Overview of Narrowband Filter

The so-called narrowband filter is a subdivision of the bandpass filter, and its definition is the same as that of the bandpass filter. This type of filter allows light signals to pass through a specific band, while blocking light signals on both sides outside of this band. The passband of a narrowband filter is relatively narrow, generally less than 5% of the center wavelength value.

Parameter description of narrowband filter:

center wavelength

The center wavelength of a narrowband filter is generally the operating wavelength of the instrument or device, which refers to the wavelength at the center of the passband. Figure 1 is a schematic diagram of the transmittance curve of a narrowband filter. In the figure, the detailed definition of the center wavelength is as follows:;

Among them, the wavelength orientations on the left and right sides of the passband corresponding to half the peak transmittance are separated;

In the practical production process, there is always a slight difference between the orientation of the center wavelength and the planned value, so a tolerance scale is generally added when specifying the center wavelength. The tolerance scale is determined by practical application conditions. Generally, the narrower the bandwidth, the smaller the tolerance. For example, for a bandwidth of around 10nm, the tolerance of the center wavelength is generally only allowed up to± 2nm， Regarding bandwidth above 30nm, it can be relaxed to± 5nm。

Figure 1 Schematic diagram of parameters for narrowband filter

bandwidth

Bandwidth refers to the distance between two directions in the passband where the transmittance is half of the peak transmittance, sometimes also known as full width at half maximum (FWHM). The detailed definition of bandwidth value is as follows:;

Bandwidth also has tolerance, and its tolerance scale is related to the size of the bandwidth itself. Generally speaking, the smaller the bandwidth, the smaller the tolerance. The selection of bandwidth is related to the light source used, the required signal wavelength scale, and the size of interference. Further discussion will be conducted on application examples.

Peak transmittance

Peak transmittance refers to the maximum transmittance of a bandpass filter in the passband. The requirements for peak transmittance vary depending on the application scenario. In the requirements of noise suppression and signal fineness, if more emphasis is placed on signal fineness and the hope is to improve signal strength, high peak transmittance is required. If more emphasis is placed on noise suppression and a higher signal-to-noise ratio is desired, some peak transmittance requirements can be reduced and the cutoff depth requirements can be increased.

Deadline scale

Cut off scale refers to the wavelength scale required to be cut off, excluding the passband. For narrowband filters, there is a front cutoff, which is the segment where the cutoff wavelength is less than the center wavelength, and a long cutoff, which is the segment where the cutoff wavelength is higher than the center wavelength. If subdivided, it is necessary to describe the separation between the two cutoff bands, but in general, as long as the shortest and longest wavelengths that the narrowband filter needs to cut off are specified, the cutoff scale of the filter can be known. When confirming the cut-off scale, it is not enough to simply say '; Except for passband, nothing else is required; This depiction is too idealistic and may encounter difficulties in the practical production process.

The selection of cutoff scale is related to the light source used, the wavelength scale of the interfering light, and the spectral response scale of the receiver used.

Cut off depth

The cutoff depth refers to the maximum transmittance allowed for light to pass through the cutoff band. Different application systems have different requirements for the cut-off depth. For example, in the case of excitation fluorescence, the cut-off depth is generally required to be below T<0.001%. In general monitoring and recognition systems, a cut-off depth T<0.5% is sometimes sufficient. The detailed cut-off depth depends on the intensity of the light source, the size of the interference light, and the requirement for signal-to-noise ratio. Similarly, when confirming the cut-off depth indicator, we cannot say '; Except for the passband, all light transmittance in the cutoff band is zero! "

For simplicity, OD value is often used to indicate the cut-off depth. The relationship between OD value and transmittance is as follows:;

For example, if T<0.01%, its transmittance is 10-4, corresponding to an OD value of 4. If T<1%, the corresponding OD value is 2

Incident viewpoint

The incident angle refers to the angle between the incident light and the normal of the filter. As shown in Figure 2.

Figure 2 Schematic diagram of incident angle, theta; 1, θ 2, θ 3 represents the incident angle;

Do not misunderstand the incident angle as the angle between the direction of the light source and the line connecting the center of the filter and the normal of the filter. If the collimated light path is not passed through, even if the light source orientation is installed on the center normal of the filter, the light will still diverge, and the incident angle will not be 0 °;.

If the angle between the incident light and the normal of the filter has a certain scale, it is necessary to specify the detailed scale of this angle, because the planning of the interference filter is very sensitive to the viewpoint. Regarding 0° The narrowband filter planned below has completely different effects if used at different viewpoints. Taking the 850nm narrowband filter as an example, Figure 3 shows that as the incident viewpoint increases, the orientation of the center wavelength continuously shifts towards shorter wavelengths. As long as it is an interference filter, this viewpoint effect cannot be avoided, which is the fundamental characteristic of interference filters. From the figure, it can be seen that when an 850nm narrowband filter operates at 50 ° C; When used, it is no longer an 850nm narrowband filter, but a 770nm narrowband filter. Some users may report that when using a wide-angle lens to take pictures of objects, after adding a narrowband filter, they find that only the central part of the object can be captured, and the edges are very dark. They believe that the narrowband filter only transmits at the center and not at the edges. This understanding is incorrect. The entire surface of the narrowband filter is uniform, mainly because when the large viewing point is incident, the transmission band of the filter shifts to shorter wavelengths, and there is no light source in the shorter wavelengths.

Figure 3: The Influence of Different Incident Angles on the Transmittance Characteristics of Interference Filters;

The above six parameters must be considered when applying and selecting narrowband filters, as different performance indicators require different filter production costs.