Spectrometer

A Spectrometer (specifically a spectroradiometer) is an advanced optoelectronic instrument that measures the spectral power distribution (SPD) of a light source — the amount of optical power emitted at each wavelength across the visible spectrum (typically 380 nm to 780 nm). Unlike a colorimeter, which uses filters to mimic the human eye's response, a spectrometer captures the full spectral fingerprint of the light. This enables:

• Precise chromaticity calculation — deriving CIE (x,y) or (u',v') coordinates with high accuracy
• Colour rendering index (CRI) — assessing how accurately colours appear under the light
• Correlated colour temperature (CCT) — quantifying the warmth or coolness of white light
• Spectral analysis — identifying unexpected spectral spikes or dips caused by LED binning, filter degradation, or optical contamination
• Metamerism detection — understanding when two lights appear to match in colour but have different SPDs, which can cause visual discrepancies under different lighting conditions

In AGL applications, spectrometers are primarily used in laboratory settings for type testing, research, and advanced troubleshooting, though portable field spectrometers are increasingly available for on‑site diagnostic work.

The spectrometer's purpose is to see past the single colour coordinate a colorimeter reports and reveal the full spectral "fingerprint" behind it, which matters because two light sources can register the same nominal colour while having very different underlying spectra — a distinction that becomes critical when diagnosing a colour that is marginally out of tolerance or verifying that the LEDs actually installed in a fixture are from the correct manufacturing bin. Its detailed spectral data is what allows engineers to trace a colour shift back to its physical cause, such as a filter that has faded in a specific wavelength band, rather than simply confirming that a problem exists. Because of this diagnostic depth, the spectrometer also serves as the primary reference used to calibrate simpler field-grade colorimeters and photometers, keeping the whole measurement chain traceable back to a single spectral truth.

Spectrometers are deployed in specialised scenarios where detailed spectral data is required:

• Type Testing and Certification: Independent testing laboratories use spectrometers to verify that new AGL fixture models meet the full chromaticity and colour quality requirements specified in ICAO Annex 14 and FAA AC 150/5345‑46.
• LED Bin Verification: Manufacturers use spectrometers to ensure that the LED chips used in AGL fixtures are from the correct chromaticity bin, as required by the design specification.
• Research and Development: When developing new AGL lighting technologies (e.g., novel LED phosphor blends), spectrometers provide the detailed spectral feedback needed for optimisation.
• Advanced Troubleshooting: When colourimeter readings show marginal compliance or indicate a colour shift, a spectrometer can reveal the underlying spectral cause — for example, a degraded filter that attenuates a specific wavelength range.
• Calibration of Colorimeters and Photometers: Reference spectrometers are used as primary standards to calibrate field‑grade photometers and colorimeters, ensuring traceability to national metrology institutes.
• Quality Assurance in Manufacturing: Some high‑end AGL manufacturers use spectrometers on production lines to perform 100% spectral inspection of critical components like optical filters and LED modules.

The spectrometer has no fixed field location — it is a bench or portable instrument kept in a calibration or photometric laboratory and brought to the measurement point as needed. Laboratory measurements are made on an optical bench in a darkened room, with the fixture or sample under test mounted on a fixed stand and the instrument's fibre-optic probe or cosine-corrected input positioned at the specified distance and angle. For field or production-line use, a portable unit with a handheld probe is positioned directly against or in front of the light source being sampled. Like other photometric reference instruments, the spectrometer is periodically sent to an accredited calibration laboratory to maintain traceability, and is itself sometimes used in that role to calibrate field-grade photometers and colorimeters.

A spectrometer typically consists of the following key components:

• Entrance Optics: A lens or fibre optic assembly that collects the light from the fixture under test and couples it into the instrument.
• Wavelength Dispersion Element: A diffraction grating or prism that separates the incoming light into its constituent wavelengths (similar to a rainbow).
• Linear Array Detector: A silicon photodiode array (e.g., CCD or CMOS) that captures the intensity of light at each wavelength simultaneously. The number of pixels determines the spectral resolution.
• Signal Processing and ADC: The detector signals are amplified, digitised, and converted into spectral data (intensity vs. wavelength).
• Microprocessor and Software: The instrument calibrates the raw data using a traceable reference source (e.g., a standard lamp with known SPD). It then calculates photometric and colorimetric quantities (luminous flux, CIE coordinates, CCT, CRI, etc.) from the SPD.

For AGL measurements, the spectrometer is typically placed at a defined distance and angle relative to the fixture (matching the geometry used for intensity measurements). The fixture is operated at the required current step, and the spectrometer captures the SPD. The software then compares the chromaticity coordinates against the relevant standard's colour boundary table to determine pass/fail. The full SPD also allows detection of spectral artefacts (e.g., spikes due to reflected ambient light or artefacts from the fixture's optics).

Like the photometer, a spectrometer used for airfield lighting or paint verification is a standalone measurement instrument rather than a networked control system, but its output flows through a defined signal chain. Light from the sample or fixture enters through the input optics, is dispersed by a grating or prism across the detector array, and the resulting per-wavelength intensity signal is digitized and processed against the instrument's stored wavelength and intensity calibration, traceable to certified reference sources, to produce a spectral output such as chromaticity coordinates or dominant wavelength. The result is compared against the applicable specification to generate a pass/fail determination, and as with any calibrated optical instrument, periodic recalibration against a certified reference source is essential since a drifted instrument corrupts every downstream pass/fail decision.

The instrument is built around a fixed or holographic diffraction grating that disperses incoming light onto a linear CCD or CMOS detector array, all mounted on a rigid optical bench inside a light-tight enclosure to prevent stray light and mechanical drift from affecting wavelength calibration. Light enters through a cosine-corrected diffuser or integrating sphere port, and where the instrument is used for luminaire testing rather than ambient light measurement, a fibre-optic input probe is used instead so the detector head can be kept away from the heat and electrical noise of the light source under test. Laboratory-grade units are housed in a benchtop enclosure suited to a temperature-controlled environment, while field-hardened versions add a sealed IP54 housing, shock-mounted optics, and a rechargeable battery pack so the instrument can be carried to a fixture under test and operated without a bench setup. Data and power connections use standard USB, Ethernet, or Wi-Fi interfaces rather than industrial connectors, reflecting the instrument's role as a portable or laboratory measurement tool rather than a permanently installed field device.