Longpass Filter

A longpass filter is a device that selectively transmits light of longer wavelengths and blocks or reflects light of shorter wavelengths. It can be used to isolate a specific spectral region or to enhance the contrast between different colors.

 

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Application of Longpass Filters

Imaging and Photography

  • Blocking UV light: Protecting camera sensors and film from harmful UV radiation.
  • Color correction: Adjusting color balance by removing unwanted short wavelengths.
  • Infrared photography: Isolating infrared light for specific imaging techniques.

Microscopy

  • Fluorescence microscopy: Separating excitation light from emission light.
  • Confocal microscopy: Removing out-of-focus light.

Spectroscopy

  • Raman spectroscopy: Filtering out Rayleigh scattered light.
  • Infrared spectroscopy: Blocking unwanted visible light.

Other Applications

  • Laser safety: Protecting eyes and sensors from harmful laser radiation.
  • Optical sensing: Removing noise from sensor signals.
  • Environmental monitoring: Filtering specific wavelengths for analysis.
  • Medical imaging: Reducing background noise in imaging systems.

Specifying a Longpass Filter

  • Cuton Wavelength (Cuton)

    The cut-on wavelength for a longpass filter is the wavelength at which the filter's transmission increases to 50% of its peak value.

    • Wavelengths below the cut-on wavelength are typically blocked.
    • Wavelengths above the cut-on wavelength are typically transmitted.
    • The steeper the transition from blocking to transmission (often referred to as "edge steepness"), the better the filter's performance.

  • Transmission Band

    This specifies the range of wavelengths that the filter allows to pass through, typically defined from the cut-on wavelength to an upper limit determined by the substrate material.

  • Blocking Band

    The range of wavelengths that the filter effectively blocks must be defined. This includes understanding how well the filter suppresses shorter wavelengths and the extent of this blocking.

  • Optical Density (OD)

    Optical Density (OD)is a measure of how much light is absorbed by a material.It's a logarithmic scale,meaning that each increase of 1 in OD corresponds to a 10-fold decrease in light transmission.

    • OD1 - 10%
    • OD2 - 1%
    • OD3 - 0.01%
    • OD4 - 0.01%
    • OD5 - 0.001%
    • OD6 - 0.0001%

  • Edge Steepness (Optional)

    This describes how quickly the filter transitions from blocking to transmitting light around the cut-on wavelength, which can be critical for applications requiring sharp spectral isolation.

  • Environmental Resistance (Optional)

    Considerations for durability under specific conditions (e.g., temperature range, humidity) are also important for practical applications

  • Size and Shape

    The physical dimensions of the filter must be specified according to its intended application, which may require custom sizes or shapes.

How to create a Longpass Filter for your Project

Example: Environmental Monitoring

Objective: To measure specific spectral features in environmental samples, such as pollutants.

Steps:

  1. Define Target Wavelengths: Determine which wavelengths are relevant for your analysis, for example, pollutants that absorb light in the near-infrared range.
  2. Select Cut-On Wavelength: For pollutants absorbing around 950 nm, choose a longpass filter with a cut-on wavelength of approximately 900 nm.
  3. Material Selection: Use materials suitable for infrared transmission, such as germanium or silicon substrates, which can withstand environmental condition.
  4. Testing and Calibration: Calibrate your measurement system with known standards to ensure accurate readings through the longpass filter.

Mounting:

  • Optical Mounts: In Raman spectroscopy setups, longpass filters are often mounted using adjustable optical mounts. These mounts allow precise alignment of the filter with respect to the laser source and detector.
  • Fixed Optical Bench: Filters can also be integrated into a fixed optical bench setup, where they are secured using clamps or brackets that hold them in place while maintaining alignment with other optical components.

Example: Raman Spectroscopy

Objective: To separate Raman scattered light from the excitation laser light.

Steps:

  1. Identify Laser Wavelength: For a laser operating at 785 nm, select a longpass filter with a cut-on wavelength just above this, such as 800 nm.
  2. Optical Density Requirements: Ensure that the filter has high optical density (OD6 or higher) at the laser wavelength to block it effectively while allowing Raman signals to pass through.
  3. Edge Steepness: Choose a filter with a steep edge (sharp transition) to maximize rejection of the laser light while allowing lower-energy Raman signals to be detected.
  4. Implementation: Place the longpass filter in front of the detector in the Raman setup.

Mounting:

  • Filter Wheel Utilize a motorized or manual filter wheel that can accommodate multiple longpass filters. Each filter should be designed for specific wavelengths relevant to the pollutants or environmental conditions being monitored.
  • Adhesive Mounting: In some cases, filters may be adhered directly to sensor housings using optical adhesives designed to maintain clarity and prevent degradation over time.

Product List in Stock

Product Cuton OD Tpk(%)
LP1300 Longpass Filter (T>90%@1380-2200nm, OD2@200-1250nm) 1300nm OD2 90
LP360 Longpass Filter (T>90%@370-1200nm, OD3@200-350nm) 360nm OD3 90
LP390 Longpass Filter (T>90%@395-1100nm, OD3@200-380nm) 390nm OD3 90
LP405 Longpass Filter (T>90%@415-1200nm, OD3@200-395nm) 405nm OD3 90
LP410 Longpass Filter (T>90%@420-1200nm, OD3@200-395nm) 410nm OD3 90
LP425 Longpass Filter (T>90%@435-1200nm, OD3@200-415nm) 425nm OD3 90
LP430 Longpass Filter (T>90%@440-1200nm, OD3@200-415nm) 430nm OD3 90
LP440 Longpass Filter (T>90%@450-1200nm, OD3@200-430nm) 440nm OD3 90
LP515 Longpass Filter (T>90%@530-650nm, OD5@400-485nm) 515nm OD5 90
LP520 Longpass Filter (T>90%@530-1200nm, OD3@200-505nm) 520nm OD3 90
LP530 Longpass Filter (T>95%@540-1200nm, OD5@200-520nm) 530nm OD5 95
LP540 Longpass Filter (T>95%@550-1200nm, OD5@200-530nm) 540nm OD5 95
LP550 Longpass Filter (T>90%@555-800nm, OD5@200-535nm) 550nm OD5 90
LP575 Longpass Filter (T>90%@580-2000nm, OD3@200-560nm) 575nm OD3 90
LP580 Longpass Filter (T>95%@590-1100nm, OD4@200-570nm) 580nm OD4 95
LP585 Longpass Filter (T>90%@590-200nm, OD3@350-565nm) 585nm OD3 90
LP590 Longpass Filter (T>90%@600-1200nm, OD3@200-570nm) 590nm OD3 90
LP600 Longpass Filter (T>90%@605-2500nm, OD5@200-585nm) 600nm OD5 90
LP610 Longpass Filter (T>90%@615-1200nm, OD4@200-600nm) 610nm OD4 90
LP620 Longpass Filter (T>90%@630-1200nm, OD4@200-610nm) 620nm OD4 90
LP635 Longpass Filter (T>90%@650-1200nm, OD4@200-620nm) 635nm OD4 90
LP660 Longpass Filter (T>95%@670-1200nm, OD4@200-645nm) 660nm OD4 95
LP670 Longpass Filter (T>95%@685-1200nm, OD4@200-655nm) 670nm OD4 95
LP680 Longpass Filter (T>90%@705-1200nm, OD3@200-670nm) 680nm OD3 90
LP690 Longpass Filter (T>90%@715-1200nm, OD3@200-690nm) 690nm OD3 90
LP710 Longpass Filter (T>90%@700-1200nm, OD3@200-690nm) 710nm OD3 90
LP740 Longpass Filter (T>95%@740-1100nm, OD5@200-725nm) 740nm OD5 95
LP770 Longpass Filter (T>90%@780-1200nm, OD3@200-750nm) 770nm OD3 90
LP780 Longpass Filter (T>90%@790-1200nm, OD3@200-760nm) 780nm OD3 90
LP795 Longpass Filter (T>90%@815-1200nm, OD3@200-785nm) 795nm OD3 90
LP800 Longpass Filter (T>90%@800-1200nm, OD2@200-770nm) 800nm OD2 90
LP830 Longpass Filter (T>90%@840-1200nm, OD4@200-815nm) 830nm OD4 90
LP835 Longpass Filter (T>95%@845-1800nm, OD4@200-820nm) 835nm OD4 95
LP840 Longpass Filter (T>90%@850-1800nm, OD5@200-810nm) 840nm OD5 90
LP950 Longpass Filter (T>90%@960-1700nm, OD5@200-930nm) 950nm OD5 90
LP970 Longpass Filter (T>90%@985-1200nm, OD4@200-950nm) 970nm OD4 90

Design longpass filter for your project