Registers – Coded record sensors – Particular sensor structure
Patent
1996-12-10
1998-10-27
Hajec, Donald T.
Registers
Coded record sensors
Particular sensor structure
235454, G06K 710
Patent
active
058280456
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention refers to the field of electronic reproduction technology and is directed to an optoelectronic recording element for a recording device, also referred to as a recorder or exposer, for the point-by-point and line-by-line recording of information on a recording material.
Such a recording element is essentially composed of a light source for generating a brightness-modulated light beam, of a drive circuit charged with an image signal for the light source, of an objective arranged on the optical axis of the light source for focussing the light beam onto the recording material, and of a diaphragm for limiting the light beam. The image signal contains the information to be recorded. The light beam modulated in brightness by the image signal undertakes the point-by-point and line-by-line exposure of the recording material, for example a film. A laser diode whose emitted light power is defined by a driver current through the laser diode is often employed as light source. The driver current is generated in the drive circuit dependent on the image signal.
For exposing two-level black/white information, the laser diode operates in switched mode wherein the light output by the laser diode is switched on and off by the driver current. In order to achieve a high recording quality, the level of the emitted light must be constant in the on-time intervals of the laser diode. Based on its very nature, a laser diode does not meet the demand for a constant light level in the on-time intervals since the emitted light power of a laser diode is temperature-dependent.
It is known to stabilize the light power emission of a laser diode with a light regulation. To that end, the light emitted by the laser diode is measured with a light detector and the measured signal generated by the light detector is returned as actual value to a regulator for the drive current via a feedback branch.
The light detector is, for example, a photodiode, also referred to as monitor diode or PIN diode. The laser diode and the monitor diode optically coupled thereto are often integrated in a common housing. The employment of an internal monitor diode has the disadvantage that the measured signal it generates is small and noise-infested, so that a good control quality and, thus, a high stability of the light power are thereby not achieved.
The employment of an external monitor diode, i.e. a monitor diode that is not accommodated in a common housing with the laser diode, is disclosed by EP-A-0 511 354. The optical coupling between the laser diode and the locally offset monitor diode therein ensues via a planar splitter mirror arranged in the beam path of the light beam that couples a portion of the light beam out and reflects it onto the external monitor diode. The use of a splitter mirror has the disadvantage that the light power of the light beam employed for the exposure is reduced and a lens is additionally required that focusses the portion of the light that has been coupled out onto the monitor diode. Moreover, a relatively long light path between laser diode and monitor diode derives in the known arrangement, so that the measured signal is correspondingly time-delayed and, thus, the regulation is considerably limited in bandwidth.
SUMMARY OF THE INVENTION
It is an object of the invention to improve upon an optoelectronic recording element with a laser diode such that a precise and fast regulation of the light power output by the laser diode is achieved.
According to the invention, an optoelectronic recording element is provided for point-by-point and line-by-line recording of information on a recording material. A laser diode is provided for generating a light beam along an optical axis. An objective is arranged on the optical axis for focusing the light beam onto the recording material. A light detector is arranged spaced from the laser diode for measuring a light power of the light beam. A reflector is arranged in the beam path of the light beam for reflecting a part of the light beam onto the light det
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Frech Karl
Hajec Donald T.
Linotype-Hell AG
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