1. Field of the Invention
The present invention relates to an optical fingerprint acquisition apparatus capable of acquiring a fingerprint image without distortion by correcting rectangular deformation and trapezoidal distortion that may occur in an optical fingerprint acquisition process using a light refractor such as a prism.
2. Discussion of Related Art
Due to its unique and unchangeable nature, a user's biological information is widely used for personal authentication with information equipment. Among the user's biological information, a fingerprint is a means for authentication that has attracted the most attention and has been generalized due to its simple structure and convenience.
A fingerprint acquisition apparatus is an apparatus for acquiring an image of a user's fingerprint for the purpose of authentication, etc., and is generally combined with a means for extracting minutiae data from the acquired fingerprint image. For example, the minutiae data obtained from the fingerprint image is used for fingerprint authentication. Although there are other fingerprint acquisition methods, optical fingerprint acquisition methods using an optical device such as a prism are most widely used.
Furthermore, the optical fingerprint acquisition methods are generally classified as a method in which a fingerprint image is determined according to absorption of light at ridges of a fingerprint (a so-called “absorption type”), and a method in which a fingerprint image is formed by light scattered by the ridges (a so-called “scattering type”).
FIG. 1 illustrates an example of an optical fingerprint acquisition apparatus using a prism. A fingerprint acquisition apparatus 100 includes a prism 10 that comes in contact with a user's fingerprint, a light source 11 that emits light toward the prism 10, an image sensor 13 that finally outputs a fingerprint image as a digital signal, and a lens 15 that projects the fingerprint image emitted from the prism 10 onto the image sensor 13.
When the user's finger comes in contact with a fingerprint contact surface 10a of the prism, light emitted from the light source 11 is incident on the fingerprint contact surface 10a through the prism 10. The light reflected, scattered or refracted according to the shape of a fingerprint in contact with the fingerprint contact surface 10a is emitted to the lens 15 to create an image and is input to the image sensor 13.
When the fingerprint acquisition apparatus 100 is the scattering type, light emitted from the light source 11 is incident on a fingerprint contact surface 10a at right angles or a smaller angle than a threshold angle for total reflection. Since the light is refracted and travels without total reflection at grooves of a fingerprint and scatters at ridges of the fingerprint, an image in which the ridges of the fingerprint are bright and the grooves are dark is acquired.
When the fingerprint acquisition apparatus 100 is the absorption type, light emitted from the light source 11 is incident on the fingerprint contact surface 10a at a greater angle than the threshold angle for total reflection and is totally reflected at the grooves. Thus, an image in which the grooves of a fingerprint are bright and ridges are dark is acquired.
In a fingerprint image emitted from the prism 10, various distortions may occur. For example, a square image having one side length of 2×a on the fingerprint contact surface 10a looks remarkably reduced in up-down direction (vertically) after passing through the prism 10.
This is because an apparent image of an image projected onto the lens 15 is distorted due to refraction and so on. Light reflected by the fingerprint contact surface 10a of the prism 10 is refracted at a surface of emission 10b and projected onto the lens 15. Thus, when an image including object points A, B and C disposed at the same intervals on the fingerprint contact surface 10a is actually projected onto the lens 15, an apparent image is formed by virtual object points A1, B1 and C 1 disposed at the same intervals of b1.
As shown in the drawing, since the interval a>the interval b1, a vertical length of the fingerprint image becomes 2×b1 and looks smaller than an actual length of 2a. On the other hand, a horizontal length 2a of the fingerprint image almost does not change and is projected as is, and thus a square image on the fingerprint contact surface 10a looks remarkably reduced in the up-down direction (vertically) after passing through the prism 10. In practice, the square image does not look like a rectangle whose vertical length is simply shortened but rather looks like a trapezoid due to distortion caused by other optical factors.
Meanwhile, a fingerprint acquisition apparatus may acquire fingerprints of one finger at a time or all fingers at once. In the latter case, a size of the prism 10 proportionally increases, but a fingerprint image is distorted far more than in the former case.