Optical design and fabrication play an ever-increasing role in our modern society as more applications for optics are developed, especially in the areas of imaging, sensing, and illumination systems. Of particular interest is the ability to utilize modern design tools to reduce cost, augment manufacturability, and enhance system performance. New materials and unprecedented capabilities to create and measure precise aspheric and freeform optical surfaces have created a whole new design space for imaging, while new computational tools have allowed more complex systems to be both designed and fabricated. These capabilities have extended to micro-optics, head-mounted displays in military aviation. Its application ranges from various kinds of optical coating such as narrow band-pass, band-reject, reflective coating, anti-reflective coating, Electromagnetic interference and electromagnetic compatibility (EMI/EMC) coating on optical surfaces, etc. to various types of optical components and mirrors used in Head-Up Display (HUD), Multifunctional Display, Standby Display Unit (SDU), Head-Mounted Display (HMD), Optical Gun Sight, Laser Designator Payload, etc. The optics in head-up display systems are used to “collimate” the HUD image so that essential flight parameters, navigational information, and guidance are superimposed on the outside world scene.
Precision optical components play a key role in avionics systemsin both civil and military aviation. Its application ranges from various kinds of optical coatings such as narrow band-pass, band-reject, reflective coating, anti-reflective coating, EMI/EMC shielded coatings on optical surfaces, etc. to various types of optical components and mirrors used in Head-Up Display (HUD), Multifunctional Display, Standby Display Unit (SDU), Head-Mounted Display (HMD), Optical Gun Sight, Laser Designator Payload, etc.
Older aircraft hosts many displays to show important flight data such as artificial horizon, navigation, airspeed, radar display, altitude, angle of attack, etc. presented on separate instruments panels. This arrangement of multiple displays and display formats require thepilot to scan various instruments panels to understand flight parameters and details causing split attention between the outside world and the cockpit displays. These displays were generally configured as Head down displays (HDD) needing pilot to do continual eye adjustments due to varying focus, changing brightness etc. resulting in longer reaction times, fatigue and reduced efficiency. For this reason, new generation aircrafts are built with glass cockpit hosting multitude of displays such as HUD, MFD, SDU, HMD and associated systems such as Mission Computer, Laser Designator Payload, etc. to aid pilot operations. These systems process data from a host of sensors to ahost of display systems for displaying critical ﬂight information like altitude, airspeed, angle of attack, artiﬁcial horizon, navigation, radar display, etc. It was found that pilots using HUDs could operate their aircraft with greater precision and accuracy than they could with conventional flight instrument systems.
Design of opto-avionic systemsis very challenging; be it cockpit display or the aircraft exterior and interior lights and so is its fabrication, assembly, metrology on individual component level and on the assembly level. The designs for systems like HUD and Pilot Display Unit for aircraft variants have been made on rotationally symmetric surface shapes conforming to military standards MIL-810-G for which the manufacturability issues are understood where manufacturing is performed iteratively. The optical system design has now been shifted to hybrid concept, utilizing spherical and aspheric optical components as it offers significant advantages over conventional flat and spherical surfacesby using aspheric optics, the number of components are reduced thereby reducing the weight and volume of the system and at the same time enhancing the optical efficiency and image quality of the avionic system.Typical optical specifications obtained are: Instantaneous field of view in elevation (IFOV-El): 18°-23°, IFOV (Azimuth): 20°, Convergence error: 1 mR, and Distortion: ± 1% for HUD variants using aspheric optics with reduction of weight by more than 30%. The further attempt is to use freeform mirror and optical components to reduce the size of system by an order of magnitude and to control astigmatism at multiple locations in the field of view and thus reduce wave front aberration.
Considering the need for high precision in HUD optics manufacturing to improve interchangeability of components, improve quality control and longer wear/fatigue life. The deterministic optical fabrication issues in terms of optical grinding and polishing parameters to minimize surface and sub-surface damage, which may get worsened during thermal, vibration and other environmental extremities experienced by military aviation products during its service time, have been characterised and modelled for machining parameters for optical semi-automatic grinding and polishing setup for glass optics and metal and plastic substrates with diamond tuning process. The optical assembly procedure is made innovative by employing silicon gasket with suitable Jigs used for spacing lenses along with suitable lens rest mechanism coupled with nitrogen purging to withstand harsh environment of temperature and mechanical shocks experienced during aircraft operation.
The flat, uniform, graded and rugate multilayer optical coatings have been designed and realized on glass substrates to get upto 75% reflection in the target wavelength region in form of single band rejection filter with central wavelength of 545 nm, bandwidth of 23 nm and total thickness of 3.25 µm for see-through displays while maintaining transmission better than 80% in other wavelength region. The anti-reflective and protective optical coatings on glass results in minimum loss of brightness and no shift in central wavelength. Controlled and deterministic freeform surfaces for LED lights reflector along with highly reflective optical coatings has resulted in focussed light in spread of 18°x13° and 10°x8° in Aircraft Taxi and Landing Modes respectively.Protective coating on outer surface ofpolycarbonate cover of Wing and Fin Navigation Lights and Taxi and Landing Lights, anti-reflective coating on inner surface and ITO coating on outside surface has made it possible to achieve harsh environmental ruggedization and electromagnetic interference and compatibility specifications.Apart from this, design of double band rejection filter with 650 layers, Central wavelength: 400, 500 nm, Bandwidth: 27 & 39 nm, Total thickness - 3.25 µm, Design of triple band rejection filter with 642 layers, Central wavelength: 400, 500 &600 nm, Bandwidth: 27, 39&49 nm, and total Thickness: 3.25 µm has been carried out.The fabrication process of depositing thin film dielectric rugate filters using ion-beam sputtering has been established.
Central Scientific Instruments Organisation (CSIR-CSIO), INDIA