Conventional capsule-type endoscopes contain, for example, an objective optical system, an illumination means, an image pickup element, and a transmission means within a capsule-shaped shell. A cover member (hereinafter termed a transparent cover) that is generally spherical in shape and transparent within the field of view of the objective optical system seals these items within the capsule-shaped shell. Typically, a capsule-type endoscope having the above-discussed structure converts into signals in vivo images that are captured on a light-receiving surface of the image pickup element. These signals, of images that have been captured using light that has passed through the transparent cover and the objective optical system, are then transmitted externally by means of the transmission means. The transmitted signals are received by an external receiver device, and are then displayed on a display device for examination.
Capsule-type endoscopes as discussed above have a problem in that illumination light emitted from the illumination means is partly reflected by the inner surface of the spherical transparent cover and enters the entrance pupil of the objective optical system, which causes flare and significantly deteriorates the image contrast.
The inventions as disclosed, for example, in the following prior art patent documents have been proposed to prevent light that is reflected by the inner surface of the transparent cover from reaching the entrance pupil of the objective optical system and causing flare.
Japanese Laid-Open Patent Application 2003-325441 discloses a capsule-type endoscope in which the transparent cover has a spherical inner surface and the center of the entrance pupil of the objective optical system coincides with the center of curvature of this surface. Light that is emitted from the illumination means and reflected by the inner surface of the transparent cover is prevented from reaching the entrance pupil of the objective optical system, thus preventing flare. Also, Japanese Laid-Open Patent Publication 2003-501704 discloses an optical device in which the transparent cover has an ellipsoidal inner surface and multiple light sources that function as an illumination means are provided along a focal curve on the focal plane of the ellipsoid. Once again, the light that is emitted from the illumination means and reflected by the inner surface of the transparent cover is prevented from reaching the objective optical system and causing flare.
However, the capsule-type endoscope disclosed in Japanese Laid-Open Patent Application 2003-325441 requires that the illumination means be positioned around the objective optical system because of the structure that the center of the entrance pupil of the objective optical system coincides with the center of curvature of the inner surface of the transparent cover. This disadvantageously increases the size of the capsule-type endoscope.
As noted above, the light emitting elements that are used as the illumination means of the optical device disclosed in Japanese Laid-Open Patent Publication 2003-501704 must be located on the focal curve. Consequently, in order to provide sufficient room in which to mount these light-emitting elements, the transparent cover has to be increased in size, which requires that the size of the capsule-type endoscope itself be larger.
Furthermore, it is desired that the capsule-type endoscope described above provide a space for carrying a battery for ensuring sufficient operation time of the capsule-type endoscope or a tank for carrying a substance in liquid form. However, when an attempt is made to provide such a space in the prior art capsule-type endoscopes discussed above, the location of such a space must be such that no interference occurs between the objective optical system and the illumination means. This requirement tends to increase the size of the capsule-type endoscope.