|Category:||Head-Up Display [HUD]|
|Object Name:||HUD Dual Holographic Combiner Assembly|
|Year of Manufacture:|
|Location:||Archive Object Store|
The Total field of view (TFoV) of a Head Up Display is defined as the “Total angle subtended by the display symbology seen with head movement” and it is largely determined by the size of the exit lens in a conventional refractive optical design.
The Instantaneous Field of View (IFoV) is the “Subtended angle of display seen from one head position using one eye” and the Binocular Instantaneous Field of View is the “Subtended angle of display seen from one head position using both eyes”. Typically a HUD TFoV will be circular and of 25° and may only have an IFOV of 13.5° in elevation and 20° in azimuth. The pilot has two eyes so the actual IFoV is two overlapping circles a bit like a Venn diagram. The exit lens is limited in size by what can be made accurately but also by the environment. The stress of operation over a very wide temperature range (-40oC to +125oC or more in an enclosed cockpit in the desert sun) and extreme temperature shock should the canopy be lost at altitude together with severe vibration especially when the aircraft gun is firing have held this lens to no more than about 6.7inches in diameter.
The vertical field of view can be extended by the addition of a second Combiner Glass forward of the first glass and extending higher up. The optical system then provides an IFoV with a second but smaller double circular field above the first. Now, the image has to transition from the aft glass to the forward glass in a smooth manner and this done by using graded optical coatings which are traditionally a neutral density coating. Thus the coating on the aft glass reduces in reflectance at the top as the forward glass coating increases in reflectance and the transmittance, of course, does the opposite. The whole effect must be achieved without any obvious display artefacts and even more important there must be no false horizon lines created. This last issue is aided by making the top of the Combiner Glass angle down in the forward direction on the sight line from the design eye position. To achieve this gradation of coatings requires a sophisticated deposition technique. The Company used this optical design on the F-5 HUD and increased the vertical FoV by about three degrees. Originally the graded coatings were made by Watshams who were experts in coating optical elements.
This is an experimental assembly, of a size for a small exit lens typical of the F-5 HUD, where the Combiner glasses employ a diffractive layer sandwiched between two glass plates. The dichromated gelatin technique is used to optically grade the coatings for a uniform transition. These coatings are also much more frequency selective in their response to the CRT p53 phosphor such that a higher symbol luminance and higher outside world transmission are both achieved albeit with a slight pink tinge to the outside world as the P53 frequency spike is removed from the outside world spectrum. The display can achieve greater than 80% photopically weighted transmission through the combiner, while maintaining up to 80% reflection of the display. This Combiner may have been made in the Company Falcon building or by Pilkingtons (now Qioptic)
ODHU0657: This is a complete dual combiner assembly from the Tornado Mid-life update HUD made by GEC Ferranti. These glasses use another technique of a special multi-layer coating called rugate technology. Rugates act like a single inhomogeneous film with a refractive index that varies continuously and periodically throughout the thin-film structure. The same impressive increase in transmission and reflection can be achieved as the ‘holographic’ system but without the bulk of the glass sandwich. Omitec originally developed thin-film rugate technology to manufacture "notch" filters of specific bandwidth. In 1991 flight tests of Head Up Displays, fitted with what was claimed to be the world's first airborne thin-film colour-selective graded combiners were commenced in a Harrier GR.7 and a Tornado GR.4 later in the year. Omitec also delivered Rugate coated Combiners to GEC Ferranti for the wide-angle HUD for the Royal Air Force's Tornado GR.4 mid-life upgrade.