In a typical aviation scenario, an external scene is acquired by a sensor, converted into an electrical signal, reproduced on a display, and then relayed optically to the eye(s). Within our definition of an HMD, the display which first reproduces the scene imagery, prior to relaying it to the eye, is referred to as the image source. In the IHADSS, the image source is a miniature, 1-inch diameter, CRT. When the concept of HMDs was first seriously pursued, the CRT was the only established display technology available. CRTs have remained the display of choice due to their attributes of low cost, easy availability, dependability, and good image quality. However, CRTs, even miniature ones, have inherent drawbacks which include weight, size (primarily depth), power requirements, high anode voltage, and heat generation. And, it is only due to these deficiencies that a new class of display technologies has been able to gain a foothold. These new technologies are collectively referred to as FP technologies, due to their flat display surface and thin physical profile. Displays based on FP technologies offer characteristics which counter the deficiencies of CRT displays. Flat panel displays (FPDs) have a greatly reduced physical profile, low power and voltage requirements, low heat output, and low weight. All of these characteristics make them very desirable for aviation use where space, weight, and power are at a premium. While types of image sources are not limited to CRTs and FP technologies, these are the most likely candidates for near-future systems (excluding I2 systems).
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