You designed a perfect optical system. You selected the ideal long pass filter. But when you assembled everything, the filter wasn't blocking the right wavelengths.
The culprit? Angle shift.
This phenomenon affects all interference-based optical filters. Understanding it can save you from costly redesigns and performance failures.
This guide explains what angle shift is, how it affects long pass filters, and how to compensate for it.
Angle shift is the change in a filter's spectral characteristics when light hits it at a non-perpendicular angle (angle of incidence > 0°).
For interference-coated filters (including most long pass, short pass, and bandpass filters), increasing the AOI causes the spectral curve to shift toward shorter wavelengths – called a blue shift.
The physics in simple terms:
An interference filter works by creating constructive/destructive interference between light reflections. When light hits at an angle, the effective distance between coating layers decreases. To maintain interference conditions, the wavelength must shorten.
The shift amount depends on three factors:
Angle of incidence (AOI) – larger angle = larger shift
Filter design – some coatings are more angle-sensitive than others
Wavelength – shift is proportional to wavelength
| AOI (degrees) | Approximate Blue Shift | Example: 550nm LPF shifts to… |
|---|---|---|
| 0° | 0 nm (baseline) | 550 nm |
| 10° | 2 – 4 nm | 546 – 548 nm |
| 15° | 5 – 8 nm | 542 – 545 nm |
| 20° | 9 – 14 nm | 536 – 541 nm |
| 30° | 20 – 30 nm | 520 – 530 nm |
| 45° | 40 – 60 nm | 490 – 510 nm |
Key takeaway: At 30° AOI, a 550nm long pass filter behaves more like a 525nm filter. If you need to block light at 530nm, it may fail completely.
Long pass filters from the professional long pass filter supplier have a single sharp cut-on edge. Angle shift moves this edge. The consequences depend on your application.
You selected a 500nm long pass filter to block 488nm excitation light. At 15° AOI, the cut-on shifts to ~493nm. Now the filter partially transmits the 488nm light you wanted to block.
| Parameter | At 0° (design) | At 20° (actual) | Consequence |
|---|---|---|---|
| Cut-on wavelength | 500 nm | ~489 nm | Excitation light leaks through |
| Transmission at 488nm | <0.01% (OD4) | ~5% (OD1.3) | Signal-to-noise ratio collapses |
Conversely, if your desired emission signal is at 520nm, a 30° AOI might shift the cut-on from 500nm to 480nm. This is fine – but the edge slope may degrade, slightly reducing transmission.
In a wide-angle imaging system, the center of the image sees near-0° AOI, while edges see 10-20° AOI. This creates a spatially varying spectral response – the center and edges of your image have different color/blocking characteristics.
A high numerical aperture (NA) objective produces a cone of light. For NA 1.4, the maximum ray angle is approximately ±67°.
| Objective NA | Max AOI | Shift on a 550nm LPF |
|---|---|---|
| 0.25 (low NA) | ~14° | ~5-8 nm |
| 0.75 (medium NA) | ~48° | ~50-70 nm |
| 1.4 (high NA) | ~67° | ~100+ nm |
Result: The same filter can block at the image center but completely fail at the edges.
Solution: Use a filter specifically designed for high NA systems, or place the filter in a collimated space (parallel beam).
A 1064nm laser passes through a long pass filter to block residual 532nm harmonics. If the beam is focused (converging), the AOI varies across the beam profile, causing uneven harmonic rejection.
Solution: Place filters in collimated beam sections, never at or near a focus.
You have four options depending on your system constraints.
If your system has a fixed, known AOI (e.g., 15°), order a filter with its cut-on specified at that angle.
How: Tell your supplier: I need a long pass filter with cut-on at 532nm at 15° AOI.
Bena Optics can adjust the coating design so the filter performs correctly at your operating angle.
Place the filter where light rays are parallel (collimated) rather than converging or diverging.
| Location | Typical AOI | Angle Shift Risk |
|---|---|---|
| Collimated beam | 0° – 3° | Very low |
| Near focus | 0° – 30°+ | High |
| Wide-angle imaging | 0° – 20°+ | Moderate to high |
Some filter technologies are less sensitive to AOI:
| Filter Type | Angle Sensitivity | Best For |
|---|---|---|
| Interference long pass (standard) | High | Collimated systems |
| Absorptive glass filters | None (no shift) | Broadband applications |
| Multi-cavity long pass | Lower than standard | High AOI systems |
Trade-off: Absorptive filters have no angle shift but offer less sharp edges and lower blocking depth.
In software-defined systems, you can:
Calibrate the spectral shift and compensate in post-processing
Use a slightly longer cut-on wavelength so the shifted value still meets requirements
| Your System's Maximum AOI | Recommended Action |
|---|---|
| 0° – 5° | Standard filter works fine |
| 5° – 15° | Standard filter acceptable for most applications |
| 15° – 25° | Consider custom design for critical applications |
| 25° – 45° | Custom filter required – specify AOI to supplier |
| >45° | Use collimated space or absorptive filter |
When ordering long pass filters for non-zero AOI applications, provide these details:
| Parameter | Why It Matters |
|---|---|
| AOI (degrees) | Determines the blue shift magnitude |
| Acceptable cut-on tolerance | How precise the shift can be (±2nm vs ±10nm) |
| Maximum acceptable transmission loss | Higher AOI can reduce peak transmission |
| Blocking depth requirement (OD) | Shift may reduce effective blocking |
| Numerical aperture (NA) of your beam | For converging systems |
At Bena Optics, we design and manufacture long pass filters for specific AOI requirements – not just 0°.
Our AOI capabilities:
Standard filters: Optimized for 0° ± 5°
Custom AOI filters: 5° to 45° (specify your angle)
High NA filters: For microscopy and converging beam systems
What we provide:
Measured spectral curves at your specified AOI
Guaranteed cut-on wavelength at operating angle
Transmission and blocking data at your exact conditions
