On FindLight, Longpass Filters are available for integration into research, industrial, and OEM platforms.

Typical Specifications

When evaluating Longpass Filters, buyers should review several key parameters. Cut-on or cutoff wavelength is usually defined at 50% transmission and may range from the UV (≈300 nm) through the visible and into the near-infrared (up to ~2 µm, depending on substrate and coatings). Steepness of the transition region and optical density (OD) on the blocking side determine how effectively shorter wavelengths are rejected.

Transmission in the passband is typically specified as an average percentage (e.g., >90% or >95%) over a defined wavelength range. Substrate materials for Longpass Filters often include fused silica, BK7, borosilicate glass, or IR-grade materials, with surface quality given in scratch-dig (e.g., 40-20) and flatness in fractions of a wavelength. Coatings may be hard dielectric, soft dielectric, or colored glass, each affecting durability, angle sensitivity, and long-term stability.

Mechanical parameters include clear aperture, outer dimensions, and thickness, which must match lens mounts, filter wheels, or microscope cubes. Many Longpass Filters are specified for normal incidence; using them at non-normal angles shifts the effective cutoff wavelength and can change polarization behavior.

Use Cases and Buyer Guidance

Typical use cases for Longpass Filters include separating excitation and emission bands in fluorescence microscopy, blocking pump light in Raman systems, creating order-sorting filters in spectrometers, and shaping spectra in broadband illumination or laser setups. In machine vision, they can suppress ambient or short-wavelength stray light to improve contrast.

When sourcing Longpass Filters on FindLight, compare cutoff wavelength, blocking range and OD, in-band transmission, substrate and coating type, and physical size against your system requirements. Check damage threshold values for high-power laser applications and confirm compatibility with your incidence angle and numerical aperture.

FindLight lists multiple Longpass Filters from different manufacturers in a single place so you can review datasheets, compare specifications, and request quotes directly from suppliers. No registration is required to initiate contact, enabling rapid procurement cycles for both prototype and production builds.