AGL PHOTOMETRY COMPLETE NOTES

WHAT IS AGL PHOTOMETRY?

Photometry is the measurement of light weighted by human visual response (radiometry measures optical radiation physically). In AGL it answers: how bright, in which direction, does the beam cover the required area, is the colour right, is it aimed correctly, and does the installed fixture still meet its specified performance?

FAA AC 150/5340-26C says regular photometric measurement is the practical way to check output and find misalignment; AC 150/5345-46F holds the fixture photometric requirements. Visual inspection finds dark fixtures, broken lenses, contamination and obvious problems; photometry gives objective measurement.

THEORY: INTENSITY, ANGLES AND THE INVERSE SQUARE

I = Φ / Ω (cd = lm / sr). For a point source, E = I / d² and I = E × d². AGL fixtures are highly directional, so follow the applicable test geometry rather than applying the inverse-square law blindly. If the measurement distance is wrong, the computed intensity is wrong.

Pilots need light in specific directions, so candela and angular distribution matter more than total lumens. A lux meter alone is not an AGL photometric system. "Candlepower" is an older term for intensity; use candela.

Wattage ≠ photometric performance: two 200 W fixtures can differ in intensity, beam, colour and distribution. A 30 W LED is not automatically the optical equal of another 30 W fixture. Higher lumens is not automatically better.

ISOCANDELA DIAGRAMS AND DATA

H-angle  V-angle    cd
 -5°       3°      420   (illustrative only)
 -2°       3°      510
  0°       3°      550
 +2°       3°      505
 +5°       3°      415

Every fixture type has an expected photometric signature (intensity, angle, colour, spread, direction). Field work checks that the installed fixture still resembles it. There is no universal AGL curve: runway centerline, edge, taxiway edge or centerline, stop bar, threshold and PAPI each have their own requirement, and the report must state the fixture type (L-850A/B/C/D, L-852 variants, L-861, L-862, etc., per AC 150/5345-46F).

COLOUR AND PAPI TRANSITION

Photometry also asks "what colour?". Chromaticity (CIE x, y) describes colour independent of brightness; correlated colour temperature is a different idea, and "3000 K" does not establish aviation colour compliance. Meaning in AGL comes from colour + direction + location + function. LED ageing can shift colour, so a bright fixture can fall outside its colour region. Colour is measured per the applicable method, for example filtered detectors or chromaticity coordinates; a spectroradiometer gives more but the standard's method governs. Coloured light also introduces detector spectral-mismatch error, so use the instrument's correction factors or a suitably matched detector.

PAPI is special: its critical feature is the red/white transition (AC 150/5345-28H sets transition-sharpness limits; take the exact angular limits and test geometry from the edition in force). PAPI photometry covers intensity, colour, transition, beam centre, aiming angle, horizontal coverage and per-unit results; aiming defines where the beam points, photometry shows what it actually looks like. See Note 11.

ALIGNMENT, TOE-IN AND BEAM ORIENTATION

Photometry detects mechanical misalignment: a fixture can be bright yet aimed wrong, giving too little intensity at the pilot's viewing angle. Intensity without correct aiming is not sufficient. Some directional fixtures have intentional toe-in; a fixture can have correct optics and incorrect toe-in. Beam orientation depends on application (runway edge, centerline, taxiway, stop bar, threshold, approach, PAPI), each with its own viewing geometry, and measurement angles are referenced to the specified coordinate system, so wrong fixture orientation means wrong interpretation.

Evaluate centre, edges and required angles, not centre intensity alone. Main-beam average (where the standard defines it) represents the beam better than a single point.

MEASUREMENT: INSTRUMENTS, CALIBRATION, QUALITY

Accuracy is closeness to the reference; precision is repeatability: 520, 523, 519 cd is good; 520, 690, 410 cd means investigate the process. Reproducibility covers different operators, instruments and sessions. Control ambient light (sun, aircraft, vehicles, apron lights, floodlights) and record weather and fixture temperature, since LED and lamp output can depend on temperature. Periodically repeat a known reference measurement; investigate calibration, sensor, software, geometry or ambient light if it drifts.

FIELD SURVEY AND DATA

Before field work review the approved layout and fixture schedule, circuit numbers, weather, access and calibration, and coordinate with operations and ATC. Verify distance, sensor height, reference direction, horizontal and vertical angle, fixture orientation and vehicle position; sensor misalignment causes wrong intensity, shifted beam and false maintenance indications. Give every fixture a unique ID matching the asset register (e.g. RWY-27-E-001); a database holds ID, circuit, type, GPS or chainage, intensity, colour, beam result, status and history.

Validate data: if neighbours read 400–500 cd and one reads 5,000, check angle, fixture ID, sensor, data entry or fixture type before accepting. Outlier detection flags very low or high values, beam shift, odd colour, missing fixtures and wrong sequence. Software can class results as pass, monitor or service, but limits must come from the governing standard. Always compare at the correct CCR brightness step and current, and keep raw data, not just a pass/fail table.

Fixture type -> Applicable standard -> Approved photometric table
  -> Operating step -> Measurement -> Comparison -> PASS / FAIL

ELECTRICAL AND OPTICAL DIAGNOSIS

Low output is not automatically a fixture fault. CCR current error, transformer problems, high-resistance connectors, cable voltage drop and driver faults all show up photometrically, so check the electrical chain before replacing a healthy fixture: CCR current → transformer → connector → secondary cable → fixture → lens/optics → alignment → re-measure. See Notes 07, 08 and 10.

Optical components (lens, reflector, filter, prism, light pipe, LED optic, diffuser) each shape the beam; damage, scratches, ageing or movement of any one changes distribution, colour or intensity.

LED AND INCANDESCENT

LED output can degrade through thermal stress, driver ageing, optical degradation and contamination, so periodic photometry reveals loss before failure. LED fixtures may respond to CCR step current differently from incandescent ones, so measure at each step the specification requires. Pulsed (PWM) drive can also upset fast-sampling photometers, so confirm the instrument's integration time suits the fixture. When converting to LED, do not assume lower power means the same optics: verify intensity, beam, colour, distribution, compatibility and electrical behaviour.

MAINTENANCE PHOTOMETRY

Before and after: measure, clean or re-lamp or replace, then measure again (AC 150/5340-26C recommends this). Example: 260 cd before, 440 cd after proves contamination was a major cause. Rubber builds up on centerline and touchdown fixtures and can need frequent attention, especially on CAT II/III runways; dirt, mud, oil, salt, insects and de-icing residue also cut output.

Balance matters too: fixtures of 100, 100 and 45 cd leave an uneven pattern even if the weak one clears a minimum. Maintenance should consider the installed design value, not only a generic minimum, where the governing criterion says so. Compare against adjacent fixtures, the same type, circuit, step and zone.

Illustrative trend values only. Here the reference is the fixture's own baseline, used for trending. The FAA 70% maintenance target is judged against the minimum required output in the applicable AC table, so check the actual requirement for the fixture type before deciding.

Water in shallow light bases, loose bolts (check torque with a calibrated wrench) and moved fixtures cause electrical and alignment faults; after water damage recheck insulation, fixture, optics and output. Trending gives condition-based and predictive maintenance: measure, assess, and maintain only where needed. Low-visibility and CAT II/III operations depend on light output; ICAO recommends in-field measurement of intensity, beam spread and orientation for CAT II/III precision-approach lighting, with mobile units where appropriate.

PROGRAM, COMMISSIONING AND PRODUCT DATA

Use the manufacturer's approved photometric distribution or the standard's required distribution for the correct fixture type; never compare a centerline fixture with a taxiway fixture's curve. Signs follow the sign specification, and pavement markings are judged by colour, geometry, contrast and condition, not luminous intensity (Notes 12 and 13). Weather affects both measurement and real pilot visibility; a measurement shows what the fixture produces under test conditions.

REPORTING AND DEFICIENCIES

A report should cover: project, airport, runway or taxiway, date and weather; instrument, model, serial and calibration; fixture type, circuit and CCR step; method, test distance and angular grid; results (intensity, colour, beam, aiming); analysis, pass/fail and deficiencies; corrective action and retest; final status. No result should be untraceable to a field fixture: report → fixture ID → location → circuit → measurement file → instrument → calibration certificate.

Fixture ID / Location / Circuit / Fixture Type
Measured / Required (design) / Deviation
Likely Cause / Recommended Action / Retest / Status

Formulas: Output % = Measured / Reference × 100 (440/500 = 88%); Degradation % = (Reference − Measured) / Reference × 100 (500 → 400 = 20%); Deviation % = (Measured − Reference) / Reference × 100. These are comparison metrics, not pass/fail criteria. A uniformity ratio (min/max) must not replace the standard's uniformity criterion. Retest after correction: FAIL → action → measure → compare → PASS or FAIL. A photometric NCR records requirement, measured value, result, action, retest and final result, which beats "light repaired".

Never accept "this fixture gives 500 cd". Ask: 500 cd at which angle, in which direction, at which CCR step, for which fixture type, under which specification?

TEAM, AIRSIDE SAFETY AND CHECKLISTS

A survey team may include a photometry engineer, AGL engineer, electrical technician, driver or operator, operations coordinator and QA/QC inspector. A survey is done in a live airport: obtain access, coordinate with ATC and operations, use an approved vehicle, keep radio contact, follow escort, speed and lighting restrictions, control FOD (loose tools, covers, cables, fasteners, packaging), and use the approved night airside procedure. Do not assume a test can go ahead because the equipment is ready. Cleaning, adjusting, re-lamping or replacing a fixture means working on a series circuit: isolate and lock off the circuit, prove it dead and follow the approved procedure; never open a fixture or disconnect an L-823 connector on an energised loop.

QUICK REVISION

PHOTOMETRY = INTENSITY + DIRECTION + BEAM + COLOUR + ALIGNMENT
             + CALIBRATION + MEASUREMENT + ANALYSIS
I = Φ/Ω   E = I/d²   Output % = Measured / Reference × 100

EXAM / INTERVIEW QUESTIONS

FINAL STUDY TAKEAWAY

AGL photometry is the optical verification layer of an airport lighting system: measuring the right amount of light, in the right direction, with the right colour, from the right fixture, under the right operating condition, and proving it with calibrated data.

ENGINEERING & EXPERT NOTES

REFERENCES & FURTHER READING

These notes are a study aid. Verify details against the primary sources below.

EXPLORE ALL AGL STUDY NOTES