Understanding Bandpass Filters

A bandpass filter is designed to pass a specified range of wavelengths with high transmittance while blocking all others. At Shanghai Optics we produce a variety of band-pass optical filters for various applications, including biotech, machine vision, laser line separation, spectral radiometry, and fluorescence applications.

Specifications of a Bandpass Filter

Bandpass filters are typically designated by the wavelength range they transmit, their passband. Aside from the passband wavelength, the three most important specifications of any bandpass filter are the center wavelength (CWL), the full width at half maximum (FWHM), and the peak transmission (T).

The center wavelength (CWL) of a bandpass filter is the wavelength at the center of the passband. The peak transmission (T) is the wavelength of light at maximum transmission. Finally, the full width at half maximum (FWHM) is the bandwidth at 50% of the maximum transmission.

Sometimes cut-on and cut-off wavelengths are used to describe the filter’s passband instead of FWHM and CWL.

The flatness of the filter’s passband is described as pass band ripple. Flat-top bandpass filter designs are more desirable but also more costly to manufacture, as they require more layers.

The transition from passband to blocked wavelengths of light is described by the edge steepness. If a filter has a steep edge, the transmission drops of quickly at the cut-off and cut-on wavelengths.

Types of Optical Bandpass Filters

Standard bandpass filters will have a passband in the 400-1100 nm range, and a full width at half maximum starting at around 10 nm. The sizes of our standard filters range from 2-90 mm. However, we can produce specialized bandpass filters per customer request if your application has special requirements.

The most commonly used optical bandpass filters are either all-dielectric or metal-dielectric. All–dielectric bandpass filters are designed with two reflecting mirrors with alternating high and low refractive index materials, and a dielectric spacer separates them. The properties of these filters depend on the number of reflective mirror layers and the thickness of the spacer. Metal-dielectric filters are similar, but they utilize metal spacers and are primarily used for UV bandpass filters.

Multiband filters are bandpass filters with several passbands: that is, a series of discrete high transmission and low transmission regions. They are used to combine multiple beams of light and can even be used to correct color blindness.

A narrowband filter provides high transmission over a narrow passband and dense blocking of other light. These narrow bandpass filters are often used as clean up filters and to reduce cross talk between signals.

Bandpass filters for signal processing

To fully understand band-pass filters it helps to look at them in the general context of band-pass filters for signal processing. As such a band pass filter passes a certain frequency range while blocking others. In electronics, an active band pass filter circuit is known as second order filter since it is designed with two capacitors.

Active filters have a peak frequency response (resonant frequency) at the center frequency of their passband. The transfer function is defined as the output signal voltage divided by the input voltage in the complex plane. The bandwidth of the filter is defined as the difference between the upper and lower cutoff frequencies, and the quality factor is considered to be the ratio of the center frequency of the bandpass over the whole passband.

Please contact us to discuss your bandpass filter needs, to receive a catalogue of our on-shelf stock or to inquire about custom options.