Laser Printer and Photocopiers By Rahul Prasad 2007PH10615

Gary Starkweather, who was a researcher at the Xerox Corporation invented the first laser printer, way back in Lasers Printers produce high quality non-impact prints using electrophotography. Photoreceptor material is generally amorphous Si or Organic Photoconductor(OPC) material. Why lasers are used? Because they are coherent source of light. So Intensity remain same over long distance and focused down to very small spot sizes for high resolution.

Laser Printing System Laser Beam Scanning Optics Unit. Electrophotography Process Unit. Paper Transporting Unit.

Working Input Signal Charging Exposure Developing Transferring Fusing Cleaning Discharging

Laser Beam Scanning Unit Laser Source. Collimator Lens. High Speed Rotating Polygon Mirror. F – Ѳ Lens. Reflecting Mirrors.

Laser Printer Light Source Must be Durable and highly Stable. Must be possible to modulate their Intensity at higher speed. Emission wavelength must be matched to the photoconductor sensitivity. Must be compact. Must be inexpensive. Must have short warm time.

Characteristics of Laser used Low Power Semiconductor Lasers(laser diode). AlGaAs Lasers commonly used. Wavelength, λ = nm. Typical power required = 5mW. Typical Spot size = 50 – 100μm(600 dpi). Can modulate laser output intensity directly by changing current.

Collimator Lens The light emitting parts of diode lasers are extremely small; light emitted from them has a large divergence angle, so it is necessary to mount a collimator lens next to laser to couple output light in the system. Converts light from laser diode to parallel rays. It aids in scanning and provides better converge to a dot.

Rotating Polygon Mirror 6 sided mirror. Deflect the laser beam, by which it scans the surface of photoreceptor drum for Individual Lines. Stable line spanning speed. Revolves at a speed of around 25,000 revolution min -1 Due to tilting of reflecting facets, there will be scan pitch irregularity. Hence, Facet Correction lens is introduced.

F-Ѳ lens Compensate image aberrations in the focused laser spot on the drum caused by the wide deflection angle of the scanned laser beam. Image velocity is proportional to the angular velocity of the scanning mirror. To have constant spanning speed across the surface of the OPC Drum. Light is refracted to parallel as it passes through the lens to ensure that dots at the edge of drum are at the same distance apart as the dots in the centre of the drum. Hardly affected at all by changes in wavelengths of the laser diode.

Colour Printers Single Photoconductor Drum. Four coloured toners of Cyan, Magenta, Yellow, Black. Same Process repeated four times. Speed decreases. To increase the speed, four drum printers(tandem systems) are used.

Future Trends Higher Speed i. Increasing Polygonal mirror speed, but have limits(for 25k, 35k, 60k rev m -1 ball bearings, Dynamic fluid bearings, Dynamic air bearings have to be used.) ii. Increasing the number of beams. Higher image Density. Higher Precision.

References Handbook of Laser technology and Applications; Vol. III Applications; Collin E. Webb, Julian D. C. Jones. Multifunction Peripherals for PC: Technology, Troubleshooting and Repair; Marvin Hobbs. IEEE Explorer; Diode Lasers for Mass Market Applications: Optical Recording and Printing; Donald B. Carlin, Yoshito Tsunoda. Sam’s Laser FAQ, Samuel M.Goldwasser Wikipedia.

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