Signal light detection device and method of detecting signal light

A signal light detection device comprising:

• an AD converter circuit which converts an electric signal, output from a photo-electric converter unit which converts signal light into an analog electric signal, into a digital signal;
• and
• a correction unit which corrects said digital signal,
• said AD converter circuit comprising;
• a comparator which judges whether said electric signal reaches or exceeds a reference voltage, or not;
• and
• a reference voltage modifier unit which elevates said reference voltage, after a predetermined time period elapses since said comparator judged that said electric signal reached or exceeded said reference voltage, and
• said correction unit calculates an incidence time which represents a time when said signal light starts to enter said photo-electric converter unit, by correcting a rise-up time of said electric signal when it exceeds said reference voltage, based on a pulse width which represents a time period from when said electric signal reaches or exceeds said reference voltage, up to when said electric signal falls below said reference voltage.