Longpass, Shortpass or Bandpass: Which One Fits Your Application?
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Longpass and shortpass names become easier to remember when you read them literally: a longpass passes longer wavelengths, and a shortpass passes shorter wavelengths. A bandpass keeps a band between two boundaries.
Those descriptions apply within each filter's specified operating range. No real filter should be assumed to pass or block wavelengths all the way to infinity.
Choose the side you want to keep
| Filter | Intended transmitted region | Useful starting question |
|---|---|---|
| Longpass | Longer wavelengths above the transition | Can I accept a broad range above this edge? |
| Shortpass | Shorter wavelengths below the transition | Can I accept a broad range below this edge? |
| Bandpass | A selected interval with two transitions | Do I need both a lower and an upper spectral limit? |
When a longpass makes sense
Suppose you want near-infrared light while reducing visible light. A suitable longpass may be a starting point if the camera should receive a broad near-infrared interval. Check how far the source and detector extend: the absence of an upper edge in the main function may admit background you did not intend.
Longpass filters can also separate a longer-wavelength signal from shorter-wavelength excitation. Whether one is adequate depends on the actual separation and required rejection, not just the words “longpass” or “fluorescence.”
When a shortpass makes sense
If the useful signal is at shorter wavelengths and longer-wavelength output is unwanted, consider a shortpass. For example, a design may retain part of the visible spectrum while reducing longer wavelengths that influence a detector.
A shortpass does not automatically block ultraviolet light. If unwanted light exists below its main transmitted region, you may need another spectral boundary.
When you need a bandpass instead
For a camera working with a particular LED, unwanted light may exist on both sides of the LED band. A bandpass provides both boundaries. Compare its width with the source emission: a very narrow band may discard useful signal, while a broad one may admit too much background.
You can sometimes create an overlapping band by combining a longpass and shortpass. That adds glass surfaces and transmission losses. Compare the combined response with a single bandpass design before deciding that stacking is the simpler solution.
The edge is a transition, not a wall
A filter does not jump instantly from full blocking to full transmission. Its curve passes through a transition region. If your useful signal sits very close to that transition, a small wavelength or angle change can affect the result.
Read how the cut-on or cut-off is defined and check transmission at the wavelengths you actually need. The same nominal edge can hide differences in slope, blocking range and secondary transmission.
Compare the complete response
Open the longpass, shortpass or bandpass collection and inspect the full curves. Consider the source, detector, angle and measured wavelength coverage together.
If you are between two families, tell us which wavelengths you want to keep and which cause trouble. That gives us a useful starting point even before you have chosen a product code.