Long Pass Filters and Short Pass Filters

Long pass filters and short pass filters are interference filters, which provide a sharp cut-off either above or below a particular wavelength. They are useful for isolating specific regions of the spectrum.  Often referred to jointly as edge pass filters (EPFs), long pass filters (LPFs) and short pass filters (SPFs) are used to pass, or transmit, a range of wavelengths and to block, or reflect, other wavelengths on one side of the passband.  In the case of long pass filters, the transmitted wavelength is long wavelength radiation, while short wavelength radiation is reflected.  Conversely, short pass filters transmit a wide spectral band of short wavelength radiation and block long wave radiation. These filters are specified by their center (50% transmission) wavelength. The transmission of long pass filters is 85% from a wavelength that is 3% greater than the specified wavelength to 1500 nm. The transmission of short pass filters is 85% from a wavelength that is 3% less than the specified wavelength to 425 nm (except for the 400 nm filter). The width of the transition from cut-on to cut-off is typically 6% or less of the center (specified) wavelength.  The cut-on wavelength is where the long pass filter stops reflecting and starts transmitting. The cut-off wavelength is where the short pass filter stops transmitting and starts reflecting.

The region of high transmittance is known as the passband and the region of high reflectance is known as the reject or reflectance band.   The roll-off region separates the pass-band and reflect-band. The complexity of long pass filters and short pass filters depends primarily upon the steepness of the transition region and also on the ripple specifications in the passband.  In the case of a relatively high angle of incidence, polarization dependent loss may also be a consideration.

Long pass filters and short pass filters are constructed of hard, durable surface materials covered dielectric coatings.  They are designed to withstand normal cleaning and handling.  They are available in circular, rectangular, and square side configurations.  While there is little specific differentiation in performance between the three shapes, they are so designed to fit into a wide variety of application requirements.

The applications of long pass filters and short pass filters include wavelength band separation and optical add-drops.  Their high rate of transition makes them an excellent choice as emission filters in fluorescence applications, and as order sorting filters in photometry. They are also useful as stray light or trim filters, and to eliminate any unwanted near-band radiation.

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