1. Field of the Invention
The present invention relates to an imaging device.
Priority is claimed on Japanese Patent Application No. 2012-208597, filed Sep. 21, 2012, the contents of which are incorporated herein by reference.
2. Description of Related Art
In recent years, as a method of diagnosing cancer, a method using a fluorescent pigment which is typified by indocyanine green (ICG) has been actively studied. A technique is known which forms a filter using Fabry-Perot interference on an image sensor using a general CMOS process to manufacture an optical element for detecting light with a specific wavelength, such as fluorescence (for example, see Non-patent Document: “Nicolaas Tack, three others, A Compact, High-speed and Low-cost Hyperspectral Imager, SPIE, 2012”). Hereinafter, a pixel in which a Fabry-Perot filter is formed on a photodiode is referred to as a Fabry-Perot filter pixel (FPF pixel).
In addition, a technique is known which applies the Fabry-Perot filter to an endoscope to capture a fluorescent image, thereby separately capturing the image of a cancer tissue part using a fluorescent pigment (for example, see PCT International Publication No. WO2009/072177). Furthermore, a technique is known which generates an RGB image that is generally captured by an imaging device, such as a digital camera, from a fluorescent image to capture a visible image sensed by the human eye (for example, see Japanese Patent No. 3781927).
The technique disclosed in Non-patent Document: “Nicolaas Tack, three others, A Compact, High-speed and Low-cost Hyperspectral Imager, SPIE, 2012” will be described with reference to FIGS. 6 and 7. FIG. 6 is a schematic diagram illustrating an example of an FPF pixel according to the related art in which a Fabry-Perot filter is formed on a pixel of a CMOS image sensor. In the example shown in FIG. 6, a plurality of pixels F1 to F8 are formed on a semiconductor substrate 5-5. A photodiode (PD) 5-4 is formed in each of the pixels F1 to F8. In addition, dielectric layers 5-1 and 5-3 and an interlayer film 5-2 are formed, as the Fabry-Perot filter, on a light-receiving surface of each of the pixels F1 to F8. The interlayer film 5-2 which is formed above each of the pixels F1 to F8 is interposed between the dielectric layers 5-1 and 5-3. The thicknesses of the interlayer films 5-2 formed above the pixels F1 to F8 are different from each other such that light components in different wavelength bands are incident on the photodiodes 5-4 provided in the pixels F1 to F8. According to this structure, the pixels F1 to F8 can detect, for example, light in a narrow transmission band, as shown in FIG. 7.
FIG. 7 is a graph illustrating the wavelength of light passing through the Fabry-Perot filter according to the related art. In the graph shown in FIG. 7, the horizontal axis is a wavelength and the vertical axis is transmittance. In the example shown in FIG. 7, the Fabry-Perot filter (the dielectric layers 5-1 and 5-3 and the interlayer film 5-2) formed on the pixel F1 transmits light with a narrow band wavelength of about 420. Therefore, the pixel F1 can detect light with a narrow band wavelength of about 420 nm. Similarly to the pixel F1, the pixels F2 to F8 can detect light with a narrow band wavelength, depending on light with wavelengths transmitted through the farmed dielectric layers 5-1 and 5-3 and the farmed interlayer film 5-2. The band of light transmitted through the dielectric layers 5-1 and 5-3 and the interlayer film 5-2 formed on each of the pixels F2 to F8 is as shown in FIG. 7.
A fluorescent image can be captured by the pixels F1 to F8 shown in FIG. 6 and an RGB image can be generated based on the fluorescent image, as described in the related art. For example, in the technique disclosed in Japanese Patent No. 3781927, the spectral characteristics of blue light are generated based on the values detected by the pixels F1 to F3, the spectral characteristics of green light are generated based on the values detected by the pixels F4 and F5, and the spectral characteristics of red light are generated based on the values detected by the pixels F6 to F8. In this way, it is possible to acquire an RGB image.
For example, in the actual surgical operation, it is necessary to capture images of a part separated by a fluorescent pigment and the other parts at the same time and present the images to a doctor. Therefore, it is necessary to attach a device capable of capturing both an RGB image and a fluorescent image to the leading end of the endoscope. In addition, it is necessary to reduce the size of the leading end of the endoscope. The use of the above-mentioned technique makes it possible to generate both a fluorescent image with narrow-band sensitivity characteristics and an RGB image while reducing the size of the leading end of the endoscope.