In optical engineering, narrowband filters have become core components in fields such as fluorescence microscopy and spectral analysis due to their exceptional spectral selectivity. The center wavelength is the most critical parameter defining the performance of such filters.
Definition and Key Parameters of Center Wavelength

The center wavelength refers to the wavelength value corresponding to the peak of a narrowband filter's transmittance curve. It determines the primary light signal wavelength permitted to pass through the filter. In practical applications, the center wavelength is always defined in conjunction with the full width at half maximum (FWHM):
Center Wavelength (CWL): The wavelength (λ₀) corresponding to the peak transmission.
Full Width at Half Maximum (FWHM): The spectral width corresponding to half the peak transmission.
For example, “CWL=1550nm, FWHM=10nm” indicates the filter exhibits maximum transmittance at 1550nm and maintains transmittance no less than half the peak value within the 1545-1555nm range.

Key Factors Affecting the Center Wavelength

The angle of incidence effect is the most significant influencing factor. When light strikes at an angle, the center wavelength shifts toward shorter wavelengths (blue shift). This shift can be described by the formula λ(θ)≈λ₀×√(1-sin²θ/n²). Therefore, beam collimation must be strictly controlled in practical systems.
Practical Applications of Center Wavelength
In fluorescence microscopy imaging, the center wavelength of the excitation filter must precisely match the absorption peak of the fluorescent dye, while the center wavelength of the emission filter should align with the dye's emission peak.

In telecommunications wavelength division multiplexing systems, dense wavelength division multiplexing employs narrowband filters with channel spacing as tight as 0.8nm. Each filter's center wavelength corresponds to an independent communication channel, and its precision and stability directly determine the system's transmission capacity and communication quality.

 The figure shows the transmittance curve of a narrow band filter. In the figure, the detailed definition of the centre wavelength is as follows: which, respectively, corresponds to the wavelength position of the left and right side of the passband when the transmittance is half of the peak value. In the practical production process, the central wavelength of the orientation and the planning value is always more or less a little difference, so in the provision of the central wavelength, generally add a tolerance scale. This tolerance scale is determined by the practical application conditions, usually, the narrower the bandwidth, the smaller the tolerance. For example, the bandwidth of about 10nm, the centre wavelength tolerance is generally only allowed to ± 2nm, on the bandwidth of more than 30nm, it can be relaxed to ± 5nm.


The center wavelength, as the core parameter of narrowband filters, is crucial for the performance of optical systems due to its precise control and stable maintenance. A thorough understanding of how factors such as the angle of incidence and temperature affect the center wavelength provides essential guidance for the proper selection and use of narrowband filters. From life sciences to communication technologies, the precise control of center wavelength continues to push the boundaries of optical applications.

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