Modular, Biocular Eyepiece Display For Thermal Systems

Many high quality biocular magnifier eyepieces for two-eye viewing have been designed and fabricated for use with image intensifier direct view systems. These eyepieces provide outstanding imagery across a large exit pupil. The phosphor screen of the image intensifier tube has sufficient brightness (10 to 100 foot lamberts) to allow for very comfortable two-eye viewing. The great advances of the Far Infrared (FIR) technology which detects temperature differences and displays that thermal image in the visible spectrum now provide a new dimension to night vision systems. Those FIR systems that use a cathode ray tube (CRT) display can capitalize on the biocular eyepieces designed for image intensifier systems. However, the majority of FIR systems use Light Emitting Diodes (LED) which are reflected off the back side of the FIR scan mirror so as to avoid the complexity of multiplexing inherent in CRT displays. The scanned LED displays typically use visible relay optics which are slow (approx f/9) and present a real image to an eyepiece lens. One cannot use the afore-mentioned biocular eyepieces since most biocular eyepieces have an f/number less than f/1.0. Therefore, current FIR systems with LED displays use only monocular eyepieces and often cause most observers to "squint" at the thermal picture. This paper proposes a solution to that situation which uses a "diode tube" consist-ing simply of a photo-emitting cathode, proximity focussed on a phosphor screen, which provides a lambertian source of increased brightness located at the reticle plane of the visual optics relay and thereby allows for use of existing image intensifier biocular eyepieces. Measured performance parameters of the biocular eyepiece and diode tube are presented.