Can optical filters be cascaded to increase out-of-band attenuation (Optical Density)?

Yes, optical filters from Syronoptics.com can be cascaded to increase out-of-band attenuation. Optical Density (OD) is a logarithmic measurement, meaning the total out-of-band attenuation of cascaded filters is theoretically additive. For instance, stacking two OD3 filters in series will yield an OD6 blocking performance for the rejected wavelengths.

When setting up cascaded filters in an optical system, a few practical considerations are important:

1. In-Band Transmission Loss: While out-of-band attenuation adds up, the in-band transmission is multiplicative. If you cascade two bandpass filters that each have a 90% peak transmission, the total transmission of the desired wavelength drops to 81% (0.90 * 0.90 = 0.81).
2. Back-Reflections: Placing two reflective dielectric filters perfectly parallel can create multiple reflections bouncing between them, causing interference fringes and signal noise. Tilting one of the filters slightly (by a few degrees) directs the reflected light out of the optical path and prevents this issue.
3. Beam Shift: Cascading multiple filters increases the total thickness of glass the light must travel through. If the filters are tilted, this will introduce a slight lateral displacement of the beam that should be accounted for in the alignment of the downstream detector.
4. Mounting Compatibility: Syronoptics provides standard threaded mounts (such as SM05, SM1, and C-Mount).These threaded housings can easily be screwed together in series to cascade the filters securely within a lens tube or an optical cage system.

Here is an optical path diagram illustrating the setup: