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The basic purpose of the optical designs for helmet and head mounted devices is to focus small image sources to provide a specific field of view to the viewer with sufficient eye clearance for spectacles and protective masks, and sufficient size eye box to compensate for pupil displacements from eye movement, vibration, and head/helmet slippage. To achieve these objectives, a series of calculations are required to determine the sizes of key HMD elements, particularly the diameter of the last optical power element(s) of the eyepiece for a given set of HMD characteristics. The sizes of the HMD elements will primarily determine the ultimate weight. This exercise will start first with a direct view system such as NVG with no see-through provision, the refractive, on-axis (such as IHADSS), and the on-axis catadioptric designs with see-through vision. The first order eyepiece calculations for the off-axis designs are unique to the particular design, and are beyond the scope of this exercise. However, they will be discussed later on in this section.
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