Bandpass Optical Filters

Introduction to Bandpass Optical Filters

What is a bandpass filter?

An optical bandpass filter is an optical filter that selectively transmits a specific range of wavelengths of light while blocking all other wavelengths. Bandpass filters are typically characterized by their central wavelength, bandwidth, transmission, and blocking.

  • Transmission: The percentage of light, T, that passes through the filter passband.
  • Blocking: Measured as Optical Density (OD), defined as OD = – log10 (T).
  • Edge Steepness: The wavelength interval needed for transition between high transmission and blocking.

While the passband often refers to the theoretical filter performance derived from design specifications, Semrock prefers to use manufacturable specifications which describe the actual filters as received by the customer.

What does “bandwidth” mean?

Semrock uses a “manufacturable specification” approach to define the bandwidth of our bandpass optical filters. We believe this approach more accurately reflects the performance of the filter in an optical system.

As shown in the diagram, the filter spectrum (red line) will, on average, lie within the unshaded regions. The average transmission will exceed the specification Tavg in the Transmission Region, which has a certain center wavelength (CWL) and a width called the Guaranteed Minimum Bandwidth (GMBW). The transmission will, on average, lie below the blocking level specifications (OD) in the Blocking Regions.

how to define bandwidth graph

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How are optical bandpass filters used?

Optical filters are used in diverse fields such as Spatial Biology, PCR, Quantum Computing, and Wearable Medical Devices, most of which rely on the fluorescence emitted by specially designed molecular probes.

  • PCR is one of the most sensitive ways to detect presence of COVID and other serious disease entities. The technique relies on amplifying sample DNA modified to emit fluorescence depending on the DNA contents. The fluorescence is passed through a bandpass filter and indicates if the sample has COVID DNA or not.
  • Spatial Biology is the study of a tissue in their native 2D or 3D environment, similar in concept to an aerial survey of the detailed ecology of an area. The tissue is populated with fluorescent probes, which emit light describing the local conditions, which can even include details on local RNA contents. Bandpass filters are used to clean up the signals so they can be interpreted correctly.
  • A Wearable Medical Device lies on the skin and shines a small beam of light into the region on or below the skin. A small injectable bead that contains a sensor responds to the light with output light that depends on the physiological signal (e.g. glucose) being monitored. This light is selectively passed by the bandpass filter to the detector.

Learn More About Our Bandpass Filter Product Families