Advanced rotorcraft Helmet Mounted Display Sighting System (HMDSS) optical distortion correction methodology

Helmet Mounted Displays (HMDs) typically utilize off axis optical systems that result in distorted images. In order to minimize the weight on the pilot's head, a pixilated display, such as an Active Matrix liquid Crystal Display (AMLCD), is utilized as the imaging source. Pixelated displays based on AMLCDs cannot correct distortions or perform spatial transformations as easily as an analog CRT-based systems using electron beam deflection. An advanced rotorcraft HMDSS is a digital system where correcting the distortion within the digital domain is desired to eliminate the inaccuracies of converting to analog, correcting the distortion and converting back to digital. Other system requirements necessitate that the input video be rescaled to provide the proper image to the optical system in order to have the FUR imagery overlay the real world as the pilot looks through the canopy. To optimize image resolution with minimum sensor size, the FUR system scans in column mode. As this is not compatible with conventional AMLCD scanning, the FUR video data must be converted to a row scan. This function, which normally results in additional frame delay, will also be described, together with methods for reducing the latency. The physical constrains of the helmet and the desire to use identical AMLCD devices meant that the devices are rotated between sides of the helmet. This rotation requires that the video image be scanned horizontally and vertically flipped creating another complexity in the design. Requirements for a helmet mounted image intensified television camera to be displayed as an image by itself or overlaid with symbology provided from external video creates additional complexity for distortion correction within the optical chain and will be discussed in this paper. All of these modes require that the video be manipulated in varying degrees of complexity. The enabling technology described in this paper is a complex integrated circuit that allows the user to program the required functions of rotation, scaling, overlays and distortion with minimum latency achieving an effective solution for an advanced rotorcraft Helmet Mounted Display Sight Subsystem (HMDSS).