Light guide for finger print reader

1. An image capture apparatus, located in an environment medium, comprising:

a light guide device, comprising:

a top surface;

a bottom surface, opposite to the top surface;

an light incident surface, connected between the top surface and the bottom surface, wherein an acute angle α is included between the light incident surface and the top surface; and

a light emitting surface, opposite to the top surface, wherein the bottom surface is connected between the light incident surface and the light emitting surface;

a transparent device, disposed on the top surface of the light guide device;

a light source, used to emit a light beam, wherein the light beam passing through the light incident surface is transmitted toward the transparent device and totally reflected by an interface between the transparent device and the environment medium; and

an image capture device, disposed on the light emitting surface of the light guide device,

wherein the acute angle α satisfies a formula (2):

θ

i

≤

sin

-

1

⁢

{

n

2

n

1

⁢

sin

⁡

[

α

-

sin

-

1

⁡

(

n

1

n

2

)

]

}

,

(

2

)

wherein θ i is an incident angle of the light beam entering the light incident surface, n 1 is a refractive index of the environment medium, and n 2 is a refractive index of the light guide device.

2. The image capture apparatus according to claim 1 , wherein the light source is disposed outside the light guide device and located in the environment medium, and wherein refractive indices of the light guide device and the transparent device are the same.

3. The image capture apparatus according to claim 1 , further comprising:

a first reflection device, disposed on the top surface of the light guide device and located between the transparent device and the light guide device; and

a second reflection device, disposed on the bottom surface of the light guide device, wherein the light beam passing through the light incident surface is sequentially reflected by the first reflection device and the second reflection device and expanded, the expanded light beam is totally reflected by the interface between the transparent device and the environment medium, and the image capture device is disposed more adjacently to the transparent device than the second reflection device.

4. The image capture apparatus according to claim 3 , wherein the light guide device further comprises:

an inner wall, wherein the light emitting surface is more adjacent to the transparent device than the bottom surface, the inner wall is connected between the bottom surface and the light emitting surface, a recess is formed by the inner wall and the light emitting surface, and the image capture device is disposed in the recess.

5. The image capture apparatus according to claim 3 , wherein the image capture device has a light receiving surface facing the light emitting surface, and the light receiving surface and the light emitting surface are inclined with respect to the top surface.

6. The image capture apparatus according to claim 1 , wherein the image capture device has a light receiving surface facing the light emitting surface, and the light receiving surface and the light emitting surface are inclined with respect to the top surface.

7. The image capture apparatus according to claim 1 , wherein the light beam is invisible light.

8. An image capture apparatus, located in an environment medium, comprising:

a light guide device, comprising:

a top surface;

a bottom surface, opposite to the top surface;

a light incident surface, connected between the top surface and the bottom surface; and

a light emitting surface, opposite to the top surface, wherein the bottom surface is connected between the light incident surface and the light emitting surface;

a transparent device, disposed on the top surface of the light guide device;

a light source, used to emit a light beam;

an image capture device, disposed on the light emitting surface of the light guide device;

a first reflection device, disposed on the top surface of the light guide device and located between the transparent device and the light guide device; and

a second reflection device, disposed on the bottom surface of the light guide device, wherein the light beam passing through the light incident surface is sequentially reflected by the first reflection device and the second reflection device and expanded, the light beam reflected by the first reflection device and the second reflection device is totally reflected by an interface between the transparent device and the environment medium, and the image capture device is disposed more adjacently to the transparent device than the second reflection device.

9. The image capture apparatus according to claim 8 , wherein the light guide device further comprises:

an inner wall, wherein the light emitting surface is more adjacent to the transparent device than the bottom surface, the inner wall is connected between the bottom surface and the light emitting surface, a recess is formed by the inner wall and the light emitting surface, and the image capture device is disposed in the recess.

10. The image capture apparatus according to claim 8 , wherein θ i is an incident angle of the light beam entering the light guide device through the light incident surface, and θ i satisfies a formula (3):

θ

i

≤

π

-

sin

-

1

⁡

(

n

1

n

2

)

,

(

3

)

wherein n 1 is a refractive index of the environment medium, and n 2 is a refractive index of the light guide device.

11. The image capture apparatus according to claim 8 , wherein the image capture device has a light receiving surface facing the light emitting surface, and the light receiving surface and the light emitting surface are inclined with respect to the top surface.

12. The image capture apparatus according to claim 8 , wherein the light beam is invisible light.