CIRCUIT FOR MEASURING AND EVALUATING OPTICAL RADIATION

• US 3,813,540 A
• Filed: 03/26/1973
• Issued: 05/28/1974
• Est. Priority Date: 03/26/1973
• Status: Expired due to Term

• Current Assignee: Peter Albrecht
• Original Assignee: Peter Albrecht
• Inventors: Albrecht, Peter
• Primary Examiner(s): Lawrence, James W.
• Assistant Examiner(s): Nelms, D. C.
• Application Number:
  US05/344,824
• Time in Patent Office: 428 Days
• Field of Search: 250/219D,219DC,219DD,206,208,214,231R,231SE,221,222 307/311 235/61.11E 340/146.3J
• US Class Current: 250/214.0RC
• CPC Class Codes: G01N 21/27 using photo-electric detection G01N21/31 takes precedence; circuits for computing concentration logarithmic circuits G06G7/24; photometric circuits in general G01J
• CPC Class Codes: G06K 7/10851 Circuits for pulse shaping, amplifying, eliminating noise signals, checking the function of the sensing device basic electronic circuitry H03

Abstract

The invention relates to a circuit for optically sensing coded data on a record medium and including a photosensitive transducing element such as a phototransistor. In order to render the circuit independent of background brightness variations, the load impedance of the transducing element includes one or more of series connected diodes which have an exponential characteristic curve, so that the voltage drop across the diode or diodes is proportional to the natural logarithm of the current flowing through the transducing element. The voltage difference resulting from sensing contrasting marks on the record medium thus depends only on the contrast in reflected light and not on the absolute value of the current in the transducing element.

Claims

1. A circuit for optically reading a data record medium on which data is recorded in the form of marks which have a different reflectivity from the record medium;

a phototransistor for sensing optical radiation, said phototransistor operating in its linear region and having a first current value associated with the background of the record medium and a second current value associated with the marks;

a load coupled to said phototransistor, the impedance of said load having an exponential voltage current characteristic such that the voltage drop across said load Is proportional to the logarithm of the current flowing through said impedance, wherein said first current value causes a corresponding first voltage value to appear across said load impedance and said second current value causes a corresponding second voltage value to appear across said load impedance;

analog storage means for storing a voltage proportional to said first voltage value;

a first differential amplifier means for receiving at one input thereof the voltage across said load and at another input thereof a portion of the output signal of said first differential amplifier means;

a voltage divider for receiving at one end the remaining portion of the output signal from said first differential amplifier means, said voltage divider including at least one diode connected in parallel with first and second serially connected resistors;

a peak voltage equalizer including a second differential amplifier means with feedback and having a diode in series with the output of said second differential amplifier means, said peak voltage equalizer connecting the other end of said voltage divider to said analog storage means;

a third differential amplifier means having one input connected to said analog storage means and to the other input of said second differential amplifier means, the other input of said third differential amplifier means being connected to a point between said first and said second serially connected resistors of said voltage divider, such that the reference voltage switching point of said third differential amplifier means is determined by the value of said stored voltage, said third differential amplifier means being switched by the application to said other input thereof of a voltage proportional to said second voltage value.

2. The circuit according to claim 1, wherein a limiting circuit and a fourth differential amplifier means operating as a bistable element are associated with the output of the third differential amplifier means.