Colorimeter

A Colorimeter is an optoelectronic instrument that quantifies the colour of light emitted by an AGL fixture. Unlike a photometer, which measures intensity, a colorimeter focuses on chromaticity — the subjective colour perceived by the human eye, expressed in standard CIE (International Commission on Illumination) coordinates. In airfield lighting, each colour (aviation red, green, yellow, blue, white) has a defined chromaticity boundary within the CIE 1931 (x,y) or 1976 (u',v') colour spaces. The colorimeter verifies that the fixture's output falls within the required tolerance box, ensuring unambiguous visual signals for pilots.

Key colour standards for AGL include:

• Aviation Red: Used for runway end lights, obstruction lights, and stop bars.
• Aviation Green: Used for threshold lights, taxiway centreline lights, and lead‑on lights.
• Aviation Yellow: Used for intermediate holding position lights and clearance bars.
• Aviation Blue: Used for taxiway edge lights.
• Aviation White: Used for runway edge, centreline, and approach lights.

The colorimeter's purpose is to confirm that a fixture's colour is not just close to the right shade of red, green, yellow, blue, or white, but falls inside the precise chromaticity boundary that gives that colour its specific operational meaning to a pilot. Because a pilot's decisions — whether they are looking at a threshold, a runway end, or a taxiway edge — depend on correctly identifying colour at a glance, a fixture whose colour has drifted outside its tolerance box, even if it still looks broadly correct to the eye, can create genuine ambiguity about what the light is signalling. The colorimeter's function is to catch that drift objectively, whether during manufacturing quality control, type certification, or a routine field check prompted by a pilot report of an off-colour light.

Colorimeters are deployed in various stages of the AGL lifecycle to ensure colour compliance:

• Manufacturing Quality Control: Used by AGL fixture manufacturers to verify that each production batch meets the specified chromaticity requirements before shipping.
• Type Testing and Certification: Independent testing laboratories use colorimeters to certify that new fixture models comply with ICAO, FAA, and IEC colour standards.
• Airport Commissioning: During new AGL system installation, colorimeters are used to confirm that every fixture emits the correct colour when powered at the required current steps.
• Periodic Maintenance Inspection: Airports perform regular colour checks (e.g., annually) to detect colour shifts due to ageing of filters, LEDs, or optical components.
• Troubleshooting: When pilots report colour confusion (e.g., a green light appearing yellowish), a colorimeter is used to pinpoint the cause — often a degraded filter or incorrect LED bin.
• Calibration of Field Test Equipment: Simpler colour‑checking devices (e.g., colour filters) are calibrated against a reference colorimeter.

The colorimeter is stored in its padded carry case between uses, with the sensor head or optical port protected by a lens cap to keep dust and fingerprints off the measurement surface. It is kept in a stable, moderate-temperature environment when not in use and allowed to acclimatise before taking readings if moved between significantly different temperatures, since thermal drift can affect sensor accuracy. Field crews carry it out to the fixture being tested rather than removing the fixture, and the unit is periodically checked against a reference calibration standard to confirm its readings remain within tolerance, with the calibration due date tracked alongside the instrument's storage record.

A colorimeter typically uses one of the following methods to measure colour:

• Tristimulus Filter Method: The instrument uses three or more photodetectors, each fitted with a filter that mimics the CIE colour‑matching functions (x̄, ȳ, z̄). By measuring the response through each filter, the instrument calculates the tristimulus values (X, Y, Z) and derives the chromaticity coordinates (x, y). This is the most common approach for field instruments due to its simplicity and speed.
• Spectroradiometric Method: A more advanced technique where the instrument measures the full spectral power distribution (SPD) of the light source using a diffraction grating and a linear array detector. From the SPD, the CIE coordinates are computed with high accuracy. This method is used in reference laboratories and for type testing.

For AGL measurements, the colorimeter is positioned at a specific distance and angle relative to the fixture (often the same geometry as intensity measurements). The fixture is operated at the required current step, and the colorimeter records the chromaticity coordinates. The results are compared against the relevant standard’s colour boundary table to determine pass/fail.

The colorimeter operates as a standalone measurement instrument rather than a controlled system component, but it sits within a broader photometric verification architecture. It is typically synchronised with the fixture's power supply (set to the specific current step being tested) and, in a test-bench or production line setup, its digital output feeds into the same data acquisition and pass/fail reporting software used for intensity measurements, so a single test run can log both luminous intensity and chromaticity coordinates against the fixture's serial number. There is no feedback loop in the electrical sense — the colorimeter does not adjust the fixture it is measuring — but the pass/fail verdict it produces is what gates whether that fixture proceeds to shipment, installation, or return for rework, making it a control point in the quality process even though it exerts no control over the electrical circuit itself.

The colorimeter is built around a precision measurement head containing either filtered tristimulus photodetectors or a miniature spectrograph, mounted behind an optical aperture that defines the instrument's field of view and fixes its viewing distance and angle relative to the fixture's optical axis. The head and body are housed in a rigid polymer or metal shell that maintains consistent internal baffling to exclude stray light, with the aperture ring precisely machined to hold that fixed, repeatable measurement geometry against the light source under test. Data interface ports (USB, Bluetooth, or RS-232) and the power/battery compartment are positioned away from the optical path to avoid contaminating readings, and the housing carries an IP54-rated seal around seams and connector covers for field use in outdoor AGL environments. A protective lens cap or calibration tile holder is typically supplied to shield the optics and provide an on-site reference check between formal calibrations.