LED PHOTOMETRY VS HALOGEN PHOTOMETRY
Same Measurement, Different Source — LED and halogen AGL fixtures are evaluated with the same fundamental photometric quantities (candela, lux, beam distribution), but the way each source generates light changes what a test needs to account for: spectrum, directionality, thermal behaviour, and ageing.
At a Glance
| Feature | LED Photometry | Halogen Photometry |
|---|---|---|
| Light source | Light-emitting diode | Tungsten-halogen filament |
| Basic photometric quantities | cd, lx, beam distribution | cd, lx, beam distribution |
| Measurement principle | Fundamentally the same | Fundamentally the same |
| Source spectrum | Typically relatively narrow/structured | Typically broad and continuous |
| Native emission pattern | More directional | More omnidirectional |
| Optical control | Often achieved with lenses/secondary optics | Often relies strongly on reflectors and optics |
| Warm-up behaviour | Usually rapid | Output changes as filament reaches thermal equilibrium |
| Ageing behaviour | Gradual lumen/intensity depreciation can occur | Filament and lamp ageing affect output |
| Colour behaviour | Depends on LED spectrum and colour system | Depends on filament temperature and filtering |
| Test consideration | Electrical drive and temperature can matter | Supply condition and lamp stabilisation can matter |
The Measurement Itself Is Not Different
For both an LED and a halogen AGL fixture, the same set of photometric quantities is measured:
LED vs halogen photometry is mainly about the behaviour of the light source and fixture — not about changing the definition of candela or lux.
How an LED AGL Fixture Produces Its Beam
Photometric testing may examine:
- Luminous intensity
- Intensity distribution
- Horizontal beam spread
- Vertical beam spread
- Colour characteristics
- Performance at specified electrical operating conditions
Because LEDs can have strongly directional emission and are often combined with optical systems, the resulting fixture beam pattern is strongly influenced by the LED-optics combination.
How a Halogen AGL Fixture Produces Its Beam
The final beam pattern is therefore often strongly dependent on:
- Filament geometry
- Reflector design
- Lens optics
- Lamp position
Directional vs Omnidirectional Emission
LED
Native LED emission tends to be more directional, with optics used to further shape and control the output.
Halogen
A halogen filament emits broadly in nearly all directions, relying on the reflector to redirect light forward.
These describe source behaviour only. The final AGL fixture distribution is determined by the complete optical design.
Two Different Routes to a Controlled Beam
| Fixture Type | Beam Formation Path |
|---|---|
| LED fixture | LED → lens / optics → controlled beam |
| Halogen fixture | Filament → reflector + lens → controlled beam |
An LED is not automatically better simply because it is directional, and a halogen lamp is not automatically unsuitable because its source emission is broad. The required comparison is between the final fixture photometric distributions.
Narrow/Structured vs Broad/Continuous
Spectral distribution depends on LED material/design, phosphor system, and colour-producing method.
Typically a broader continuous spectrum, governed by filament temperature.
This spectral difference can affect colour measurement and the relationship between physical optical radiation and photometric quantities.
Photometry vs Colorimetry
For AGL applications, colour performance must be considered independently from intensity — photometry asks "how much?", colorimetry asks "what colour?"
LED
Colour can depend on:
- LED spectral characteristics
- LED binning/design
- Temperature
- Drive conditions
- Optical components
Halogen
Colour characteristics can depend on:
- Filament operating temperature
- Electrical operating conditions
- Filters
- Optical components
Thermal Behaviour During Testing
LED
LED optical performance can be influenced by junction temperature, so the thermal condition during measurement can be important.
Halogen
Halogen output also depends on thermal equilibrium: power drives filament heating, temperature stabilises, and light output stabilises in turn.
Both technologies require measurements to be performed under the defined operating and stabilisation conditions.
How Output Changes Over Operating Hours
| Stage | LED | Halogen |
|---|---|---|
| New source | New LED | New lamp |
| Over operating hours | Gradual optical depreciation | Filament / lamp ageing |
| End state | Depreciation rate depends on LED system, thermal management, operating conditions, fixture design | Output changes and eventual filament failure |
Comparing Stability Factors
| Factor | LED | Halogen |
|---|---|---|
| Start-up | Typically rapid | Requires thermal stabilisation |
| Source ageing | Gradual depreciation possible | Lamp ageing and eventual filament failure |
| Temperature sensitivity | Important | Important |
| Supply sensitivity | Depends on driver/system design | Lamp output can be strongly affected by operating conditions |
| Optical degradation | Possible | Possible |
| Final beam stability | Depends on complete fixture | Depends on complete fixture |
Runway Edge Light: Two Technologies, Same Question
| Fixture | Path to Beam |
|---|---|
| LED fixture | Power → driver → LED → optics → beam |
| Halogen fixture | Power → filament → reflector → optics → beam |
The photometric test ultimately asks the same question for both: does the completed fixture provide the required luminous intensity within the required angular sectors and other applicable optical characteristics? The internal technology does not change the fundamental compliance objective.
Myths vs Facts
Not fundamentally. The measurement may require different operating, thermal, spectral, or stabilisation considerations, but the core photometric quantities remain the same.
Not necessarily. A well-designed reflector and optical system can produce the required beam distribution.
Not necessarily. Candela depends on how much luminous intensity is produced in a particular direction, not simply on source efficiency.
Full Engineering Parameters
| Parameter | LED Photometry | Halogen Photometry |
|---|---|---|
| Fundamental quantities | Same | Same |
| Candela measurement | Yes | Yes |
| Beam distribution | Yes | Yes |
| Horizontal photometry | Yes | Yes |
| Vertical photometry | Yes | Yes |
| Spectrum | Often structured/technology-dependent | Broad/continuous |
| Source emission | Often more directional | Broad |
| Beam formation | LED + optics | Filament + reflector/optics |
| Thermal condition | Important | Important |
| Stabilisation requirement | Defined by test procedure | Defined by test procedure |
| Ageing behaviour | Gradual degradation possible | Lamp ageing/failure mechanisms |
| Compliance basis | Final fixture performance | Final fixture performance |
LED Photometry vs Halogen Photometry
LED photometry and halogen photometry measure the same fundamental optical quantities. The photometric measurement itself is not fundamentally different between the two technologies.
The main difference is how the light source generates light, and how operating conditions, spectrum, temperature, ageing, and optics influence the measured result. For AGL compliance, the final question is essentially technology-independent: does the complete fixture, operated under the specified conditions, produce the required intensity, beam distribution, and colour characteristics?
For AGL: choose the source technology on engineering and lifecycle grounds — the photometric compliance question stays the same either way.
Engineering & Expert Notes
Allow for Thermal Stabilisation
MethodLED output changes as the fixture warms. Define the wait.
Confirm CCR Step and Current Before Measuring
SettingsRecord the step with each reading.
Align the Optical Axis
AlignmentDifferent optics can be sensitive to aiming.
Report Instrument and Setup
ReportingInclude calibration and geometry.
Frequently Asked Questions
No. Both are evaluated using the same quantities — candela for luminous intensity, lux for illuminance, cd/m² for luminance, and intensity-versus-angle for beam distribution.
Because LED optical performance can be influenced by junction temperature, so the thermal condition of the LED during measurement can affect the recorded output and colour.
A halogen filament's output changes as it heats toward thermal equilibrium, so measurements are taken once the lamp has stabilised under the defined operating conditions.
Not automatically. Candela depends on how much intensity the complete fixture produces in a specific direction, not simply on the source's efficiency or technology.
Yes. LEDs can show gradual lumen/intensity depreciation over operating hours, while halogen lamps experience filament and lamp ageing that can also change output over time.
No. The compliance question is essentially technology-independent — does the completed fixture provide the required intensity, beam distribution, and colour characteristics under the specified conditions?
Governing Documents
For an actual project design, always verify the current edition of the applicable documents.
FAA AC 150/5340-30
Design and Installation Details for Airport Visual Aids — photometric requirements for runway and approach lighting, applicable to both LED and halogen fixtures.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — photometric characteristics of aeronautical ground lights, intensity and colour requirements.
CIE Photometry Standards
International Commission on Illumination definitions and measurement methods for luminous intensity, illuminance and luminance.
ANSI/IES LM-79
Approved Method: Optical and Electrical Measurements of Solid-State Lighting Products — the specific test method for LED source/luminaire photometric and electrical measurement referenced on the LED side of this comparison; halogen sources are instead measured under conventional goniophotometric practice.
FAA AC 150/5345-46
Specification for Runway, Taxiway, Heliport, and Vertiport Light Fixtures — sets the actual photometric requirement (candela tables per L-type) that a given fixture's LED or halogen photometry must be measured against.
Final Engineering Note
Do not assume source technology changes the definition of the measured quantity. Confirm stabilisation, thermal, and drive conditions required by the governing test procedure for the specific source in use.
LED and halogen photometry ask the same question of the fixture — they just require different care in how the source is operated during the test.
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