There are three types of cone cells in the retina that are called L-cones (sensitive to red light), M-cones (sensitive to green light), and S-cones (sensitive to blue light). The coordinated response of cone cells to visible light in the wavelength range of 400-780 nm lets us see the colorful world. In the wavelength range of invisible X-ray (1 pm—10 nm), X-ray detectors perform the same function as “eyes.” However, X-ray detectors generally apply charge-integration-mode detection, which cannot distinguish wavelength and energy. Therefore, digital radiography and CT examinations can only obtain grayscale imaging.
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Source: Phys.org