Classification of filters:
- Absorption filter
Absorption filters are made by adding special dyes to plastic or glass substrates. According to the same material’s ability to absorb light of different wavelengths, it can serve the purpose of filtering. The more common one is the color glass filter, which has the advantages of stability, uniformity, good optical quality, and relatively low price, but the disadvantage is that its passband is relatively large, generally rarely less than 30nm.
- Interference filter
The interference filter is made by forming an optical film with a certain thickness on the surface of the glass by a vacuum coating method. It is usually composed of a multilayer film. The interference principle is used to pass light in a specific spectral range. There are many types of interference filters with different uses. Common interference filters include cut-off filters, band-pass filters, and dichroic filters.
- Cut-off filter
The spectrum can be divided into two regions. The light in one region cannot pass (cut-off region), while the light in the other region can fully pass (passband region). Typical cut-off filters are: long-wave pass filter and short-wave pass Filter
- Bandpass filter
Only allow the light of a specific band to pass, and the light outside the passband is cut off. The optical indicators of the bandpass filter are mainly center wavelength and half bandwidth. According to needs, the passband of the bandpass filter can be from infrared to ultraviolet, according to the bandwidth Divided into:
Narrowband filter, narrow passband (bandwidth less than 30nm)
Broadband filter, with a bandwidth greater than 60nm;
- Dichroic filter
A small range of colors used to selectively pass light while reflecting other colors.
- Neutral density filter
Also called attenuator, is used to prevent strong light from damaging the camera sensor or other optical parts. It will absorb or reflect the part of the light that is not transmitted to uniformly reduce the transmittance of a certain part of the spectrum. .
- Fluorescence filter
is a key component used in biomedical and life science instruments. Its main function is to separate and select the characteristic band spectra of the excitation light and emission fluorescence of the substance in the biomedical fluorescence inspection and analysis system.
Technical indicators of the filter
Explanation of several confusing terms related to optical filters:
- Bandpass: In the spectral curve, the area through which light actually passes is called the passband.
- Cut-off range: indicates the wavelength interval of the energy spectral region that is attenuated by the filter, which is simply understood as the range outside the passband.
- Center wavelength: In a bandpass filter, the midpoint between the wavelengths when the transmittance is equal to 50% of the peak transmittance.
- Half-bandwidth: In a band-pass filter, the difference in wavelength at which the transmittance is equal to 50% of the peak transmittance.
- Transmittance: The transmittance of the target band, usually expressed as a percentage. The larger the value, the better the transmittance.
- Cut-off rate: also called cut-off depth, which represents the transmittance T corresponding to the cut-off area. The cut-off rate can be expressed in terms of transmission percentage or optical density (OD).
- Transition bandwidth: according to the filter cut-off depth value, the specified filter cut-off depth and the 50% transmittance point allow the larger spectral width
- Edge steepness: [(λT80-λT10)/λT10] *100%
Selection of filter：
To choose a suitable product from many filter products, you can generally refer to the following steps:
1. According to the desired function of the filter in the optical path, such as cut-off, band pass, light splitting, attenuation, etc., determine the type of filter required;
2. Choose suitable products according to the requirements of optical transmission/reflection/attenuation spectral range, transmittance, size, etc.