This page covers optical notch filters — components that reject light, not the electronic or audio notch filters that reject signals. In Raman spectroscopy the laser line scattered from the sample is vastly stronger than the Raman signal sitting a few nanometres away from it, and in fluorescence and laser-viewing instruments the same geometry repeats. A notch filter centred on the laser line suppresses it while passing the spectrum on both sides, which a longpass edge cannot do when the signal of interest lies on both sides of the line.
Notch filters appear in the manufacturer's documented filter group as a family of their own, and a combined narrow band pass / notch listing at a 520 nm centre wavelength for medical tests shows the two rejection-and-selection disciplines sharing one instrument catalogue. The specification conversation for a notch is dominated by three numbers: the centre wavelength to be rejected, the width of the rejection band, and the blocking depth at the line — plus the transmission required immediately outside the notch, which is where the measurement lives.
Specifying a Notch Filter
- Centre wavelength — normally the laser line to be rejected; state the laser, not just the number, if the line has width.
- Notch width — how much spectrum around the line can be sacrificed.
- Blocking depth — how far the line must be suppressed for the detector to work.
- Passband requirement — transmission on both sides, and how close to the notch the signal sits.
State where the signal sits relative to the line, and the band the filter must reject, in the enquiry; whether the form to price is a notch or a longpass filter is confirmed against those stated conditions in the quotation. Buyers building a complete laser-instrument filter set can name the excitation bandpass and the notch against the same wavelength plan in one enquiry; each filter's supply is confirmed item by item in the quotation. Minimums and pricing are confirmed per specification in the written quotation.
Frequently Asked Questions
Is this an electronic notch filter?
No — this is an optical component that rejects a band of light, typically a laser line. It has nothing to do with audio or circuit filters.
Notch or longpass for Raman?
State whether the signal you measure exists on one side of the laser line or both — including whether it is only the red-shifted (Stokes) side — and the form, notch or longpass, is confirmed against that stated geometry in the quotation.
What does the quotation need?
The laser line wavelength, acceptable notch width, required blocking and the size and quantity.
Living Next to the Notch: Passband Quality
The specification number to watch is not the notch depth but the passband beside it. Raman and fluorescence signals sit close to the laser line by physical necessity, so the filter's transmission a few nanometres outside the rejection band — and the steepness of the transition between the two — decides how much of the wanted signal survives. A notch that blocks superbly and transmits indifferently has solved the easy half of the problem. When you enquire, give the closest wavelength to the line that carries signal, not only the line itself; that offset is part of the design's specification and the honest basis of the quotation. It is also worth stating the laser's own linewidth and stability, because a drifting source needs a wider notch than a locked one, and width bought unnecessarily is signal thrown away on both sides of the line.
Request a Quotation
Tell us the component, material, dimensions and coating you need. Pricing and lead time for your specification are confirmed in the written quotation.