Optical device, image display apparatus and head-mounted display unit

A head-mounted display unit, comprising:

• (A) a frame of the eyeglasses type adapted to be mounted on the head of an observer;
• and
• (B) an image display apparatus attached to said frame;
• said image display apparatus including light source and scanning means for scanning a light beam emitted from said light source;
• said scanning means including
• (a) a first mirror mounted for pivotal motion around a pivotal motion axis thereof provided by a first axis extending in a first direction and adapted to receive the light beam emitted from said light source and incident thereto,
• (b) first light deflection means having an axial line extending along a second direction different from the first direction for receiving the light beam emitted from said first mirror and incident thereto at a first incidence angle and emitting parallel light at a predetermined first emergence angle with respect to the second direction depending upon the first incidence angle of the light beam by the pivotal motion of said first mirror,
• (c) a second mirror mounted for pivotal motion around a pivotal motion axis thereof provided by a second axis extending in a third direction and adapted to receive the parallel light emitted from said first light deflection means and incident thereto, and
• (d) second light deflection means having an axial line extending along a fourth direction different from the third direction for receiving the parallel light emitted from said second mirror and incident thereto at a second incidence angle and emitting parallel light at a predetermined second emergence angle with respect to the fourth direction depending upon the second incidence angle of the parallel light by the pivotal motion of said second mirror;
• said second light deflection means including an external light receiving face provided in an opposed relationship to a light emitting face provided in parallel to the fourth direction for receiving external light incident thereto;
• said second light deflection means having a plurality of translucent films provided inside thereof;
• said translucent films having a light reflectivity R2 at a wavelength of the light beam which satisfies:
• R2 ≦k×{(P2/t2)×tan(ζ2)}1/2
• where k is a constant greater than 0 but lower than 1, P2 an array pitch of said translucent films, t2 a thickness of said second light deflection means, and ζ2an angle formed between said light emitting face and said translucent films.

An optical device having:

• a light receiving face provided in parallel to one direction and adapted to receive light from a light source incident thereto;
• a light emitting face provided in parallel to an axial line extending in a direction different from the one direction;
• and
• an external light receiving face provided in an opposing relationship to said light emitting face and adapted to receive external light incident thereto;
• said optical device having a plurality of translucent films provided therein;
• said translucent films being arrayed in parallel to each other in a spaced relationship from each other along the axial line and disposed in an inclined relationship to the axial line;
• light from said light source incident to the light receiving face being reflected by said translucent films and emitted from said light emitting face while external light incident from said external light receiving face is emitted from said light emitting face;
• said translucent films having a light reflectivity R at a wavelength of the light from said light source which satisfies:
• R ≦k×{(P/t)×tan(ζ)}1/2
• where k is a constant greater than 0 but lower than 1, P an array pitch of said translucent films, t a thickness of said optical device, and ζan angle formed between said light emitting face and said translucent films.