The present invention relates to a personal identification apparatus using a living body. In particular, the present invention relates to a biometric personal identification apparatus based on a blood vessel (vein) pattern of a finger.
As a security scheme that does not need carrying a key or the like, that is high in convenience, and that is little fear of illegal use such as loss and theft, attention is paid to biometrics using a part of an individual's body, such as a finger print, iris, a blood vessel (vein) pattern. Among them, the identification method using a blood vessel pattern does not remind of criminal investigation unlike the identification method using a fingerprint. The identification method using a blood vessel pattern does not irradiate light directly to an eyeball unlike the identification method using iris. Therefore, the identification method using a blood vessel pattern causes little psychological resistance feeling. Furthermore, since the blood vessel pattern is not a feature of a living body surface which can be observed easily, but it is an inside feature, the identification method using a blood vessel pattern has an advantage that the residual property is not present and forgery is difficult.
Such a blood vessel pattern within a living body is obtained by illuminating a target region with near-infrared light sources and picking up an image of the target region with an image pickup system, such as a camera or an image sensor, having a sensitivity for near-infrared light (see, for example, US2004/0184641). Since hemoglobin in blood absorbs near-infrared light well, light is absorbed by a blood vessel and the blood vessel is taken dark as compared with a peripheral tissue. A pattern generated by the difference between brightness and darkness becomes the blood vessel pattern.
In such a scheme for picking up an image of a blood vessel pattern using light, existence of light other than light from the infra-red light sources for image pickup, i.e., external light remarkably affects the quality of the blood vessel pattern image picked up. In most biometric schemes using light used heretofore, therefore, restrictions have been imposed on the use condition under the external light. For example, when the biometric scheme is used outdoors in clear weather, light having near-infrared wavelengths is also included in naturally existing sunlight and its intensity is far higher than that of the output of the light sources for blood vessel pattern image pickup. If sunlight streams into an identification apparatus including a camera adjusted so as to be able to optimally pick up an image of a slight difference between bright light and dark light as a pattern, luminance of many pixels is saturated at its maximum value because of intense light, and an image including a saturated highlight region is obtained. In such a saturated highlight region, the luminance is uniquely at the maximum value. Therefore, the difference between brightness and darkness in the blood vessel portion does not appear, and a correct blood vessel pattern cannot be obtained.
In the invention described in US2004/0184641, a method of adjusting the brightness of the light sources for image pickup according to various finger states has been disclosed as the method for picking up an image of a blood vessel pattern clearly. However, the method is premised on the indoor use, and the supposed range of the external light variation is also limited. Under the intense external light as described above, the saturated highlight region cannot be eliminated in many cases even if the intensity of the light sources is minimized. Furthermore, in the case described above, light irradiated to the finger cannot be utilized suitably although it has a quantity enough to pick up an image of the blood vessel pattern. As a result, energy is supplied to the light sources dedicated to image pickup unnecessarily.
A conceivable solution to the problem is to eliminate the saturated highlight region by using sensitivity adjustment, such as the exposure or iris of the image pickup camera, together. In ordinary photographing as well, the sensitivity, such as the exposure or iris, of the camera is adjusted and the brightness of illumination such as the light is suitably changed. Basically, this should be conducted automatically. Not a few cameras have a mechanism called AGC (auto gain control) as a standard component. The AGC is a function of amplifying an output of image pickup elements, such as CCDs (Charge-Coupled Devices), in a camera to bring the output into a predetermined voltage range. Specifically, the amplification factor of the amplifier is automatically adjusted. The amplification factor is increased when the output of the image pickup elements as a whole is low. Conversely, the amplification factor is decreased when the output is high. As a result, it becomes possible to automatically obtain an optimum picture quality according to the brightness of the image pickup target.
However, the mere application of the AGC to the camera for picking up an image of a blood vessel does not make it possible to obtain a clear blood vessel pattern. The blood vessel pattern cannot be obtained until light of a suitable quantity is transmitted. Therefore, suitable image pickup is impossible unless the sensitivity is controlled after it is ascertained well that light for picking up an image of the blood vessel pattern is sufficiently supplied. Conversely, if the light source power is fixed without conducting light source power control at all, light continues to be added from the light sources even if intense external light is present. As a result, the saturated highlight region in the image pickup region is further aggravated, and departure from a range in which the saturated highlight region can be eliminated by the sensitivity control is also caused. It is necessary to suitably control the power of the light sources dedicated to irradiation for image pickup and the sensitivity of the camera according to the situation so as to cause a living body region to be picked up as an image to always produce a constant picture quality. Unless there is such cooperation, the power of the light sources and the sensitivity of the camera might interfere with each other in attempting a suitable picture quality and eventually convergence to a suitable picture quality might be not attained. Furthermore, in the ordinary AGC, picture quality control is exercised to optimize the picked up image as a whole. Therefore, the finger portion in the picked up image does not necessarily become optimum in picture quality. For example, when a great difference is caused in brightness between the inside of the finger and the background by external light, or when only a portion of the finger directly exposed to light from the light sources for image pickup is locally bright, the gain is set to an excessively low value by strong influence of the brightness and consequently the finger portion for which an optimum picture quality should be originally obtained becomes too dark in its image. It is also conceivable to restrict an image range in which it is determined whether the picture quality is optimum. At the time of identification, however, the finger is placed or detached. Even if the finger is placed stationarily, the finger is not always placed in the same specific position. Therefore, the brightness of the picked up image has an infinite variety according to states of the finger.
Even if external light having intensity enough to obtain the blood vessel pattern is applied to the finger, it cannot be utilized sufficiently in situations where the light source power control and the sensitivity control in the AGC or the like operate asynchronously. For example, if the AGC operates when the power of the light sources for image pickup is high and an optimum picture quality is attained, there was a possibility that the light source power could be lowered if the gain was raised. This is wasteful energy consumption. Especially in the case of energy supply using a battery or the like, the sustaining time of the battery is shortened and there is a problem from the viewpoint of preservation of the global environment as well.