The so-called narrow band optical filter is subdivided from the bandpass filter, and its definition is the same as that of the bandpass filter. It allows light signals to pass through a specific wavelength band, while the light signals outside this band are blocked. The passband of narrow band optical filters is relatively narrow, generally less than 5% of the center wavelength value. The parameters of narrow band optical filters are described as follows.
The center wavelength of the narrow band optical filter is generally the working wavelength of the instrument or equipment. It refers to the wavelength in the center position of the passband. In the actual production process, the position of the center wavelength is always slightly different from the design value, so when specifying the center wavelength, a tolerance range is generally added. This tolerance range is determined by actual usage conditions. Usually, 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 to be ±2nm, and for a bandwidth of over 30nm, it can be relaxed to ±5nm.
The bandwidth refers to the distance between two positions where the transmittance of the passband is half of the peak transmittance. Sometimes it is also called the half-width (not the half bandwidth). The bandwidth also has a tolerance, and its tolerance range is related to the size of the bandwidth itself. Generally speaking, the smaller the bandwidth, the smaller the tolerance. The choice of bandwidth depends on the light source used, the required signal wavelength range, and the size of interference.
The peak transmittance of the bandpass filter refers to the highest transmittance in the passband of the narrow band optical filter. The high or low requirements for the peak transmittance of narrow band optical filters depend on the specific usage situation. In noise suppression and signal intensity requirements, if the signal size is more concerned and the strength of the signal is hoped to be improved, it requires high peak transmittance. If noise suppression is more concerned and a higher signal-to-noise ratio is expected, some requirements of the peak transmittance can be decreased, while the requirements for the cut-off depth need to be raised.
The cut-off range refers to the wavelength range required to be cut off except for the passband. For narrow band optical filters, there is a front cut-off and a back cut-off. The cut-off wavelength of the front cut-off is less than the center wavelength, while the cut-off wavelength of the back cut-off is greater than the center wavelength. If subdivided, both cut-off bands need to be described separately. However, in general, only indicating the short and long cut-off wavelengths required by the narrow band optical filter can determine the cutoff range of the filter. When determining the cut-off range, it is not simply to say that "everything except the passband does not need to be cut off," because this description is too idealistic and can cause problems in the actual manufacturing process. The choice of cut-off range depends on the light source used, the range of interference wavelengths, and the spectral response range of the instrument used.
The cut-off depth refers to the maximum transmittance allowed in the cut-off band. Different application systems have different requirements for the cut-off depth. For example, in the case of using fluorescence excited by the light, the cut-off depth is generally required to be below T ＜ 0.001%. In ordinary monitoring and identification systems, a cut-off depth of T ＜ 0.5% is sometimes sufficient. The specific 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.
The incident angle refers to the angle between the incident light and the normal direction of the narrow band optical filter. Do not misunderstand the incident angle as the angle between the position of the light source and the center of the narrow band optical filter and the normal direction of the filter. Even if the position of the light source is installed on the center normal line of the filter, the light beam is still divergent if it has not passed through a collimated light path, so the incident angle will not be 0°.
If the incident angle between the incident light and the normal direction of the filter has a certain range, specify the specific range of this angle. Because the design of interference filters is very sensitive to angles, narrow band optical filters designed under 0° have completely different effects when used at different angles. Some users may find that when using a wide-angle lens to shoot the object, after adding a narrow band optical filter, only the middle part of the object can be photographed, and the edges are very dark. They think that the narrow band optical filter only transmits the middle part and opaque on the edges, which is not correct. The entire surface of the narrow band optical filter is uniform. The main reason is that when the incident angle is large, the passband of the filter moves towards the shorter wavelength, and the short-wave part has no light source to cause the effect.
The above six parameters must be considered when using and choosing a narrow band optical filter. Different performance indicators bring different manufacturing costs to filters.