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).

• Center Wavelength (CWL): The wavelength at the center of the passband.
• Peak Transmission (T): The wavelength of light at maximum transmission.
• Full Width at Half Maximum (FWHM): The bandwidth at 50% of the maximum transmission.
• Pass Band Ripple: Describes the flatness of the passband. Flat-top bandpass filters are preferred for their stability, though they are costlier to manufacture due to the added layers required.
• Edge Steepness: The transition from passband to blocked wavelengths. A steep edge ensures that transmission drops off quickly at the cut-off and cut-on wavelengths.

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

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.

• All-Dielectric Bandpass Filters: Designed with two reflecting mirrors of alternating high and low refractive index materials, separated by a dielectric spacer. The properties depend on the number of reflective mirror layers and spacer thickness.
• Metal-Dielectric Bandpass Filters: These utilize metal spacers and are primarily used for UV bandpass filters.
• Multiband Filters: These have multiple passbands, allowing the combination of multiple light beams. They can also be used to correct color blindness.
• Narrowband Filters: Provide high transmission over a narrow passband with effective blocking of other light, ideal for clean-up and reducing 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.

Custom Optical Bandpass Filters

We offer a wide range of bandpass filters suitable for diverse optical and photonic applications. Our standard filters cover the 400-1100 nm range, with sizes from 2-90 mm. For applications with specific requirements, we can produce custom bandpass filters tailored to your needs.

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