PAPI Lamp/LED Module

The PAPI Lamp or LED Module is the replaceable light‑generating component within a PAPI light unit. It is the primary source of the high‑intensity light that the optical assembly (reflector, prism, filters) shapes into the characteristic glide slope beam. Two main technologies are used:

• Halogen Lamp: A tungsten‑halogen incandescent lamp, standardised at 200W (6.6A, PK30d base), with a colour temperature of 2800K–3200K. Halogen lamps provide high luminous flux and excellent colour rendering but have a relatively short lifespan (~1,000 hours at full intensity) and degrade over time.
• LED Module: A solid‑state assembly containing multiple high‑power LEDs with an integrated driver and heat sink. LED modules offer 50,000+ hours of life, instant‑on capability, lower power consumption, and consistent output over their lifetime, but require careful thermal management and have higher upfront cost.

Both types are designed to be field‑replaceable, with the lamp or module fitting into a standardized socket or holder within the PAPI unit. The choice between halogen and LED affects the photometric performance, maintenance schedule, and total cost of ownership of the PAPI system.

The lamp or LED module’s purpose is to sustain, over its entire service life, the exact colour and intensity that the pilot depends on to read a sharp red/white split. PAPI works by contrast: a pilot judges glide slope by the crisp boundary between red and white, not by either colour in isolation, so if one unit's light source degrades or shifts in colour temperature relative to the other three, the transition point stops being consistent across the array even though the optics and angle haven't changed at all. This is precisely why halogen lamps, which dim and yellow measurably as they age, need comparatively frequent replacement to keep every unit matched, while LED modules hold their output and colour far more consistently across a much longer service life. Whichever technology is used, the module's job is the same: keep the light source itself from becoming the thing that quietly undermines an otherwise correctly aligned and powered PAPI unit.

Each light source technology has distinct characteristics that influence selection for new installations or retrofit projects:

Many airfields are transitioning to LED PAPI systems to reduce maintenance costs and improve reliability. However, halogen remains a proven, cost‑effective option for low‑traffic or budget‑constrained operations.

The PAPI lamp or LED module is a universal component used in every PAPI system, regardless of the installation type:

• Standard 4‑unit PAPI – on precision runways (CAT I, II, III) worldwide.
• APAPI (2‑unit) systems – using a single lamp or LED per unit.
• Helicopter PAPI – with steeper angle optics, often using LED for rapid start and reliability.
• Portable PAPI – typically LED for low power consumption and battery operation.
• Retrofit projects – replacing halogen lamps with LED modules in existing PAPI fixtures to extend life and reduce maintenance.
• As a spare part – stocked by airfield maintenance teams for scheduled or unscheduled replacements.

The lamp or LED module fits into the optical housing at the rear of each PAPI unit on the wing bar, secured with a bayonet, threaded, or bracket mount depending on the fixture design, and positioned so its light source sits precisely at the optical system's focal point. Correct seating is critical, since even a small offset from focus will blur the sharp red/white transition the whole PAPI depends on, so the module is pushed fully home and locked before the housing is resealed against moisture. Replacement is done in the field at the fixture itself, with the wing bar's alignment re-checked afterward to confirm the module hasn't shifted the beam's aim, and spare modules are kept on hand at the airfield precisely because this is the component most likely to need swapping between full unit replacements.

The PAPI lamp or LED module converts electrical energy into visible light, providing the raw output for the optical system:

• Halogen Operation: An electric current passes through a tungsten filament sealed in a quartz envelope with halogen gas. The filament heats to ~3000°C, emitting a continuous spectrum of light. The halogen cycle redeposits evaporated tungsten back onto the filament, extending life compared to standard incandescent lamps.
• LED Operation: A semiconductor diode emits light when forward‑biased. Multiple LEDs (often in a chip‑on‑board array) are mounted on a metal‑core PCB with a phosphor coating to produce white light. An integrated driver regulates the current, and a heat sink dissipates thermal energy. The module may include secondary optics to tailor the beam shape before it enters the PAPI’s main optical assembly.
• Intensity Control: Both types are controlled by the CCR via the series circuit. Halogen lamps dim smoothly by reducing current. LED modules require either current reduction (if the driver is linear) or pulse‑width modulation (PWM) – the module’s driver translates the CCR current into the appropriate LED drive signal, providing the required brightness steps (day, twilight, night).
• Mounting & Electrical Interface: The lamp or module fits into a socket or connector within the PAPI unit. Halogen lamps use a PK30d prefocus base, while LED modules have a proprietary connector or screw terminals for the driver. Both must be handled carefully to avoid contamination or damage.

The lamp or LED module sits at the end of the control chain, not inside it — it has no control logic of its own and simply converts whatever current or drive signal it receives into light output. A halogen lamp dims smoothly because reducing the series current directly reduces filament heat; an LED module needs an integrated driver to translate the same series current (held constant) into the PWM or regulated-current signal its diodes actually require, since LEDs don't dim proportionally to current the way a halogen filament does.

This is why LED retrofits into legacy CCR circuits need a driver stage the halogen lamp never required: the CCR's control logic (step selection, loop monitoring) is unchanged either way, but the module itself has to do more translation work when the light source is a semiconductor rather than a filament.

A halogen lamp is built as a sealed quartz or hard-glass envelope containing a tungsten filament and a halogen gas fill, mounted on a standardised base or bi-pin holder that plugs directly into the socket inside the PAPI unit's optical assembly; a dichroic or reflective coating on some versions helps direct heat away from the beam path. An LED module, by contrast, is a small circuit assembly: an array of high-power LED die mounted on a metal-core or ceramic circuit board for heat spreading, bonded to a finned aluminium heat sink that draws heat away from the junctions, with an integrated driver circuit regulating current to the array and a connector or flying leads bringing in primary power. Both formats are built to the same mechanical footprint and mounting interface as the socket or holder they replace, so a technician can swap a failed lamp or module for a new one of the same type without modifying the optical assembly around it, though moving between halogen and LED typically requires a compatible holder or driver change.