Shortpass Filter

A Shortpass filter is an edge filter designed to transmit light at wavelengths shorter than a specified cut-off wavelength, while blocking longer-wavelength light. It is fabricated using thin-film interference coatings: alternating high- and low-index layers are deposited onto a substrate to create a multilayer interference stack. For wavelengths below the cut-off, the layer thicknesses produce constructive interference, enhancing transmission; for wavelengths above the cut-off, they produce destructive interference, reflecting or blocking the light.

• Life-Science Instruments: Separating excitation and emission bands in fluorescence microscopes and enabling multiplexed dye imaging.
• Spectral Imaging: Capturing short-wavelength signals in spectrometers and hyperspectral cameras by rejecting long-wavelength stray light.
• Optical Protection in Laser Systems: Blocking unwanted long-wavelength reflections to protect downstream optics and detectors.
• Industrial Inspection: Enhancing machine-vision contrast by filtering out ambient long-wavelength light and highlighting surface defects.

Consider your system’s optical and environmental requirements:

• Cut-off Wavelength: Set between the desired short-wave signal and the unwanted long-wave band.
• Transmission & Blocking: Specify the minimum transmission (e.g. ≥90%) below cut-off and the required blocking optical density (e.g. OD ≥3) above it.
• Transition Width & Slope: A narrower transition (steeper slope) ensures cleaner separation between pass and stop bands.
• Angle of Incidence: Non-zero AOI shifts the cut-off wavelength (“blue shift”)—choose filters specified for your operating angle.
• Substrate & Coating: Select fused silica or optical glass and hard-coated dielectric stacks to match temperature, humidity, and laser-damage-threshold needs.

• Longpass Filter: Transmits wavelengths longer than its cut-on wavelength (50% transmission point) and blocks shorter wavelengths.
• Shortpass Filter: Transmits wavelengths shorter than its cut-off wavelength (50% transmission point) and blocks longer wavelengths.

Although both use the same multilayer-interference principle, their layer designs are mirrored so that one favors long-wave transmission and the other favors short-wave transmission.

We can custom-fabricate shortpass filters across the ultraviolet, visible, and infrared regions. Specifications including cut-off wavelength, substrate material, dimensions, and coating stack can all be tailored to your application requirements.