Calibrated image-sensor-based ambient light sensor

An electronic device may be provided with an image sensor for capturing digital images. The image sensor may be used as part of image-sensor-based ambient light sensing circuitry for producing ambient light sensor readings. The image-sensor-based ambient light sensing circuitry may include a reference array. The reference array may be formed from an array of light sensor elements that are matched to elements in the image sensor but that are covered with a light blocking material. Control circuitry can measure current flow into the reference array and the image sensor array and can use current measurements from these arrays in producing a calibrated ambient light sensor reading. The control circuitry may make current measurements by measuring a decay time associated with the voltage of a discharging capacitor. A comparator, pulse generator, and switch may be used in periodically recharging the capacitor. The capacitor may be adjusted to ensure accurate readings.