Freeform OST-HMD system with large exit pupil diameter and vision correction capability

Abstract

Compactness and light weight, large exit pupil diameter and distance, small distortion for virtual image, and see-through light paths are pivotal factors to achieve a better, wearable experience of optical see-through head-mounted displays (OST-HMDs). In addition, light efficiency of the virtual image light path is an important factor for heat dissipation in HMD devices. This paper presents a new type of OST-HMD optical system that includes three wedge-shaped freeform prisms and two symmetric lenses. Based on a 0.71 in. microdisplay, an OST-HMD prototype with a diagonal field of view (FOV) of 45.3°, an F -number ( F /#) of 1.8, an exit pupil size of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1"> <mml:mrow> <mml:mn>12</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>mm</mml:mi> <mml:mo>×</mml:mo> <mml:mn>8</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>mm</mml:mi> </mml:mrow> </mml:math> , and an eye relief of 18 mm is demonstrated. The maximum value of distortion of the final system is 0.6% and 0.4% for virtual image and see-through light path, respectively. The overall dimension of the optical system per eye is no larger than <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m2"> <mml:mrow> <mml:mn>30</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>mm</mml:mi> <mml:mtext> </mml:mtext> <mml:mo stretchy="false">(</mml:mo> <mml:mtext>width</mml:mtext> <mml:mo stretchy="false">)</mml:mo> <mml:mo>×</mml:mo> <mml:mn>40</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>mm</mml:mi> <mml:mtext> </mml:mtext> <mml:mo stretchy="false">(</mml:mo> <mml:mtext>height</mml:mtext> <mml:mo stretchy="false">)</mml:mo> <mml:mo>×</mml:mo> <mml:mn>14</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>mm</mml:mi> </mml:mrow> </mml:math> (thickness), and the weight of the optical module including lenses, holder, and microdisplay is 12.8 g. The light efficiency of the virtual image light path is up to 50% higher than those of other OST-HMD optical solutions.