A time-of-flight, TOF, imaging system (100, 400, 1100), comprising: an illuminator to transmit light pulses to illuminate a target object for determining a distance to the target object; an image sensor (130, 432, 1132) having a light-sensitive pixel array (200, 250, 440, 1136) to receive optical signals from the light pulses, the pixel array (200, 250, 440, 1136) including an active region and a feedback region; and an optical feedback device (140, 420) for directing a portion of the light from the illuminator to the feedback region of the pixel array (200, 250, 440, 1136), the optical feedback device (140, 420) including a preset reference depth; wherein the imaging system is configured to: in a calibration period, transmit a group of calibration light pulses to illuminate the target object; sense, in the feedback region of the pixel array (200, 250, 440, 1136), light from the optical feedback device (140, 420), using a sequence of calibration shutter windows characterized by delay times representing a range of depth; and calibrate TOF depth measurement reference information based on the sensed light in the feedback region of the pixel array (200, 250, 440, 1136); in a measurement period, transmit a first measurement light pulse to illuminate the target object; sense, in the active region of the light-sensitive pixel array (200, 250, 440, 1136), light reflected from the target object, using a first measurement shutter window with a first measurement delay time relative to the first measurement light pulse; transmit a second measurement light pulse to illuminate the target object; sense, in the active region of the light-sensitive pixel array (200, 250, 440, 1136), light reflected from the target object, using a second measurement shutter window with a second measurement delay time relative to the second measurement light pulse; and determine the distance of the target object based on light sensed using the first measurement shutter window and the second measurement shutter window and the calibrated TOF depth measurement reference information. an illuminator to transmit light pulses to illuminate a target object for determining a distance to the target object; an image sensor (130, 432, 1132) having a light-sensitive pixel array (200, 250, 440, 1136) to receive optical signals from the light pulses, the pixel array (200, 250, 440, 1136) including an active region and a feedback region; and an optical feedback device (140, 420) for directing a portion of the light from the illuminator to the feedback region of the pixel array (200, 250, 440, 1136), the optical feedback device (140, 420) including a preset reference depth; in a calibration period, transmit a group of calibration light pulses to illuminate the target object; sense, in the feedback region of the pixel array (200, 250, 440, 1136), light from the optical feedback device (140, 420), using a sequence of calibration shutter windows characterized by delay times representing a range of depth; and calibrate TOF depth measurement reference information based on the sensed light in the feedback region of the pixel array (200, 250, 440, 1136); transmit a group of calibration light pulses to illuminate the target object; sense, in the feedback region of the pixel array (200, 250, 440, 1136), light from the optical feedback device (140, 420), using a sequence of calibration shutter windows characterized by delay times representing a range of depth; and calibrate TOF depth measurement reference information based on the sensed light in the feedback region of the pixel array (200, 250, 440, 1136); in a measurement period, transmit a first measurement light pulse to illuminate the target object; sense, in the active region of the light-sensitive pixel array (200, 250, 440, 1136), light reflected from the target object, using a first measurement shutter window with a first measurement delay time relative to the first measurement light pulse; transmit a second measurement light pulse to illuminate the target object; sense, in the active region of the light-sensitive pixel array (200, 250, 440, 1136), light reflected from the target object, using a second measurement shutter window with a second measurement delay time relative to the second measurement light pulse; and determine the distance of the target object based on light sensed using the first measurement shutter window and the second measurement shutter window and the calibrated TOF depth measurement reference information. transmit a first measurement light pulse to illuminate the target object; sense, in the active region of the light-sensitive pixel array (200, 250, 440, 1136), light reflected from the target object, using a first measurement shutter window with a first measurement delay time relative to the first measurement light pulse; transmit a second measurement light pulse to illuminate the target object; sense, in the active region of the light-sensitive pixel array (200, 250, 440, 1136), light reflected from the target object, using a second measurement shutter window with a second measurement delay time relative to the second measurement light pulse; and determine the distance of the target object based on light sensed using the first measurement shutter window and the second measurement shutter window and the calibrated TOF depth measurement reference information.