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LED Printhead Technologies

Electro-photography printers and photocopiers form latent images by exposing a charged photoreceptor drum to dots of light. As shown in Figure 1, one of the following two image-writing systems is generally used to form latent images: a raster output scanner (ROS), which uses a laser and rotating polygon mirror, or an LED printhead, which incorporates a light-emitting diode (LED) as a light source. Since LED printheads do not require a scanning optical system, they offer certain advantages over the conventional and widely deployed ROS system, including smaller dimensions and essentially noise-free operation. However, LED printheads also feature certain disadvantages, including the difficulty of maintaining uniform exposure due to the LED printhead's many light sources. There are also additional difficulties in maintaining uniform print density.
Tackling these challenges, Fuji Xerox has developed a high-precision, multifunctional light intensity calibration technology and achieved commercial production of a new LED printhead that provides high image quality while making it possible to build even smaller devices.

Figure 1: ROS System and LED Printhead
Figure 1: ROS System and LED Printhead

As shown in Figure 1, the LED printhead offers the following principle advantages over the ROS system:

  • Each light-emitting element forms a single pixel, eliminating the need for a scanning optical system and reducing product weight and dimensions.
  • No motor unit is required for a polygon mirror, eliminating the associated vibration and operating noise.

Using the newly-developed Self-scanning Light-emitting Devices (SLED), which reduce the number of wiring required, Fuji Xerox has succeeded in developing a new, even smaller LED printhead. These advances have resulted in an overall printhead volume 1/40 that of the conventional Fuji Xerox ROS system. Implementation of this new LED printhead into multifunction devices will enable even further reductions in size, saving space equivalent to a paper tray.

Figure 2: Light-Emitting Part of the LED Printhead
Figure 2: Light-Emitting Part of the LED Printhead

Figure 3: Comparison: Volume of ROS System and LED Printhead
Figure 3: Comparison: Volume of ROS System and LED Printhead

An all-in-one driving IC chip was developed concurrently to address issues related to exposure non-uniformity, a problem that had resisted resolution, particularly at higher resolutions. This new chip combines two separate circuits: an LED driving circuit and a newly-developed algorithm circuit DELICIS (Digitally-Enhanced Lighting Control Imaging) that adjusts the properties of the LED and lens. With this chip, the new LED printhead can achieve image quality and resolution (1200 x 2400dpi) equivalent to or better than ROS systems.

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