Optical Filter Orientation

Authors: Daniel Obeid

What Direction Should My Filter Face?
Impact of Angle of Incidence
Impact of Cone Half Angle
General Guidelines for AOI and CHA’s Impact on Filters
What You Can Do to Mitigate These Effects

Optical filter performance can be maximized by proper filter orientation and understanding the impact that angle of incidence (AOI) and cone half angle (CHA) have on spectral performance. These specifications, if unaccounted for, can severely degrade the performance of an optical system, so it is beneficial to take these variables into consideration when establishing an optical system incorporating optical filters.

AOI refers to the angle between a collimated beam incident on the optic and the surface normal of the filter’s first surface, while CHA is the angle between the direction of the light incident on the filter and the outermost ray of the light bundle (details below and in Figure 2 and Figure 3).

What Direction Should My Filter Face?

One of the most common questions regarding optical filters, such as bandpass and dichroic filters used in fluorescence microscopy, is which side should face the input light source. The orientation of the filter directly affects transmission, so the filter should be oriented to maximize throughput and performance.

Some filters have a filter coating on one side and an antireflection coating on the other. In those cases, the side with the filter coating should face the incident light source (i.e., pointing in the opposite direction of the light’s path through the system). However, other filter designs will have a primary filter coating on one surface that should face the incident light source and a secondary filter coating on the other surface.

To simplify optical filter orientation, all TECHSPEC® mounted sputtered thin-film interference filters from Edmund Optics® feature an arrow on the side of the mount pointing to the primary filter surface that should face the incident light source (Figure 1).

Figure 1: Filter oriented such that arrow points to primary filter-coated surface S1, which should point towards the incident light source to optimize optical performance

However, some other filters in our Marketplace feature arrows pointing in the direction that light is traveling, which is the opposite orientation of TECHSPEC®. In all cases, the correct orientation will be specified on the product’s webpage and should be checked before use.

Similarly, all unmounted sputtered thin-film interference filters have a small pencil or ink mark on the edge outside the clear aperture to indicate the surface that should face the incident light source.

Impact of Angle of Incidence

The angle of incidence (AOI) is the angle between a collimated beam incident on the optic and the surface normal of the filter’s first surface (Figure 2). Changing AOI outside a filter’s specified range will shift the peak transmission towards shorter wavelengths due to the change in optical path length. This occurs because the optical path length within the multilayer coating shortens, causing constructive interference to favor shorter wavelengths. This shift towards shorter wavelengths is known as blue shift.

**Figure 2:** Illustration of the angle of incidence (AOI) of an optical filter

In addition to this shift to shorter wavelengths, increasing AOI directly impacts transmission efficiency, causing a noticeable reduction in overall light throughput. At more extreme angles, not only does the peak position shift, but the entire spectral shape deforms, altering the filter’s ability to perform as specified (Figure 3).

**Figure 3:** Examples of how increasing the AOI of a filter shifts spectral performance to shorter wavelengths (known as blue shift), decreases transmission, and distorts the spectral profile

At more extreme angles, the filter’s spectral profile distorts and sidebands or secondary peaks may appear, significantly altering the filter’s spectral response. Unwanted wavelengths may be incorrectly passed while desired wavelengths are blocked. These distortions can reduce performance in applications requiring precise wavelength control, such as fluorescence microscopy, optical sensing, laser systems, or imaging applications.

Therefore, factoring in the AOI that filters in your system will experience is critical. In some cases, this can mean that custom coating designs are required to compensate for blue shift and provide optimal optical performance. Typical AOI specs are at 0°, with a tolerance of +/- 5°.

For applications sensitive to AOI effects, absorptive filters, such as color glass filters, can be a beneficial option because they are an exception to this connection between AOI and spectral performance. Absorptive filters incorporate a dye to select desired wavelengths rather than thin film coatings. While this may make color glass filters preferable in instances that involve large angles of incidence, their spectral performance lags behind thin-film interference filters.

It is possible to theoretically estimate the amount of shift to shorter wavelengths as AOI increases. The blue-shifted wavelength $(λ_θ$ ) is given by:

(1)

λ_{θ} = λ_{0} \sqrt{1 - (n_{0} n_{i}^{- 1} s i n θ)^{2}}

$λ_θ=λ_0 \sqrt{1-(n_0 n_i^{-1} sinθ)^2 }$

Where:

$λ_0$ = Original wavelength at 0° AOI

$n_0$ = Refractive index outside of the filter (typically 1 for air)

$n_i$ = Effective refractive index of the filter

$θ$ = AOI

Impact of Cone Half Angle

The cone half angle (CHA), sometimes known as half cone angle (HCA), experienced by an optical filter also plays a critical role in its spectral performance, especially when dealing with divergent or non-collimated light sources. CHA is the angle between the direction of the light incident on the filter and the outermost ray of the light bundle. In other words, the CHA is the angular range of the light, with collimated light having a CHA of 0° (Figure 4).

**Figure 4:** Illustration of the cone half angle (CHA) of an optical filter

Similar to the impact of AOI on filter performance, variations in the angular distribution of incoming light can significantly impact its performance. Most optical filters are designed for collimated light, assuming a 0° CHA for optimal performance. However, as CHA increases, transmission characteristics degrade, causing a shift towards shorter wavelengths, reduced overall transmission, and distortions in spectral peak shape—often more severe than the effects of AOI (Figure 5).

**Figure 5:** Examples of how increasing the CHA of a filter shifts spectral performance to a shorter wavelength, decreases transmission, and distorts the spectral profile, just like increasing AOI

Like the effects of increasing AOI, the shifts resulting from increasing CHA can significantly impact the performance of optical systems. To maintain optimal performance, engineers must carefully specify CHA requirements and adjust coating designs accordingly, ensuring the filter functions as intended across varying illumination conditions.

General Guidelines for AOI and CHA’s Impact on Filters

The orientation of filter sets (like excitation, fluorescence, and emission filters in fluorescence applications) must all be coordinated for optimum performance
The larger the deviation from the design AOI and CHA, the greater the blue shift (spectral curve moving to shorter wavelengths)
The larger the deviation from the design AOI and CHA, the lower the transmission will be
Most bandpass filters are designed for AOI 0°, many with an AOI tolerance of ±5°, whereas dichroic filters are typically designed for use at 45°

What You Can Do to Mitigate These Effects

Know your filters’ specified AOI and CHA when integrating them into your system
We can design filter coatings for your application-specific needs
Talk to Edmund Optics engineers to ensure that your filters’ AOI, CHA, and configuration are optimized to work together and you are not over- or under-specified for your application, whether you are purchasing off-the-shelf filters or custom filter designs.

Understanding your system’s AOI and CHA requirements ahead of time is critical for maximizing performance and accounting for real-world variations. Prints and theoretical calculations will often use an implied 0° AOI and CHA while understanding that real systems may often deviate from this will help you choose the right filters.

Please Contact Us for more information and guidance from our engineers on selecting the right filters and other optics for your system.

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