What centers trap the electrons to form the latent image?

• A. Halogen vacancies (F centers)
• B. Europium ions
• C. Interstitial impurities
• D. Conduction band

Answer: (A)

Latent-image centers are created when electrons get trapped at defect sites in a crystal. Halogen vacancies, known as F centers, are exactly that kind of trap: an empty halide site in the lattice captures an electron. The trapped electron sits in the vacancy and forms a color center, preserving the exposure information as a latent image until development reveals it.

This is different from europium ions, which act as dopants to create luminescent centers but aren’t the primary electron-trapping sites for latent images. Interstitial impurities can create traps too, but the classic latent-image centers used to store the exposure are F centers at halogen vacancies. The conduction band, by contrast, is the range of energies for free, mobile electrons, not the localized traps that store the latent image.