| 摘要 |
A light emitting device and light receiving device are positioned to face each other to sandwich the developer cartridge. A pair of light transmitting tubes is installed in the casing of a developer cartridge. Light emitted from the light emitting device travels inside the casing via the light transmitting tubes and is received by the light receiving device. The direction of light traveling through light transmitting tubes changes along the light path to the light receiving device. When a developer cartridge is attached to the housing, toner blocks light from traveling through the toner chamber to the light receiving device. When the toner is consumed and the developer cartridge is ready to be replaced, light passes through the toner chamber and is received by the light receiving device, prompting exchange of the developer cartridge.
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