an image sensor having an array of pixel cells , each including a photo - conversion device . the array has first , second , and third groups of pixel cells . the first group of pixel cells receives light and the second and third groups are shielded from light . each pixel cell of the second group is configured to output a black reference signal for determining a black level of the array . each pixel cell of the third group has at least one first transistor coupled to the photo - conversion device , and each transistor coupled to the photo - conversion device has a gate coupled to a power supply voltage .