Can I stack longpass and shortpass to make a custom band?

If you've ever needed a bandpass filter with a specific center wavelength or bandwidth and struggled to find it off-the-shelf, you might wonder: Can I make my own by stacking standard filters? The answer is yes! You can create a custom bandpass filter by stacking a longpass and a shortpass filter, quickly and cost-effectively, to achieve a targeted spectral window that standard filters may not provide.

Longpass filters transmit light that is longer than a specific cutoff wavelength, blocking (reflecting) the shorter ones.
Shortpass filters do the opposite, transmitting shorter wavelengths and reflecting/blocking the longer ones.
When you look at their transmission graphs, they're almost like mathematical 'step' functions—transmitting light above or below a certain point, with a relatively sharp transition.

By stacking a longpass and a shortpass filter, only light that passes both filters is transmitted. This doesn't just sum their effects—it multiplies them: you get a new filter that only transmits where the two original filters' transmission ranges overlap.
In math, this is like multiplying two 'Heaviside step functions':
Place the longpass filter first to block everything below its cutoff.
Add the shortpass filter to block everything above its cutoff.
The resulting window is your custom band.
For example, if you stack:

• A 450 nm longpass (transmits above 450 nm)
• A 500 nm shortpass (transmits below 500 nm)

• Customization: Achieve tight wavelength bands not available from stock bandpass filters.
• Simplicity: No need for a complex custom coating run.
• Speed: Buy stock filters and stack, instead of waiting for a custom part.
• Cost: Often cheaper—especially for prototyping, research, or low-volume needs.

• Make sure the filters are coated on suitable substrates—stacked dichroic or interference filters offer steeper transitions and higher blocking outside the passband.
• Ensure filters are oriented correctly (coated surfaces facing the light source) to minimize unwanted reflections or ghosting.
• Stack tolerance: Real-world filters have sloped—not perfectly vertical—cutoff edges, so your transmission window will have a finite slope.

• Transition Width: The band edges might not be as steep as a purpose-designed bandpass filter.
• Out-of-band Blocking: Stacked filters may not block out-of-band light as efficiently as a dedicated single-piece bandpass filter.
• Physical Constraints: Adding filters can increase thickness and potential reflection losses.

• Prototyping new instruments or testing wavelength ranges
• Building one-off scientific setups
• When cost, speed, or availability make custom bandpass unfeasible
• When you need a quick fix to achieve a unique or 'non-standard' spectral range

• Stacking a longpass and a shortpass filter is a proven and practical way to create a custom bandpass filter, ideal for situations where standard options don't fit your requirements or custom coatings would be too costly or slow.
• If you're unsure about your specific application or want to optimize your filter stack, reach out to an optics supplier or seek professional advice to match spectral needs with filter options.