Choosing a bandpass filter for your optical setup

PHOSLENS selection notes · Prepared 12 October 2026

Choose a bandpass filter by defining the signal to transmit and the unwanted light to reject, then compare catalogue candidates under the conditions of your optical setup. A matching center wavelength is a starting point; bandwidth, blocking, incidence angle and usable aperture still need review.

Start with two wavelength lists

Write down the signal wavelength or interval you need to retain. Separately, identify the unwanted wavelengths that matter to the measurement. Include the source, detector and purpose of the filter in your request, so that the discussion has context beyond a nominal wavelength.

If either list is incomplete, mark it as an open question. A request for a filter “around” a wavelength leaves room for interpretation; explain which requirements are fixed and where alternatives may be considered.

Shortlist by center wavelength and bandwidth together

Center wavelength locates the middle of the passband. FWHM is its width between the two half-maximum transmission points; it is relative to the peak rather than necessarily 50% absolute transmission. These definitions explain catalogue labels, not the performance of a particular model. Reference: optical filter terminology.

Start with the published bandpass filter families. Save the exact model names of plausible candidates. If your requirement is not covered by the listed fields, carry that gap into the inquiry instead of treating the nearest model as an agreed substitute.

For a specific series, compare the HNIF narrowband interference filter models by their listed center wavelength, bandwidth, optical density and housing dimensions. This series includes both 10 nm and 20 nm FWHM entries; check the individual model rather than assuming a uniform bandwidth or blocking range. Add suitable models and quantities to your quote list, and include the operating conditions that matter to your selection.

Use a comparison worksheet before choosing a model

Questions to resolve when comparing bandpass filters

What you find	What to check next
Same center wavelength, different bandwidths	State the signal interval to retain and the nearby wavelengths to reject. Ask which candidate meets both requirements.
A blocking value without the band you need	Request the applicable blocking interval and limit. Do not extend a quoted value to wavelengths outside its stated range.
A curve measured under different illumination	Provide your incidence angle and beam geometry; request confirmation for those conditions.
An outside diameter that fits the holder	Check the required clear aperture, thickness and mounting constraints separately.
A typical curve or an incomplete field	Ask which requirements will be confirmed in the quotation and which documentation is available.

Optical density describes attenuation, while the blocking range identifies where it applies. Higher OD means lower transmission. Interference-filter spectra can also shift with incidence angle; a reference curve should not be applied unchanged to another geometry. Reference: blocking and incidence angle.

Use the optical curve reading guide for terminology and measurement conditions. If two candidates remain plausible, keep their exact identities and list the unresolved conditions so that they can be reviewed together.

Turn the comparison into an inquiry

Include the following information where known:

• Candidate model or a description of the required filter, and quantity.
• Signal wavelength or interval, acceptable bandwidth and required transmission.
• Wavelengths to reject and the required rejection limits over each interval.
• Incidence angle, polarization where relevant, and whether the beam is collimated, converging or diverging.
• Outside dimensions, thickness, required clear aperture and holder constraints.
• Any required acceptance records, destination and requested delivery date.

Do not replace an unknown field with a guessed value. If a drawing, a changed band or another geometry is needed, describe it through Custom Optics. For a catalogue request, review the selected models and quantities in your quote list. The optical inquiry checklist explains how to keep technical requirements and commercial details identifiable.

These notes support selection discussions. Published model information and the individually agreed quotation determine the scope of supply; feasibility, availability and final terms require confirmation.