Imaging apparatus including light source, photodetector, and control circuit

#The present disclosure relates to imaging apparatuses.

#In the field of biometry, materials analysis, and so on, a method is used in which a subject is irradiated with light and information on the inside of the subject is acquired from information on the light that has been transmitted through the inside of the subject. In this method, there arises a problem that a component reflected from the surface of the subject and a component scattered directly underneath the surface are mixed into the aforementioned light. In particular, in biometry, the intensity of a surface reflection component and a subsurface scattering component is four to five orders of magnitude larger than the intensity of a component scattered inside an organism, and it is desirable that these surface reflection component and subsurface scattering component be removed as much as possible in order to acquire the component scattered inside the organism. In the field of biometry, for example, methods of acquiring only the desired information on the inside by removing the aforementioned components include a method disclosed in Japanese Unexamined Patent Application Publication No. 8-103434. Japanese Unexamined Patent Application Publication No. 8-103434 discloses a method in which measurement is carried out in a state in which a light source and a photodetector are in tight contact with a site to be measured with a certain space being provided between the light source and the photodetector.

#In one general aspect, the techniques disclosed here feature an imaging apparatus that includes a light source that includes a diffusion plate and, in operation, emits pulsed light toward a subject, the pulsed light diverging at a divergence angle greater than 0 degrees; a photodetector including a photoelectric converter that, in operation, receives light from the subject and converts the light to an electric charge and an electric charge accumulator that, in operation, accumulates the electric charge, the photodetector, in operation, generating an electric signal based on the electric charge accumulated in the electric charge accumulator; and a control circuit that, in operation, controls the light source and the photodetector. The control circuit, in operation, causes the electric charge accumulator to start accumulating the electric charge when a predetermined period of time has passed after the control circuit has caused the light source to start emitting the pulsed light, and thus causes the electric charge accumulator to accumulate the electric charge corresponding to a component, among the light from the subject, that is scattered inside the subject. The pulsed light satisfies the following expression: 81.5≤R+d/(2 tan θ), where R (mm) is a distance from the diffusion plate to the subject, θ (degree) is the divergence angle, and d (mm) is a spot size of the pulsed light on the diffusion plate.

#FIG. 1 is a schematic diagram illustrating an imaging apparatus according to a first embodiment;

#FIG. 2 illustrates a time response waveform of an optical signal that reaches a photodetector, a relationship between a shutter timing and a detected optical signal, and a relationship between a shutter start phase and the detection intensity of a detected optical signal;

#FIG. 3A illustrates a change in the density of irradiation light associated with movement of a subject;

#According to the detailed study by the inventor of the present application, it is possible to reduce the proportion of the subsurface scattering component contained in a detection signal and to increase the detection amount of the scattered component of light that has reached the brain with the method disclosed in Japanese Unexamined Patent Application Publication No. 8-103434. However, this method requires that the irradiation point and the detection point be spaced apart by 3 cm, and the spatial resolution of the obtained brain activity distribution may decrease.

#In the meantime, an image sensor capable of high-speed imaging for measuring the distance to a subject through a time-of-light (TOF) system has been developed in recent years. Such an image sensor has a high time division resolution. Thus, by controlling the shutter of the image sensor at high speed, a component scattered inside the brain may be detected while greatly reducing the surface reflection component and the subsurface scattering component having a high intensity. Specifically, in a case in which an organism is irradiated with pulsed light and the light reflected by the organism is imaged, the light reflected near the surface of the organism has a relatively shorter optical path and thus reaches the image sensor at an earlier timing, whereas the light reflected inside the organism has a relatively longer optical path and thus reaches the image sensor at a later timing. Therefore, by adjusting the shutter such that the trailing end portion of the pulsed light returning to the image sensor is detected, the component scattered inside the brain having a relatively longer optical path length and having a time delay can be detected efficiently. This method of detecting components with a temporal separation makes it possible to detect a brain signal directly underneath the irradiation point, and thus it is considered that the brain activity distribution with a higher resolution can be acquired as compared to the method disclosed in Japanese Unexamined Patent Application Publication No. 8-103434.

#However, when a signal from a subject is detected in a contactless manner, movement of the subject leads to a change in the amount of an optical signal detected by the sensor, which results in an output of an erroneous detection value. In particular, when a very tiny amount of a component scattered inside the brain is to be detected as in the cerebral blood flow measurement, an influence of a noise component associated with movement is large, and the S/N ratio in the detection of the component scattered inside the brain is reduced. In view of the issue described above, the inventor of the present application has conceived of an imaging apparatus having a novel structure. The overview of the imaging apparatus according to the present disclosure is as follows.