Collection: 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.
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
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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.
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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.
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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.
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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%
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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.
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Environmental Resistance (Optional)
Considerations for durability under specific conditions (e.g., temperature range, humidity) are also important for practical applications
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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:
- Define Target Wavelengths: Determine which wavelengths are relevant for your analysis, for example, pollutants that absorb light in the near-infrared range.
- Select Cut-On Wavelength: For pollutants absorbing around 950 nm, choose a longpass filter with a cut-on wavelength of approximately 900 nm.
- Material Selection: Use materials suitable for infrared transmission, such as germanium or silicon substrates, which can withstand environmental condition.
- 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:
- 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.
- 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.
- 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.
- 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.
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