Optical sensor and apparatus comprising an optical sensor

An ambient light sensor, comprising:

• a plurality of pixels;
• wherein each pixel comprises a photodetector;
• and
• a readout circuit configured to determine an ambient light level, said readout circuit comprising:
• a reset transistor coupled between a reset voltage node and the photodetector;
• a source-follower transistor having a control terminal coupled to the photodetector and an output terminal configured to provide a photodetector voltage;
• a comparator having a first input configured to receive the photodetector voltage and a second input configured to receive a threshold voltage, wherein the comparator is configured to assert a reset signal applied to a control terminal of the reset transistor;
• and
• a first counter circuit having an input configured to receive the reset signal, said counter circuit configured to count a number of times the reset signal is asserted over an exposure time, wherein the number of times counted is indicative of the ambient light level.