How to create a Longpass Filter for your Project
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.
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.
