HUD Combiner Assembly

The F-22 HUD has a single element combiner using new computer generated holographic techniques to take out the distortion. The Combiner assembly features a single element comprising two glass plates bonded to produce a flat parallel sided assembly. A complex reflective holographic optical element (HOE) is recorded on photosensitised gelatine on the spherical interface sandwiched in the assembly which acts as the collimating Combiner. The construction optics to manufacture the HOE incorporate a CGH that generates a phase shape that could not be done using conventional lenses.
The combiner is also flat which minimises real world distortion and provides a total field of view (TFoV) of 30deg x 25deg with an instantaneous field of view (IFOV) of 30deg x 20deg.
The F-22 design is similar to that of the Typhoon HUD but is limited to cursive operation. The support structure for the Combiner is more robust than that in the Typhoon as the polycarbonate canopy on the F-22 can deform under birdstrike and may strike the HUD. A rubber buffer was fitted to the Combiner to prevent damage to canopy or Combiner.
The key feature is integrity such that the pilot no longer needs to monitor the HUD against other instruments thus reducing workload and this HUD is classified as a primary flight instrument.

During F-22 canopy birdstrike tests, it was found that the HUD combiner glass would shatter the canopy. To solve this problem during the Engineering & Manufacturing and Development (EMD) Phase (where a system is developed and designed before going into production), the F-22 HUD had a rubber buffer strip on it that effectively shields the polycarbonate of the canopy when it flexes during a birdstrike from hitting the optical glass in the HUD and shattering. This is seen in the later Production systems where the Combiner support encloses the glass.

In 1991 the Company was selected to provide a cursive only variant of this design for the Advanced Tactical Fighter which became the USAF F-22 Raptor. The HUD order for 400 systems was part of a £300M package which also included the Pilot’s Control Stick and the integrated Vehicle Management System.

In 2017 BAE Systems was awarded a contract to replace the CRT in these HUDs with the Digital Light Engine.

During 1986 the Company research team (FARL) were beginning to make significant achievements on the use of Computer Generated Holograms (CGH) for HUD optics. A theoretical design showed the feasibility of a design with a 35deg off-axis angle being capable of installation in the F-16 envelope. Additional benefits were Combiner transmission of 80% and optical system end to end transmission of 40% compared to 20% on the conventional F-16C/D desin resulting in a much brighter display. This work involved collaboration between FARL, ADD and the Hirst Research Centre at Wembley and Marconi Research Centre at Great Baddow who provided the CGH and Hologram manufacturing facilities respectively. By 1989 this design was targeted at the proposals for EFA (European Fighter Aircraft ultimately to be the Typhoon) and also for the ATF (Advanced Tactical Fighter ultimately to be the F-22). ADD had begun the fabrication of a demonstrator which was completed by the end of 1991. This demonstrator consisted of a complex optical system containing tilted and de-centered lenses, aspheric components and a single holographic Combiner designed with the aid of CGH. The ‘double bounce fold ‘ arrangement was novel and was patented.