Will AOI change the cut-on/cut-off of my edge filter?

Will AOI change the cut-on/cut-off of my edge filter? Absolutely—understanding the impact of angle of incidence (AOI) is essential for achieving reliable optical filtering. AOI refers to the angle at which light arrives at the filter relative to its surface normal. As you increase this angle, several effects can change your filter's spectral performance in important ways:

1. Blue Shift of Cut-On/Cut-Off Wavelength
As AOI increases, the filter's edge (cut-on or cut-off) shifts to a shorter wavelength—a phenomenon known as a blue shift. For example, a longpass filter designed to block below 830 nm at normal incidence (0° AOI) may only block below about 810 nm or even shorter as the AOI increases. This blue shift is more pronounced at larger angles and can impact applications like Raman or fluorescence spectroscopy, where precise blocking or passing of specific wavelengths is required.

2. Polarization Dependence
The shift is not uniform for all light. S-polarized and P-polarized light shift differently, causing polarization splitting. P-polarized light typically experiences a greater blue shift than S-polarized. In practical terms, if your source is unpolarized or mixed, you may see a shelf or dip in your filter's transmission curve—reducing the sharpness and efficiency of the filter's edge.

3. Reduced Blocking at the Edge
AOI doesn't just shift your filter's wavelength; it also reduces optical density (OD) at the laser line or blocking wavelength. A filter with 6 OD blocking at 0° AOI may only provide 4 OD blocking at 4°, letting more unwanted light through. This is critical for high-sensitivity instruments using edge filters for Rayleigh rejection or spectral purity.

4. Why Does AOI Cause These Effects?
The physics comes from the change in effective optical thickness through the multi-layer coatings as the beam hits the filter at an angle. The relationship for how the cut-on/cut-off wavelength shifts is:

λ_θ = λ₀ √[1 - (n₀/n_i · sin θ)²]

• λ_θ: Wavelength at AOI θ
• λ₀: Wavelength at normal incidence
• n₀: Refractive index of ambient (typically 1 for air)
• n_i: Effective refractive index of the filter
• θ: Angle of incidence

5. Additional Real-World Effects

• Most edge filters are optimized for 0° AOI; deviations should be minimized unless designed otherwise.
• Dichroic filters may be intentionally designed for higher AOI (e.g., 45°), but still show polarization dependent effects.
• Light cones with a large cone half-angle (CHA) exaggerate the blue shift and reduce OD blocking.
• For best results, minimize both AOI and CHA, or work with manufacturers for custom coating solutions.

6. Practical Tips

• Always specify AOI to your filter supplier for optimal performance in your setup.
• Mount filters per manufacturer guidance (correct orientation).
• Test your filter's blocking or passing at your actual AOI and setup conditions.
• If significant AOI is unavoidable, request filters engineered for that condition.

• Increasing AOI shifts cut-on/cut-off edges to shorter ('bluer') wavelengths and can degrade blocking—always consider AOI in your optical system design for best filter performance.