A cut-off filter is an optical filter that selectively transmits or blocks light of a specific wavelength. It can be divided into two main types according to the wavelength range it transmits:

• Long-wavelength pass filter: transmits long-wavelength light and blocks short-wavelength light.
• Short-wavelength pass filter: transmits short-wavelength light and blocks long-wavelength light.

The working principle of cut-off filter is mainly based on the interference effect of optical thin film. By depositing multiple layers of thin films with different refractive indices on a substrate, the reflection and transmission of light can be precisely controlled, thereby achieving selective transmission or blocking of light of a specific wavelength.

• Central wavelength: The wavelength range that needs to be transmitted or blocked.
• Cut-off wavelength: refers to the wavelength at which the filter begins to significantly block or transmit light.
• Transmittance: refers to the ratio of the light intensity passing through the filter to the incident light intensity.
• Cut-off slope: refers to the rate at which the transmittance changes from high to low or from low to high.
• Environmental resistance: the temperature and humidity of the working environment.

Cut-off filter film has a wide range of applications in optical instruments, imaging systems, laser technology and other fields, such as:

• Microscope: to isolate excitation light or fluorescence of a specific wavelength.
• Camera: to remove unwanted infrared light or ultraviolet light.
• Laser: to filter out stray light from the laser.
• Spectrometer: to separate light of different wavelengths.

According to the manufacturing materials and processes, cut-off filters can be divided into the following categories:

• Absorption type: Use the absorption characteristics of the material to block light of a specific wavelength.
• Interference type: Use the interference effect of the thin film to achieve band selection.
• Composite type: Combines the advantages of absorption type and interference type.