1. Field of the Invention
The present invention relates to an apparatus for optically reading information directly from an uneven surface, for example, the inner surface of a fingertip when a fingerprint is to be taken, without the need of applying ink or the like to the surface.
2. Description of the Related Art
FIG. 14 in the accompanying drawings shows a conventional technique for taking a fingerprint. As illustrated in the figure, with the fingertip pressed against the surface 101a of a transparent plane-parallel plate 101, e.g., glass plate, the inner surface 50 of the fingertip is illuminated by a light source 102 disposed below the reverse surface 101b of the plane-parallel plate 101. In this case, light that is scattered by recesses the fingertip inner surface 50, which are not in contact with the surface 101a of the plane-parallel plate 101, enters the plane-parallel plate 101 after passing through an air layer.
Accordingly, the scattered light travels through the plane-parallel plate 101 at an angle smaller than the critical angle, as shown by the broken line, and hence passes through the reverse surface 101b of the plane-parallel plate 101, which is in contact with an air layer, and emerges into the air without being totally reflected at the reverse surface 101b.
However, light that is scattered by projections on the fingertip inner surface 50, which are in close contact with the surface 101a of the plane-parallel plate 101, travels through the plane-parallel plate 101 in all directions. Light that reaches the reverse surface 101b of the plane-parallel plate 101 at an angle larger than the critical angle is totally reflected back into the plane-parallel plate 101. Therefore, only the totally reflected light is taken in by an optical system comprising optical elements 103 to 107, to form an image on the image receiving surface of a solid-state imaging device 106, for example, thereby enabling dactylographic information to be read from the fingertip inner surface 50.
However, if there is water, e.g., sweat, rain, etc., (hereinafter referred to simply as "water") 51 in a recess in the fingertip inner surface 50, since the refractive index of water 51 is larger than that of the air, light that emerges from the water 51 travels through the plane-parallel plate 101 at an angle larger than in the case of incidence from the air, as shown by the arrow A. Accordingly, the scattered light from the recesses is totally reflected at the reverse surface 101b of the plane-parallel plate 101, thus making it impossible to accurately read dactylographic information from the fingertip inner surface 50.
When a trough portion of the fingertip inner surface 50 is filled with sweat, a false ridge appears between a pair of adjacent ridges on the fingertip inner surface 50 to form a bridge between the adjacent ridges. As a result, troughs which are filled with sweat are erroneously detected as ridges, as shown by reference symbol B in FIG. 15.
When dactylographic features are extracted from information read from the fingertip inner surface 50 wet with sweat as described above, bridge portions (indicated by B) are undesirably recorded as branch points. Since "bridges" produced by sweat have no reproducibility, if they are recorded as features, the fingerprint collating efficiency is lowered.
FIG. 16 shows one example of conventional approaches to cope with the above-described problem. As illustrated in the figure, light that is scattered back into a transparent member 201 by projections on the fingertip inner surface and then emerges into the air from a side surface of the transparent member 201 is detected at a position (in the hatched region 300) that cannot be reached by light that enters the transparent member 201 through a water layer filling a recess in the fingertip inner surface, and the detected light is subjected to image formation, thereby reading dactylographic information. In FIG. 16, reference numeral 202 denotes a light source, and 207 a camera [for example, see Japanese Patent Application Laid-Open (KOKAI) No. 63-211468 (1988)].
However, when an image-forming optical system is disposed in the region 300 that can be reached only by light that is scattered back into the transparent member 201 by projections on the fingertip inner surface and then emerges into the air from the transparent member 201, the image-forming optical system must be tilted with respect to the transparent member 201. Therefore, the conventional technique suffers from the disadvantage that a troublesome operation is required to handle the apparatus, for example, positional adjustment in actual use, and thus the conventional apparatus is difficult to use. In addition, the transparent member 201, which is formed, for example, from a prism or a plane-parallel plate, must be large in size. In the ease of a plane-parallel plate, a thickness of 17.47 mm or more is required.