Choosing an optical filter begins with the optical system, not with the filter itself.
A wavelength value may provide the starting point, but it does not define what the finished component needs to achieve. The useful signal has to be identified, unwanted wavelengths need to be understood, and the filter must operate alongside the source, detector, optics and environment of the complete system.
This is why two applications working at the same nominal wavelength can require very different filters. One may need a broad transmission region with moderate blocking, while another requires a narrow passband, steep spectral edges and high rejection immediately outside it. The angle at which light reaches the filter, the sensitivity of the detector and the operating temperature can introduce further differences.
The aim is therefore not to select the filter with the narrowest bandwidth, highest transmission or deepest blocking in isolation. It is to define the spectral and physical performance necessary for the optical system to make the required measurement reliably.
Start with the Optical System
Before specifying wavelengths or filter types, establish what the optical system is intended to detect, measure or image.
In an imaging system, the objective may be to increase contrast between features with different spectral responses. A fluorescence instrument may need to separate a weak emission signal from a much stronger excitation source. Spectroscopic equipment may need to isolate a wavelength region associated with a particular absorption or emission feature. An infrared system may need to restrict the radiation reaching a detector to a defined spectral band.
These applications require different forms of spectral control, even though the underlying principle is the same.
The source also needs to be considered. Its spectral output determines what radiation is available to the system in the first place, while the optical properties of the target or sample determine how that radiation is modified before reaching the detector.
The detector completes this relationship. Detectors respond over defined spectral regions, and that response is rarely uniform with wavelength. Radiation outside the useful measurement band can still produce a detector signal if it falls within the detector’s sensitivity range.
The filter therefore sits within a chain consisting of the source, subject or sample, optical path and detector. Its specification should be developed around that complete chain.
Identify the Wavelengths Carrying the Useful Signal
Once the measurement objective is understood, the next stage is to establish which wavelengths contain the information the system needs.
In some applications this is a single, well-defined spectral feature. In others, the useful information extends across a broader wavelength range or several separated regions of the spectrum.
The distinction affects the type of filter required and the width of its transmission region.
A filter should transmit enough of the useful spectrum to preserve the required signal. Making the passband unnecessarily narrow can remove optical information that would otherwise contribute to the measurement, while making it too broad may allow unwanted wavelengths to reach the detector.
It is therefore important to distinguish between the nominal wavelength associated with an application and the actual spectral region the system needs to collect.
Source spectrum, target response and detector sensitivity should be considered together when establishing that region.
Determine What Needs to Be Blocked
Defining the passband alone does not provide a complete filter specification.
Unwanted radiation reaching the detector can contribute background signal, reduce contrast or interfere directly with the measurement. The wavelengths that need to be rejected must therefore be considered alongside those that need to be transmitted.
This requires looking beyond the immediate region surrounding the useful signal.
A detector may remain sensitive over a much wider spectral range than the required measurement band. Even relatively low levels of unwanted radiation elsewhere in that range can become significant where the source is strong or the required signal is weak.
Blocking requirements should consequently be established from the spectral environment of the system rather than assigned as an arbitrary optical-density value.
The wavelength range over which blocking is required is particularly important. Specifying OD4, for example, has limited meaning unless the spectral region over which that level of rejection must be maintained is also defined.
The objective is to suppress unwanted optical energy sufficiently for it no longer to compromise the measurement.
Choose the Appropriate Filter Function
Once the transmitting and blocking regions have been established, the required spectral function becomes clearer.
A bandpass filter is appropriate where a defined wavelength region needs to be isolated between blocking regions. Longpass and shortpass filters separate portions of the spectrum around a spectral edge, while notch filters reject a defined region within an otherwise transmitting range. More complex systems may require multiple passbands or filters that divide spectral information between different optical paths.
The filter configuration should follow the spectral requirement rather than being selected first and then made to fit the application.
There can also be more than one way of achieving a particular system objective. The appropriate solution depends on the spectral separation required, available optical layout and the performance expected from the complete system.
The principal configurations and their spectral behaviour are covered separately in Types of Optical Filters.
Define Centre Wavelength and Bandwidth
For a bandpass filter, centre wavelength and bandwidth establish the position and width of the transmitting region.
The required centre wavelength should correspond to the useful spectral information identified earlier in the specification process. The tolerance applied to that wavelength must then reflect how much spectral movement the system can accommodate without affecting performance.
Bandwidth requires similar consideration.
A narrow passband provides greater spectral selectivity, but narrower is not automatically better. If useful signal exists across a wider region, reducing the bandwidth unnecessarily can decrease the optical power reaching the detector without providing a corresponding improvement in measurement quality.
Narrow filters are also more sensitive to spectral shifts caused by factors such as angle of incidence and manufacturing tolerance.
The appropriate bandwidth is therefore the one that separates the required signal from unwanted spectral information while retaining sufficient useful optical power.
Centre wavelength, FWHM and the other parameters used to define spectral response are explained in more detail in Optical Filter Performance Characteristics.
Consider Transmission and Blocking Together
High transmission within the passband is generally desirable because it allows more of the useful optical signal to reach the detector. It should not, however, be considered independently of blocking performance.
A filter with excellent peak transmission may still be unsuitable if unwanted wavelengths are insufficiently suppressed. Conversely, demanding exceptionally deep blocking across an unnecessarily large spectral range may make the filter considerably more complex without improving system performance.
The balance depends on the relative strength of the useful and unwanted signals.
Where the required signal is weak and the unwanted source is intense, blocking can become particularly demanding. In other applications, maximising transmission may have a greater influence on the available signal-to-noise ratio.
Peak transmission alone may also provide an incomplete picture. Depending on the application, minimum or average transmission across the required passband can be more relevant than the highest transmission achieved at a single wavelength.
Transmission and blocking should therefore be specified according to their effect on the measurement rather than treated as independent figures to be maximised.
Account for Angle of Incidence and Beam Geometry
The angle at which light reaches an interference filter can alter its spectral response.
A filter characterised at normal incidence will not necessarily exhibit the same centre wavelength or spectral-edge position when used at an oblique angle. Increasing angle of incidence generally shifts spectral features towards shorter wavelengths.
The mounting angle therefore needs to be known when the filter is specified.
Beam geometry can be equally important.
In a collimated beam, rays reach the filter over a relatively restricted range of angles. In a converging or diverging beam, different rays encounter the filter at different angles of incidence. Each can experience a slightly different spectral response, potentially broadening or changing the effective response seen by the system.
This becomes increasingly important as the filter bandwidth narrows or spectral tolerances become tighter.
The relevant question is consequently not simply whether the filter is mounted at zero or a specified number of degrees, but what angular distribution of light it will encounter in its actual position within the optical system.
Consider Polarisation
Polarisation may have little practical influence in some filter applications but become important in others.
At oblique incidence, s- and p-polarised light can exhibit different spectral responses in an interference filter. As angle increases, differences in transmission and spectral position between the two polarisation states can become more pronounced.
If the incoming light is polarised, or the optical system itself is sensitive to polarisation, this behaviour needs to be included in the specification.
For systems using unpolarised light at relatively small angles of incidence, a separate polarisation requirement may not be necessary. The important point is to establish whether polarisation can affect the measurement rather than assuming that it is either always significant or always irrelevant.
Select a Suitable Substrate Material
The substrate must be capable of transmitting the wavelength range required by the filter.
This sounds straightforward within the visible spectrum but becomes increasingly important as systems extend into the ultraviolet and infrared. Materials have different spectral transmission regions, and a substrate that performs well at one wavelength may become strongly absorbing at another.
Transmission is only part of the decision.
Refractive index, thermal expansion, hardness, mechanical strength and environmental durability can all affect the suitability of a substrate. High-index infrared materials can introduce substantial surface reflection, while systems exposed to changing temperatures may place greater emphasis on thermal behaviour.
Thickness also needs to be considered where the substrate exhibits measurable absorption within the operating region.
The appropriate material is therefore determined by the wavelength range and the physical requirements of the system rather than by a general preference for a particular optical substrate.
The properties of optical glass, fused silica, sapphire, silicon, germanium and zinc selenide are discussed further in Optical Filter Materials.
Define the Physical Requirements
Spectral performance describes what the filter must do to the light, but the component also has to fit and function within the optical assembly.
The required diameter or external dimensions, thickness and clear aperture should therefore be established alongside the spectral specification.
The clear aperture is particularly important because it defines the region over which the specified optical performance is required. Mechanical mounting around the edge of the component may reduce the usable optical area, and this should be accounted for when determining the filter size.
Surface quality, flatness and transmitted wavefront requirements depend on the position and function of the filter within the system.
A filter used in a precision imaging or collimated optical path may require tighter control of these characteristics than a component positioned close to a detector where small surface or wavefront errors have little influence on the final measurement.
As with spectral tolerances, these requirements should reflect the optical system rather than defaulting to the tightest specification available.
Consider the Operating Environment
Laboratory performance does not necessarily describe the conditions a filter will encounter throughout its working life.
Temperature is one of the most obvious considerations. Changes in temperature can affect refractive index and physical dimensions in both substrate and coating materials, potentially altering spectral performance.
Other systems may expose the component to humidity, vibration, contamination, repeated cleaning or abrasive environments.
The significance of these conditions depends entirely on the application. A filter enclosed within controlled instrumentation has different requirements from an externally exposed component used in an industrial or aerospace system.
Environmental conditions should therefore be established early enough to influence material, coating and mechanical decisions rather than being considered only after the optical design has been completed.
Avoiding an Over-Specified Optical Filter
One of the most important parts of specifying an optical filter is deciding which tolerances genuinely affect system performance.
It can be tempting to request the narrowest possible bandwidth, steepest spectral transitions, highest transmission, deepest blocking and tightest wavelength tolerances simultaneously. Each may appear desirable when considered independently.
Together, however, they can create a specification considerably more demanding than the application requires.
For example, very deep blocking across a wide spectral region may provide no practical benefit if the detector has negligible sensitivity across much of that range. Similarly, an extremely narrow passband may reduce useful signal unnecessarily where the measurement does not require that degree of spectral discrimination.
Tighter specifications can increase coating complexity and place greater demands on manufacturing control. In some cases, requirements can also conflict: improving one characteristic may constrain what can realistically be achieved elsewhere in the design.
The aim should therefore be to establish the performance necessary for the optical system rather than pursuing maximum values for every individual parameter.
A well-specified filter is not necessarily the one with the most demanding tolerances. It is the one whose tolerances correspond to the requirements of the measurement.
Building a Complete Optical Filter Specification
By the time a filter specification is ready to move into detailed design, the principal optical and physical requirements should be understood.
The useful wavelength region establishes what must be transmitted, while the source, detector and surrounding spectral environment determine what needs to be blocked. Bandwidth and spectral tolerances define the required selectivity, and transmission establishes how much useful signal must reach the detector.
Angle of incidence, beam geometry and polarisation describe how light will interact with the component within the actual optical system. Substrate material, dimensions, clear aperture and surface requirements define the physical component, while temperature and environmental conditions establish the circumstances under which it must continue to perform.
Not every application requires every parameter to be tightly specified. In many cases, identifying which characteristics can remain flexible is just as useful as defining those that are critical.
Bringing this information together allows the filter to be designed around the measurement the optical system needs to make rather than around a collection of isolated optical specifications.
How to Choose an Optical Filter – FAQs
What information is needed to specify an optical filter?
The starting information normally includes the wavelengths that need to be transmitted and blocked, the required transmission and blocking levels, angle of incidence and physical dimensions. Depending on the application, bandwidth, polarisation, substrate material, surface quality, operating temperature and environmental conditions may also need to be defined.
How do I know what bandwidth an optical filter should have?
The required bandwidth depends on the spectral width of the useful signal and how closely it lies to unwanted wavelengths. The passband should be wide enough to retain the information required by the system while narrow enough to provide the necessary spectral separation.
Should I choose the highest optical density available?
Not necessarily. Optical density should be sufficient to reduce unwanted radiation to a level that no longer compromises the measurement. Specifying substantially deeper blocking than the detector, source and application require can increase filter complexity without improving system performance.
Does the detector affect which optical filter I need?
Yes. Detector sensitivity is an important part of filter selection because a detector may respond to wavelengths outside the required measurement band. Blocking requirements should therefore consider the detector’s broader spectral response as well as the wavelength carrying the useful signal.
When does angle of incidence need to be specified?
Angle of incidence should be defined whenever it can influence the spectral response of the filter, particularly with interference filters. Beam geometry should also be considered because converging and diverging beams contain a range of incidence angles rather than a single value.
Can an optical filter be made to a custom specification?
Yes. Where a standard filter does not provide the required spectral or physical performance, a filter can be designed around application-specific requirements. The achievable specification depends on the combination of wavelength, bandwidth, transmission, blocking, angle of incidence, materials, dimensions and other operating requirements.