Product Announcement from Cascade Optical Corp.

Bandpass Filters-Image

Band-pass filters are usually divided in two groups: 1. Broadband pass filters, and 2. Narrowband pass filters. Although there are no bandwidth values that define narrow or broad filters in UV or visible or in IR, it is common to consider filters that have bandwidth 2% or less of the central or peak wavelength as narrowband. Thus, according to this convention, the band-pass filter centered at 500 nm is considered as narrowband if the pass band has bandwidth of 10 nm or less. Band-pass filters are defined by the bandwidth or full width at half-transmission maximum of the pass zone, half-maximum wavelengths, minimum required transmission in the pass band, the spectral range of the rejection zones below and above pass spectral band, and by the maximum allowed transmission in the rejection zones. In addition, slope of the transmission curve on both sides of the pass band can be defined together with so-called cut-on and cut of points. Cut-on point is where T just becomes higher than 5% between shorter-wavelength rejection zone and pass-band, and cut-off point is where T just turns below 5% between pass band and longer-wavelength rejection zone.

For example, band-pass filter with spectral curve to the right meets the following specs:

  • Band-pass transmission
    T > 80% @ 880 - 920 nm,
  • FWHM = 60 ± 10 nm
  • T = ½ MAX @ 870 ± 5 nm and @ 930 5 nm
  • Shorter-wavelength rejection zone
    Tav < 0.5% @ 600 - 800 nm,
  • Longer-wavelength rejection zone
    Tav < 0.1% @ 1000-1300 nm
  • Cut-on point
    T 5% @ 860 ± 5 nm
  • Cut-off point
    T 5% @ 945 ± 5 nm

According to 2% convention this filter is categorized as broadband.

Most of the band-pass filters are designed either by combining long-wave and short-wave pass filters (broadband) or by utilizing Fabry-Perot multi-cavity design approach (narrowband). In vacuum ultraviolet, below 200 nm where very few materials transmit design of efficient transmission band-pass filters is basically impossible. One solution is to utilize several band-reflection filters and use them as multiple reflectors filtering system where incident radiation is reflected from each reflection filter in the system, thus, resulting in exponential filtering of the out-of-band spectrum.

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