Which Optical Filter Do I Need? A Practical Selection Guide
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You know what is wrong with your image or light source, but you may not know the name of the filter that could help. Perhaps a camera is seeing too much background light. Perhaps a fixture looks too green. Or perhaps you need to send visible and infrared light along different paths.
Start with the change you want to make to the light. That is usually more useful than starting with a product code.
First, name the problem
| What you need | Family to explore | First check |
|---|---|---|
| Keep one wavelength band and reject others | Bandpass | Does the passband cover your useful signal? |
| Keep wavelengths above or below an edge | Longpass or shortpass | Which side of the edge should pass? |
| Suppress an unwanted band while retaining surrounding wavelengths | Notch | How wide is the unwanted band? |
| Reduce brightness | Neutral density | How much reduction, over which wavelengths? |
| Adjust the color of a light source | Color correction | Is the issue warmth, coolness, tint or spectral balance? |
| Separate light into two paths | Beam splitters or wavelength-selective mirrors | What should transmit, and what should reflect? |
Match the filter to the useful light
Imagine a monochrome inspection camera illuminated by a red LED. If room lighting is affecting the image, a bandpass filter may help retain the LED band while reducing background at other wavelengths. It will also pass background light inside that same band. Good shielding and lighting geometry can still matter.
Now imagine a color camera that is simply overexposed. A narrow red bandpass would change the colors it sees. A neutral density filter may be the better starting point if you want to reduce intensity while retaining the visible spectral balance.
These are different problems, even though both images might initially look “too bright.”
Use the whole curve to compare models
A family name tells you the general function. The transmission curve tells you where a particular design passes light. Look beyond the main peak: additional transmission bands can matter if the source emits there and the detector responds there.
Check the measured wavelength range too. A curve ending at 780 nm does not establish what happens at 850 nm. Our chart-reading guide explains how to interpret these details.
Check the physical setup before choosing
Two filters with similar curves may suit different installations. Tell us the working angle, whether rays arrive nearly parallel or in a cone, the required clear aperture, and the available space. Include operating temperature and optical power where relevant. A filter must fit the light path as well as the holder.
If glare is the main problem, also review illumination direction and polarization. A wavelength filter does not automatically remove a reflection carrying the same wavelengths as your subject.
You can start without a finished specification
A helpful first message is: “We are inspecting this material, using this light and camera, and this is the problem in the image.” A source spectrum or sample image helps, but you do not need to know every optical term.
Browse KUPO filter families, or tell us what you want your light to do. We can use that information to discuss a starting design and the requirements that still need to be defined.