Optical Filter
A transparent or coloured optical element that selectively transmits specific wavelengths of light while blocking others. Used to achieve the precise colours required for threshold (green), end (red), taxiway (blue/green), stop bars (red), and obstruction (red/white) lighting, ensuring compliance with ICAO colour specifications.
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What Is It?
An Optical Filter is a component that modifies the spectral content of light by selectively transmitting certain wavelengths while absorbing or reflecting others. In AGL fixtures, filters are used to produce the required colours — green for threshold and taxiway centreline, red for runway end and stop bars, yellow for clearance bars, and blue for taxiway edge lights. Filters can be absorptive (dyed glass or plastic that absorbs unwanted wavelengths) or dichroic (interference coatings that reflect unwanted wavelengths). Dichroic filters offer higher transmission and better colour stability but are more expensive. The choice depends on the application, cost, and environmental conditions.
Purpose & Function
The optical filter's purpose is to give a fixture the exact colour that carries operational meaning to a pilot — green marking the threshold, red marking the runway end or a stop bar, yellow marking a clearance bar, and so on. These colours are not decorative: a pilot who sees red or green light on a runway is meant to draw a specific conclusion about their position or clearance to proceed, so the filter's job is to hold the transmitted wavelengths within the tight colour-coordinate tolerances that ICAO and FAA specifications require, and to keep doing so despite years of UV exposure and heat that would otherwise fade an absorptive filter or degrade a dichroic coating's thin-film layers. Getting this wrong is not a minor optical defect — a colour that has drifted outside tolerance could be misread by a pilot as a different fixture type entirely.
Where Is It Used?
Optical filters are used in virtually every colour-specific AGL fixture:
- Threshold Lights (Green): Green filters for runway threshold and wing bar lights.
- Runway End Lights (Red): Red filters to indicate the end of the runway.
- Taxiway Centreline Lights (Green): Green filters for centreline guidance.
- Taxiway Edge Lights (Blue): Blue filters for edge delineation.
- Stop Bars (Red): Red filters for runway holding position lights.
- Clearance Bars (Yellow): Yellow filters for taxiway clearance lights.
- Obstruction Lights (Red/White): Red filters for obstruction marking, often combined with clear sections.
- PAPI & VASI: Specialised filters that create the red/white transition via colour mixing.
Installation & Location
The filter is fitted between the light source (or LED module) and the lens or prism, either as a coated layer bonded to another optical element or as a discrete disc held in a retaining groove or clip within the optical assembly. Its position is fixed by the housing's internal geometry, so there is generally no field adjustment; correct installation means confirming the filter is the correct colour and grade for that fixture type and that it's seated fully home in its groove or clip, without gaps that would let unfiltered light leak past the edge. Because colour compliance is safety-critical (a threshold light showing the wrong colour is a serious defect), the filter is always sourced and fitted as a matched replacement part rather than substituted, and the fixture's colour output is verified after any filter is fitted or replaced.
How Does It Work?
An optical filter works by either absorbing or reflecting unwanted wavelengths. Absorptive filters contain dyes or pigments that absorb specific colours, transmitting only the desired wavelengths. They are simple, robust, and inexpensive, but can fade over time due to UV exposure and heat. Dichroic filters use multiple thin-film interference layers that reflect certain wavelengths and transmit others. They offer high transmission (often >90%) and excellent colour stability, but are more delicate and costly.
In AGL fixtures, the filter is typically placed between the light source and the lens or prism, or integrated into the lens itself. The filter must be designed to withstand the fixture's operating temperature, UV radiation, and mechanical stresses while maintaining precise colour coordinates as defined by ICAO Annex 14 and FAA specifications.
Absorptive vs. Dichroic Filtering & Control Logic
Broad-Spectrum Light From Source
LED or halogen output enters the filter before reaching the lens or prism
Dyed Glass or Plastic
Pigments absorb the unwanted wavelengths directly
Simple & Inexpensive
Can fade over time from UV exposure and heat
Thin-Film Interference Layers
Unwanted wavelengths are reflected rather than absorbed
High Transmission, Stable Colour
Often >90% transmission, but more delicate and costly
Whichever Method Is Used, the Output Must Hold ICAO Annex 14 / FAA Colour Coordinates Over the Fixture's Life
Physical Construction
Absorptive filters are made from glass or plastic dyed throughout its thickness with a pigment tuned to absorb unwanted wavelengths, so colour performance depends on the material's bulk optical density rather than a surface treatment. Dichroic filters instead start as clear optical-grade glass onto which multiple thin-film interference layers are vacuum-deposited, each layer tuned to reflect specific wavelength bands while passing the target colour with minimal loss; because the effect depends on precise, uniform layer thickness, dichroic filters require tighter manufacturing tolerances than dyed equivalents. Whichever type is used, the filter is cut to the exact aperture of the fixture's optical path and mounted in a dedicated slot or retaining ring — either bonded in place or held by a light clip — positioned between the light source and the prism or lens so it intercepts the full beam without introducing stray reflections at its edges.
Technical Parameters
| Parameter | Typical Value |
|---|---|
| Material | Optical glass (with dyes) or polycarbonate (with dyes); dichroic coated glass |
| Transmission (Peak) | >80% for green/red/yellow, >70% for blue |
| Colour Coordinates (CIE 1931) | Defined by boundary equations, not a simple x/y box — e.g., per FAA Engineering Brief #67 (ICAO-modified), LED aviation green is bounded by y = 0.768 − 1.306x (blue boundary), y = 0.600 (white boundary), y = 3.470 − 9.200x (yellow boundary) |
| Temperature Range | -40°C to +100°C (glass), -40°C to +80°C (polycarbonate) |
| UV Resistance | UV-stabilised (polycarbonate) or inherent (glass) |
| Coating | Anti-reflective (optional), hard coating (for plastic) |
| Thickness | 1mm to 3mm (typical) |
| Environmental Protection | Sealed against moisture and dust (IP67 when integrated) |
Applicable Standards
ICAO Annex 14
Colour specifications for AGL lights (chromaticity coordinates)
FAA AC 150/5345-46
General FAA fixture specification; colour and photometric requirements are tested as part of the qualified fixture, filter included
IEC/TS 61827
Characteristics and test procedures for inset and elevated luminaires used on aerodromes and heliports
SAE AS25050
Chromaticity and transmission requirements for aeronautical lights and light-transmitting materials
Installation Requirements
Colour & Grade Verification
Pre-FitConfirm the filter is the correct colour and grade for the fixture type before fitting, sourcing it only as a matched replacement part rather than a substitute, since colour compliance is a safety-critical property.
Surface & Edge Inspection
Pre-FitInspect the filter for chips, delamination, or coating damage at the edge before fitting, since edge defects can let unfiltered light leak past the aperture.
Full Seating in Groove or Clip
FitSeat the filter fully home in its retaining groove or clip with no gaps around the edge, since the position is fixed by the housing geometry and there is no field adjustment once fitted.
Correct Optical Path Placement
FitConfirm the filter sits between the light source and the lens or prism as designed, intercepting the full beam without introducing stray reflections at its edges.
Colour Output Verification
Post-FitVerify the fixture's colour output after any filter is fitted or replaced, since a wrong-colour indication (for example on a threshold light) is a serious defect that isn't obvious from a visual check alone.
Pre-Energisation Sign-off
DocumentationRecord the filter part number and colour verification result before the fixture is closed up, since correct seating is not inspectable once assembly is complete.
Testing & Commissioning
Colour & Transmission Verification
Pre-InstallationVerify chromaticity coordinates against the applicable ICAO/FAA colour specification using a spectrophotometer before the filter is fitted.
Coating Condition Inspection
Pre-InstallationInspect dichroic coatings or dyed material for scratches, delamination, or discolouration before installation.
Mounting & Seating Check
InstallationConfirm the filter seats correctly in its retainer without point-loading that could induce stress cracking.
Photometric Commissioning Test
Final AcceptanceMeasure the filtered beam's intensity and chromaticity on the assembled fixture against ICAO/FAA commissioning tolerances.
Documentation & As-Built Sign-off
Final AcceptanceRecord chromaticity and photometric test results in the commissioning dossier for handover.
Inspection & Maintenance
Colour & Transmission Check
AnnualVerify colour output and light transmission remain within spec, since UV exposure fades filters gradually.
Thermal Stress Inspection
Semi-annualCheck for cracking caused by repeated thermal cycling, especially near mounting edges.
Cleaning & Contamination Check
QuarterlyClean the filter surface of dust, oil, or moisture film that can distort colour and reduce output.
Coating Condition Check
AnnualInspect dichroic coatings for delamination or peeling, which changes the filter's colour response.
Common Field Issues
Colour Fading (UV Degradation)
Absorptive filters (especially plastic ones) can fade over time due to UV exposure, shifting colour coordinates outside specification. This compromises the visual cue for pilots. Regular photometric testing and replacement of faded filters is essential.
Thermal Stress Cracking
Glass filters can crack due to thermal shock from rapid temperature changes (e.g., halogen lamp heat-up). Use of borosilicate glass or proper thermal management in the fixture design is recommended.
Contamination (Dust, Oil, Moisture)
Filters can become contaminated with dust, oil, or moisture, reducing light transmission and altering colour. Regular cleaning with approved solvents and soft cloths is required.
Dichroic Coating Delamination
Dichroic filters can suffer from delamination of the thin-film coatings due to humidity or thermal cycling. This leads to reduced transmission and colour shift. Use of high-quality coatings and proper sealing is critical.
Troubleshooting Guide
Colour Outside Chromaticity Specification
Cracked Glass Filter
Reduced Light Transmission / Dim Beam
Hazy or Cloudy Appearance on a Dichroic Filter
Failure Modes & Root Causes
Filter Material / Coating Degradation
Thermal-Shock Cracking
Surface Contamination
Dichroic Coating Delamination
Safety & Operational Notes
De-Energise Before Filter Removal
Electrical SafetyIsolate the fixture and allow the lamp or LED module to cool before removing the filter; a live circuit or hot lamp behind the filter is a shock and burn hazard during replacement.
Thermal Shock Avoidance
HandlingDo not apply a cold cleaning solution to a hot filter or install a filter immediately after a halogen lamp change; allow the fixture to reach ambient temperature to avoid thermal-shock cracking.
Coating-Safe Handling
WorkmanshipClean the filter with a soft cloth and manufacturer-approved cleaner rather than an abrasive pad or solvent, and avoid touching a dichroic-coated surface directly, since fingerprints and abrasion accelerate coating delamination.
Seal Integrity on Reinstall
WorkmanshipInspect the filter's housing seal or gasket for compression set or cracking before reinstalling, since a poor seal accelerates humidity-driven coating delamination.
FOD Control
OperationalAccount for the removed filter, gasket, and fasteners at all times during handling, and secure or remove any loose material before returning the fixture to service to prevent foreign object debris on the movement area.
Verify Chromaticity After Replacement
ComplianceAfter a filter replacement, verify the light output still meets the applicable chromaticity limits before closing out the work, since a mismatched or degraded filter can put the fixture out of tolerance.
Restore to Compliant State
ComplianceDo not return a fixture to service with a cracked, hazed, or delaminating filter; confirm the filter is fully seated and the fixture meets its photometric requirement before closing out the work.
Engineering & Expert Notes
Dichroic Coatings Degrade Before the Substrate Does
ComplianceA dichroic-coated filter's spectral performance can drift measurably from coating degradation years before the glass or coating shows any visible defect; verify chromaticity at overhaul rather than relying on a visual check of the coating's appearance.
Fingerprints Are a Long-Term Failure Mode, Not Just a Cosmetic Issue
WorkmanshipOils from bare-hand handling can etch into a dichroic coating over time under heat cycling, leaving a permanent mark that a cleaning afterward cannot remove; handle coated filters by the edges with gloves from the first touch, not just during cleaning.
Coating Adhesion Weakens With Repeated Thermal Cycling Near a Hot Source
HandlingFilters mounted close to a hot halogen or high-drive LED source undergo more thermal cycling than the fixture housing itself, which can accelerate coating delamination compared to a filter's rated lifespan under lower-heat conditions.
Humidity Ingress Attacks the Coating Edge First
DurabilityDichroic coating delamination typically starts at the filter's edge where moisture can wick in under a compromised seal, rather than across the coated face; inspect edge condition specifically when checking a filter for early degradation.
A Colour Complaint May Be the Filter, Not the Source
TroubleshootingA fixture reported as showing an off colour is often assumed to be a lamp or LED module issue first; check filter chromaticity and coating condition before replacing the light source, since a degraded filter produces the same symptom.
Reinstall in the Same Orientation, Even for Symmetric-Looking Filters
WorkmanshipSome optical filters have a coating applied to only one face; installing one reversed can change both the spectral output and the coating's exposure to heat from the source side, even if the filter looks identical from either side.
Frequently Asked Questions
Short answers about AGL optical filters: colour control, filter types, chromaticity, standards, maintenance and faults.
Q1What is an optical filter in an AGL fixture?
An optical filter selectively transmits certain wavelengths and absorbs or reflects others to give the fixture its required colour, such as green for threshold and taxiway centreline, red for runway end and stop bars, and yellow for clearance bars.
Q2Why does the exact colour matter?
These colours carry operational meaning to pilots and are not decorative. A pilot who sees red or green light on the runway is meant to draw a specific conclusion, so the colour must stay inside the specified chromaticity limits.
Q3What are the main types of optical filter?
Absorptive filters use dyes or pigments to absorb unwanted colours. They are simple, robust and inexpensive but can fade with UV and heat. Dichroic filters use multiple thin-film coatings that reflect unwanted wavelengths by interference.
Q4Where are optical filters used?
In virtually every colour-specific AGL fixture, including green threshold and wing bar lights, red runway end lights, green taxiway centreline lights, red stop bars and yellow clearance bars.
Q5What are typical optical filter specifications?
Peak transmission is above 80% for green, red and yellow and above 70% for blue. Materials are optical glass or polycarbonate with dyes, or dichroic coated glass, typically 1-3 mm thick, with an operating range of -40°C to +100°C for glass and -40°C to +80°C for polycarbonate.
Q6How is filter colour specified?
Colour is defined by chromaticity boundary equations in the CIE 1931 diagram rather than a simple x/y box. For example, FAA Engineering Brief #67 (ICAO-modified) bounds LED aviation green with three boundary lines for the blue, white and yellow sides.
Q7Which standards apply to optical filters?
ICAO Annex 14 (chromaticity coordinates), FAA AC 150/5345-46 (colour and photometric requirements tested with the qualified fixture, filter included), IEC/TS 61827, and SAE AS25050 (chromaticity and transmission for aeronautical lights and light-transmitting materials).
Q8How often should optical filters be checked?
Typical intervals are quarterly for cleaning dust, oil or moisture film, semi-annual for thermal-stress cracking near mounting edges, and annual for colour and transmission checks and dichroic coating condition.
Q9Why is a fixture's colour outside the chromaticity specification?
Likely causes are UV degradation fading an absorptive plastic filter, or delamination of a dichroic coating from humidity or thermal cycling. Run a photometric colour check against the ICAO/FAA limits and visually inspect the filter for fading or peeling.
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