| 摘要 |
PROBLEM TO BE SOLVED: To provide an optical device having a simple structure and little loss in light quantity which can be driven at low voltage and which can change optical characteristics by incorporating a liquid crystal having a helical structure and satisfying specified conditions. SOLUTION: This optical device with variable optical characteristics contains a liquid crystal having a helical structure and satisfying|Γ/2Φ|<=πwithΓ=2π(ne-no)d/λandΦ=2πd/P, wherein P is the twisting pitch of the helical structure,λis the center wavelength of the used incident light, d is the max. thickness in the optical axis direction of the liquid crystal part, (ne) is a refractive index of the liquid crystal for abnormal light, and (no) is the refractive index of the liquid crystal for normal light. Considering that the twisting pitch P of a nematic liquid crystal 1 is extremely small compared with a wavelength of the used light, the refractive index of the liquid crystal in a lens with variable focal length using this optical device with variable optical characteristics does not depend on polarized light of the incident light but effectively has the refractive index n'=(ne+no)/2, and therefore, the liquid crystal layer acts as a flat- concave lens having the refractive index n'.
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