Choosing a Bandpass Filter for Your LED and Camera

Your LED and filter both say “850 nm,” yet the camera image becomes unexpectedly dark. The labels match, so what went wrong?

An LED emits over a range of wavelengths. A bandpass filter transmits another range. What matters is how those ranges overlap in your actual setup. Matching the nominal wavelength is a useful first step, but it is not the whole selection process.

Start with the LED spectrum

Ask for the light source's spectral output, including its bandwidth and expected operating conditions. The center wavelength alone does not describe how its energy is distributed. Do not assume that every LED with the same nominal wavelength has the same spectrum.

Next, check the camera's spectral response. If it contains a built-in infrared-cut filter, an external near-infrared bandpass filter cannot make that camera transmit infrared through the internal filter. Confirm the complete optical path.

Wider and narrower solve different problems

A wider bandpass can retain more of a broad LED's emission and give more room for wavelength shifts. It can also admit more background light. A narrower band can reject more surrounding wavelengths, but it may remove useful LED light as well.

There is no universal bandwidth that is best for every 850 nm system. An indoor inspection cell with controlled lighting and an outdoor camera facing changing sunlight have different background conditions and exposure limits.

Also distinguish bandwidth from peak transmission. Narrowband designs can have high peak transmission. Losing a portion of a broad source spectrum is different from low transmission at the center of the passband.

A simple overlap example

Imagine a source that emits equally across a 40 nm interval. An ideal filter transmitting 100% over only the central 10 nm would retain 25% of that source's light, even though the filter's peak transmission is perfect.

This is an illustrative calculation, not a model of a particular LED or KUPO product. Real source spectra are not rectangular; use measured spectra when estimating your system signal.

The practical question is whether the reduction in unwanted light is worth the useful signal you give up. Compare image contrast and repeatability at the exposure time the application actually permits.

Use the controls to explore the calculation. The shapes are illustrative, not measured KUPO spectra. Open the explorer in a separate tab.

Allow for the rays that reach the filter

Interference-filter spectra depend on angle. A filter in a converging beam sees a range of angles even when the holder looks perpendicular to the lens. A wide field of view can also make the response differ across the image.

Share the working angle and lens arrangement with us. Include temperature changes that may affect the source or filter. A narrow spectral match with no margin may be difficult to maintain in production.

Build a shortlist and test it

1. Identify the useful source band and detector response.
2. Compare candidate filter curves over that band and the unwanted wavelengths.
3. Check transmission and blocking at the operating geometry.
4. Evaluate image contrast, exposure and stability with representative samples.

A filter cannot reject ambient light that falls in the same transmitted band. Shielding, controlled illumination and suitable camera timing may still be part of the solution.

Start with our bandpass filters and broad bandpass filters. Send your LED spectrum, camera model and working angle if you would like help narrowing the choice.