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The reason for ion traps is that before aluminized screens(a coating of vaporized aluminum over the phosphor which reflects light towards the viewer and shields the phosphor from ions) the unprotected phosphor screen was vulnerable to damage from being hit by ions. This effect known as ion burn is common in pre-ion trap era CRTs and manifests itself as a browning of phosphors in the center region of the screen(this brown region also does not get as bright as the surrounding area).
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