Browse Prior Art Database

SCANNING BEAM DETECTOR

IP.com Disclosure Number: IPCOM000024692D
Original Publication Date: 1981-Oct-31
Included in the Prior Art Database: 2004-Apr-02
Document File: 2 page(s) / 122K

Publishing Venue

Xerox Disclosure Journal

Abstract

In a radiant energy beam scanning system, a detector positioned at a pre-determined position along the path scanned by the beam for sensing the center position of the power distribution within the beam and generating a signal indicative thereof. When utilized in an information recording system, the detector is positioned adjacent the start position of the scan path so that the generated indicator signal provides a reference for system timing and synchronization. In such a recording environment, synchronization of the system relative to the center position of power distribution within the beam compensates for variations in beam power level and focus.

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XEROX DISCLOSURE JOURNAL

SCANNING BEAM DETECTOR Dale C. Thomas

Proposed Classification
U.S. Cl. 250/236 Int. CI. HOlj 3/14

+ 5v

44

FIG. 2

Volume 6 Number 5 September/October 1981 227

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SCANNING BEAM DETECTOR (Cont'd)

In a radiant energy beam scanning system, a detector positioned at a pre- determined position along the path scanned by the beam for sensing the center position of the power distribution within the beam and generating a signal indicative thereof. When utilized in an information recording system, the detector is positioned adjacent the start position of the scan path so that the generated indicator signal provides a reference for system timing and synchronization. In such a recording environment, synchronization of the system relative to the center position of power distribution within the beam compensates for variations in beam power level and focus.

In a preferred form, a single diode detector partially masked in a cross-scan direction is located at a predetermined position in a scan path of the beam, e.g., adjacent the start of scan position. Figure 2 illustrates such a detector 10, across which there is shown a beam spot 20 traveling in the direction of arrow 22. Mask 24 covers a portion of the active diode area 12. As shown in Figure 1, the output signal produced when the diode detector is irradiated by the scanning beam is coupled to an input 32 of op amp voltage follower 30. The output of the op amp is filtered and stored as a reference signal on capacitor 38. Since the output of the voltage follower is a unity gain, in phase replica of the applied diode output signal, the stored reference signal represents the average total power within the incident beam. This reference signal is divided by the fixed ratio of the network comprising resistors 33 and 35 (preferably a division in half) and supplied to the non-inverting input of comparator 40. The diode output signal, which is coupled to the inverting input 44, initially sets the comparator low. As the beam crosses the edge of mask 24, the detector output signal decreases rapidly towards the zero level, pulling the non-inverting terminal 44 with it. When this falling output signal reaches the level of the fixed ratio of the reference signal stored on capacitor 38, comparator...