Can I Stack Optical Filters? How to Check Transmission, Reflections and Focus

You have a bandpass filter that selects the right light, but the camera is still too bright. Adding a neutral density filter sounds sensible. In another setup, a longpass and shortpass might create the band you need from available parts.

Both can be useful approaches. Before stacking, check what the combination does at each useful wavelength and what the extra glass does to the image.

Multiply transmissions at the same wavelength

If one filter transmits 50% and another transmits 25% at the same wavelength, a simple first-pass calculation gives 0.50 × 0.25 = 0.125, or 12.5% transmission through the pair.

Optical densities add in that calculation. For example, OD 0.3 plus OD 0.6 gives OD 0.9, corresponding to about 12.6% transmission. The small difference from 12.5% comes from rounding the individual OD values.

Do not multiply unrelated averages

Two filters can each have a high average transmission and still overlap poorly. One may transmit mainly shorter wavelengths while the other transmits mainly longer wavelengths. Their shared transmitted region can be small.

Use matching wavelength points on the two curves. Multiply those values to estimate the combined curve, then compare it with the source and detector. An overall camera signal also depends on the source spectrum and detector response.

This simple model neglects interactions such as multiple reflections. Polarizing components need particular care because the first element changes the polarization arriving at the next one.

Extra surfaces can create extra images

Each added plate introduces surfaces that can reflect light. Reflections between filters, the lens and the sensor may appear as ghosts or reduced contrast, particularly around bright features.

Suitable anti-reflection treatment can help, but coating performance depends on wavelength and angle. Changing spacing or tilt may affect reflections as well as the spectrum; evaluate the full assembly rather than applying the same mounting rule to every stack.

Leave room to check focus

Adding glass can change focus when it is placed in a converging or diverging beam. The effect depends on position, thickness, refractive index and imaging geometry. A filter ahead of the lens and one close to the sensor are not equivalent cases.

Refocus if needed, then inspect image quality across the field. A sharp center alone does not establish that measurement accuracy is unchanged at the edges. Do not use a universal working-distance correction without defining the optical arrangement.

Does filter order matter?

In the simple scalar multiplication model, swapping the order does not change the product of the transmissions. In a real system, order can matter because of polarization, reflections, heating, focus or the angular distribution of light.

That is why “always put the ND closest to the camera” is not a dependable general rule. Choose the arrangement for the system you actually have.

When to consider a single custom design

A stack is useful for evaluating a spectral idea. If it becomes a permanent production solution, compare it with a single design that combines the required functions. Consider space, assembly tolerance, transmission and qualification needs as well as component cost.

Browse ND filters and wavelength-selection filters. Send us the proposed filter codes and their positions in the light path so we can discuss the combination.