Filter Selection Guide
The filter can select the spectrum or adjust the beam transmittance. It is widely used in biological imaging, machine vision, aerospace and so on.
How to select a suitable filter has always been a problem that we need to consider in our research. We usually choose the type of filter according to this idea: first, we choose the type of filter through the target function, and second, we choose specific products through the target performance.
Filter type
According to the principle of filter, it can be divided into absorption type and interference type.
Absorbing filter is a kind of filter that absorbs a specific wavelength by doping ions on a specific material substrate. The utility model has the advantages of low cost, no need for coating, but limited transmissivity.
The interference filter is made by coating multilayer optical films with alternating high and low refractive index on the substrate to form a multi-stage series Fabry Perot interferometer. The coating can realize various functions and achieve high transmittance, but the cost is increased. At present, there are more kinds of interference filters and their applications are relatively wider.
According to the spectral modulation characteristics of filters, they can generally be divided into band-pass filters, notch filters, edge pass filters, dichroic mirrors, neutral density filters, etc. Next, we will explain and distinguish them through their spectral curves.
Bandpass filter
The band-pass filter is used under normal incidence. Only the light of specific waveband is allowed to transmit, and the light of other wavebands is reflected or absorbed.
Notch filter
The notch filter is used under normal incidence, which only blocks the light of specific waveband and allows light transmission of other wavebands.
Edge pass filter
Edge pass filter, also known as cut-off filter, can make light on one side of a specific wavelength transmitted and light on the other side reflected or absorbed when used under normal incidence.
Dichroic filters
When the dichroic mirror is used at 45 ° incidence, the light on one side of a specific wavelength can be transmitted and the light on the other side can be reflected. In addition, the dichroic mirror can realize the beam splitting/combining function at the same time.
When the neutral density filter is used under normal incidence, it can attenuate the light within the working wavelength range at the same time and reduce the light energy.
summary
If only a certain wavelength is required to pass through, the band-pass filter is selected.
If only a certain wavelength does not pass, select a notch filter.
If one side of a wavelength needs to pass and the other side does not, the edge selection passes through the filter or dichroic mirror. If there are beam splitting/combining requirements at the same time, the dichroic mirror is selected.
If wavelength independent light attenuation function is required, select the density filter.
performance parameter
When selecting appropriate filter products, in addition to basic parameters such as material, size and appearance, different performance parameters should be paid attention to for different types, which will be discussed in categories below.
Band pass/notch filter
The parameters to be concerned are: central wavelength, half width, passband transmittance, stopband optical density and damage threshold.
Central wavelength: the midpoint of the stopband of the passband/notch filter of the bandpass filter, which generally reaches the maximum/minimum transmittance near the central wavelength. In general, the central wavelength of the filter needs to match the light source. According to the central wavelength, the band-pass filter can be divided into ultraviolet band-pass filter, visible band-pass filter and infrared band-pass filter.
Half width: the bandwidth corresponding to half of the peak transmittance. According to the size of half width, this type of filter has a more detailed classification, such as broadband, narrowband, etc. The smaller the half height and width, the finer the filter.
Pass band transmissivity: describes the transmissivity in the transmission band. The higher the pass band transmissivity, the better.
Stopband optical density: describes the transmissivity in the blocked wave band. The lower the stopband transmissivity, the better. It is generally described by optical density (OD). The formula of optical density is as follows. High optical density value indicates low transmissivity, while low optical density indicates high transmissivity, that is, the higher the stopband optical density, the better.
Damage threshold: reflects the ability to withstand light energy, and the damage threshold should be greater than the incident light energy to avoid damage.
Edge pass filter/dichroic filter
The parameters to be concerned include: cut-off type, cut-off or starting wavelength, transition band bandwidth, passband transmittance, stopband optical density/reflectivity, damage threshold, etc.
Cut off type: there are two common types: long wave pass and short wave pass, namely, light transmission/reflection that exceeds/falls below specific wavelength. There is a special kind of multi band dichroic mirror that can transmit at multiple bands.
Cut off or start wavelength: in the long wave pass type, it is the transmission start reflection cut off wavelength, which generally refers to the wavelength when the transmissivity rises to 50%; In the short wave pass type, it is called the transmission cut-off reflection starting wavelength, which generally refers to the wavelength when the transmissivity drops to 50%.
Transition band bandwidth: it is used to describe the bandwidth of the filter from high cut-off to high transmittance. It is generally recommended that the wavelength range of the transition band does not include the wavelength of the light source.
Pass band transmissivity: describes the transmissivity in the transmission band. The higher the pass band transmissivity, the better.
Stopband optical density: the side pass filter describes the transmissivity in the blocked band. The lower the stopband transmissivity, the better, and the higher the stopband optical density, the better.
Reflectivity: the reflectivity in the reflection band is described in the dichroic mirror. The higher the reflectivity, the better.
Damage threshold: reflects the ability to withstand light energy, and the damage threshold should be greater than the incident light energy to avoid damage.
Neutral density filter
Neutral density filter is also called attenuator, and the parameters to be concerned are: operating wavelength, transmissivity (optical density), optical density tolerance, damage threshold, etc.
Working wavelength: mainly determined by the substrate material and coating, generally broadband, can also be divided into ultraviolet attenuator, visible attenuator and infrared attenuator.
Transmittance: It is expressed by optical density, that is, OD value. The optical density is also described by attenuation (A). The formula of attenuation is as follows. The unit is dB. The larger the OD value, the lower the transmittance, and the greater the attenuation.
At present, in addition to the common uniform filter (only one OD value is provided), there are also step type and gradual type options, that is, the OD value can change step or continuously on the component surface.
The contrast relationship between transmittance, optical density and attenuation is shown in the following table.
Optical density tolerance: reflects the uniformity of light attenuation within the operating wavelength range. The smaller the tolerance, the better the uniformity.
Damage threshold: reflects the ability to withstand light energy, and the damage threshold should be greater than the incident light energy to avoid damage. Generally, the damage threshold of reflective attenuator is higher than that of absorption attenuator.
