| 摘要 |
An observation optical system has an entrance surface, a reflecting surface, and an exit surface. At least one of these surfaces is configured as a free-formed surface, to which a volume holographic optical element (HOE) is closely adhered. The observation optical system satisfies the following conditions: <?in-line-formulae description="In-line Formulae" end="lead"?>-0.20<phix/Px<0.30 <?in-line-formulae description="In-line Formulae" end="tail"?> <?in-line-formulae description="In-line Formulae" end="lead"?>-0.20<phiy/Py<0.20 <?in-line-formulae description="In-line Formulae" end="tail"?> where phix is the power of only the HOE in an x direction at the point of intersection of an axial principal ray with the surface of the HOE, phiy is the power of only the HOE in a y direction (parallel to the direction of decentration) at the point of intersection of the axial principal ray with the surface of the HOE, Px is the power of the whole of the observation optical system in the x direction, and Py is the power of the whole of the observation optical system in the y direction.
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