Bandpass Filter (FWHM = 11-40nm)

An optical bandpass filter is a device that allows a specific range of wavelengths to pass through while blocking the rest. It can be used for various applications such as fluorescence microscopy, spectroscopy, clinical chemistry, or imaging.
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Specifications of Bandpass Filter

Center Wavelength (CWL)

The wavelength mid-point of transmission range.It is usually measured in nanometers (nm) and it indicates the color of the light that passes through the filter.

Peak transmission (Tmax)

The maximum transmission value within the transmission band. It depends on the coating quality, substrate material, and angle of incidence.

Blocking Region

The blocking region for a bandpass filter is the range of wavelengths that the filter blocks or attenuates.It is the opposite of the passband, which is the range of wavelengths that the filter transmits.

FWHM Explanation

FWHM

FWHM stands for full width at half maximum, which is a measure of how wide the transmission band of a filter is. For example, a FWHM of 10 nm means that the filter transmits wavelengths within 5 nm on both sides of its center wavelength at half of its peak transmission value.

Notes:

Optical density (OD)

The measure of how much light is blocked by the filter outside the transmission band. It is expressed as a logarithmic ratio of incident and transmitted power. A higher optical density means a deeper blocking.

Notes:

  • Soft Coating can reach OD2-OD4
  • Hard Coating can reach OD4-OD6
  • Stacking Multiple OD6 Can reach OD8 or Higher

Angle of Incidence (AOI)

Bandpass Filters are very sensitive to Angle of Incidence, usually the AOI is 0 deg. for bandpass filter. Using different AOI may result in very different spectral performance.

Other thing need to know...

  • Soft Coating

    Soft coating is suitable for applications that do not require high performance and reliability, such as general illumination or color separation.Soft coating can produce filters with broad or narrow bandwidths, but with less steep edges and lower optical densities than hard coating.

  • Hard Coating

    Hard coating is a type of coating technology for optical bandpass filters.It involves depositing thin layers of dielectric materials on a glass substrate using a process called magnetron sputtering.

    Hard coating provides high transmission, deep blocking, and long durability for the filters.

  • Cavity

    A cavity in the filter coating process is a basic component of a bandpass filter that consists of two layers of reflective stacks separated by a spacer layer.

    A single-cavity filter has a triangular-shaped transmittance spectrum, while a multi-cavity filter has a rectangular-shaped transmittance spectrum.A multi-cavity filter can achieve higher transmission, narrower bandwidth, and steeper edges than a single-cavity filter, but it is more difficult to fabricate and align.

  • Steepness (or Slope)

    The steepness or slope of a bandpass filter refers to how quickly the filter transitions from transmission wavelength to blocking wavelength.