In an indirect capture flat-panel detector, which component converts visible light into electrons?

• A. Thin-film transistor (TFT) array
• B. Photodiode
• C. Scintillation layer
• D. Amorphous Selenium detector

Answer: (B)

In indirect capture detectors, the visible light produced by the scintillation layer is converted into electrons by the photodiode. The scintillator first turns X-rays into visible photons, and the photodiode absorbs those photons to generate electron–hole pairs, creating an electrical signal that the thin-film transistor (TFT) array then reads and stores. The TFT array handles the readout, while the scintillation layer provides the light and amorphous selenium is used in direct detectors (not indirect) to convert X-rays directly to charge. So the photodiode is the layer that converts visible light into electrons.