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An implantable image sensor device is extremely small and lightweight owing to the use of a specially designed complementary metal oxide semiconductor (CMOS) image sensor. It is intended to be applied in in-vivo brain function imaging under free behavior of an observation target such as a mouse. Because of its low invasiveness, it is suitable for the simultaneous imaging of the deep brain along with several other regions of the brain, and for long-term observations. In order to realize low invasiveness, it is required to perform contact imaging without using a lens. This gives rise to a different set of challenges from that faced by microscope systems that use a general lens. In this study, we developed a light source suited for the in-vivo implantation of a fluorescence imaging system to facilitate green fluorescence observation. A thinned InGaN-based blue LED was used as an excitation light source. Normally, an LED has an emission bandwidth that is too broad for fluorescence imaging. In the proposed device, in addition to the interference filter, absorption layers are added in order to remove the unwanted light emission component by using a different strategy. The interference filter used was obtained by transferring a long-pass filter formed on a quartz glass filter image onto a fiber optic plate (FOP) with a low numerical aperture (NA) by the laser lift-off (LLO) method. The interference filter showed sufficiently high performance as an excitation filter even after it was transferred to the light source. The absorption layers composed of a low-NA FOP and a blue absorption layer are used to remove unwanted components. Light rays with a high incident angle, leak through the interference filter when the absorption layers are not present. On the other hand, the unwanted components in the emission bandwidth were successfully reduced in the proposed structure.
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