Ocular optical system

1. An ocular optical system, adapted for imaging of an imaging ray from a display screen which enters an eye of an observer via the ocular optical system, a side facing towards the eye being an eye-side, and a side facing towards the display screen being a display-side, wherein the ocular optical system comprises a first lens element and a second lens element in order from the eye-side to the display-side along an optical axis, each of the first lens element and the second lens element of the ocular optical system comprises an eye-side surface facing towards the eye-side and allowing the imaging ray to pass through and a display-side surface facing towards the display-side and allowing the imaging ray to pass through,

wherein a maximum distance between the display-side surface of the first lens element and the eye-side surface of the second lens element parallel to a direction of the optical axis is less than 5 millimeters (mm), and the ocular optical system satisfies:

6≤DLD/EPD≤20,

wherein DLD represents a diagonal length of the display screen corresponding to one single pupil of the observer, and EPD represents an exit pupil diameter of the ocular optical system.

2. The ocular optical system according to claim 1 , wherein the ocular optical system further satisfies: 2≤EFL/(T1+G12+T2), wherein EFL represents a system focal length of the ocular optical system, T1 represents a thickness of the first lens element along the optical axis, T2 represents a thickness of the second lens element along the optical axis, and G12 represents an air gap between the first lens element and the second lens element along the optical axis.

3. The ocular optical system according to claim 1 , wherein the display-side surface of the second lens element has a concave portion in a vicinity of a periphery of the lens element.

4. The ocular optical system according to claim 1 , wherein the ocular optical system further satisfies: 1.2≤TTL/G2D, wherein TTL represents a distance from the eye-side surface of the first lens element to the display screen along the optical axis, and G2D represents a distance from the display-side surface of the second lens element to the display screen along the optical axis.

5. The ocular optical system according to claim 1 , wherein the ocular optical system further satisfies: 2.5≤T1/T2, wherein T1 represents a thickness of the first lens element along the optical axis, and T2 represents a thickness of the second lens element along the optical axis.

6. The ocular optical system according to claim 1 , wherein the ocular optical system further satisfies: 3.5≤(T1+G12)/T2, wherein T1 represents a thickness of the first lens element along the optical axis, T2 represents a thickness of the second lens element along the optical axis, and G12 represents an air gap between the first lens element and the second lens element along the optical axis.

7. The ocular optical system according to claim 1 , wherein the ocular optical system further satisfies: 3.0≤EFL/T1, wherein EFL represents a system focal length of the ocular optical system, and T1 represents a thickness of the first lens element along the optical axis.

8. The ocular optical system according to claim 1 , wherein the ocular optical system further satisfies: 10.0≤EFL/T2, wherein EFL represents a system focal length of the ocular optical system, and T2 represents a thickness of the second lens element along the optical axis.

9. The ocular optical system according to claim 1 , wherein the ocular optical system further satisfies: 0.5≤ER/(T1+T2), wherein ER represents a distance from the pupil of the eye of the observer to the eye-side surface of first lens element along the optical axis.

10. The ocular optical system according to claim 1 , wherein the ocular optical system further satisfies: 3.0≤TTL/(T1+T2), wherein TTL represents a distance from the eye-side surface of the first lens element to the display screen along the optical axis, T1 represents a thickness of the first lens element along the optical axis, and T2 represents a thickness of the second lens element along the optical axis.

11. The ocular optical system according to claim 1 , one of the eye-side surface and the display-side surface of the first lens element is a Fresnel surface.

12. The ocular optical system according to claim 11 , wherein the display-side surface of the first lens element is the Fresnel surface.

13. The ocular optical system according to claim 1 , wherein the ocular optical system further satisfies: G2D/T2≤15.0, wherein G2D represents a distance from the display-side surface of the second lens element to the display screen along the optical axis, and T2 represents a thickness of the second lens element along the optical axis.

14. The ocular optical system according to claim 1 , wherein the ocular optical system further satisfies: 3.5≤D1/T1, wherein D1 is an optical effective diameter of the eye-side surface of the first lens element, and T1 represents a thickness of the first lens element along the optical axis.

15. The ocular optical system according to claim 1 , wherein the ocular optical system further satisfies: 15.0≤D2/T2, wherein D2 is an optical effective diameter of the eye-side surface of the second lens element, and T2 represents a thickness of the second lens element along the optical axis.