Photo-detecting apparatus

A photo-detecting apparatus comprising:

• a photodetector to output a signal current corresponding to an incident light intensity;
• an integrating circuit to store an electric charge corresponding to the signal current output from the photodetector and output a signal voltage corresponding to the amount of stored charge;
• a first CDS circuit having a first coupling capacitor and a first amplifier both connected serially in that order between its input terminal and output terminal, the input terminal being adapted to receive the signal voltage output from the integrating circuit, a first integrating capacitor parallelly connected between an input and an output of the first amplifier, and a first switch means for storing in the first integrating capacitor an amount of electric charge corresponding to a change in the signal voltage;
• a second CDS circuit having a second coupling capacitor and a second amplifier both connected serially in that order between its input terminal and output terminal, the input terminal being adapted to receive the signal voltage output from the integrating circuit, a second integrating capacitor having a capacitance value equal to that of the first integrating capacitor and parallelly connected between an input and an output of the second amplifier, and a second switch means for storing in the second integrating capacitor an amount of electric charge corresponding to a change in the signal voltage;
• and
• a difference calculation circuit to determine a difference between the amounts of charges stored in the first integrating capacitor of the first CDS circuit and in the second integrating capacitor of the second CDS circuit and output a signal voltage corresponding to the difference,
• a timing control circuit for controlling the operations of the integrating circuit, the first CDS circuit, the second CDS circuit and the difference calculation circuit, the photo-detecting apparatus being used along with a light emitting means for throwing a spotlight toward an object;
• wherein the timing control circuit performs the steps of:
• in a first period when the light emitting means is throwing the spot light onto the object, storing a first amount of electric charge in the first integrating capacitor of the first CDS circuit based on the change in the signal voltage output from the integrating circuit when the photodetector detects the spotlight component and a background light component;
• in a second period when the light emitting means is not throwing the spotlight onto the object, storing a second amount of electric charge in the second integrating capacitor of the second CDS circuit based on the change in the signal voltage output from the integrating circuit when the photodetector detects the background light component;
• and
• in a third period following the first and second periods, having the difference calculation circuit calculate a difference between the amount of charge stored in the first integrating capacitor of the first CDS circuit and the amount of charge stored in the second integrating capacitor of the second CDS circuit and then output a signal voltage corresponding to the difference from the difference calculation circuit.